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After a short historical account, and a discussion of Hill and Meyerhof’s theory of the energetics of muscular exercise, we analyse steady-state rest and exercise as the condition wherein coupling of respiration to metabolism is most perfect. The quantitative relationships show that the homeostatic equilibrium, centred around arterial pH of 7.4 and arterial carbon dioxide partial pressure of 40 mmHg, is attained when the ratio of alveolar ventilation to carbon dioxide flow ($${\dot{V}}_{A}/{\dot{V}}_{R}{CO}_{2}$$ V ˙ A / V ˙ R CO 2 ) is − 21.6. Several combinations, exploited during exercise, of pertinent respiratory variables are compatible with this equilibrium, allowing adjustment of oxygen flow to oxygen demand without its alteration. During exercise transients, the balance is broken, but the coupling of respiration to metabolism is preserved when, as during moderate exercise, the respiratory system responds faster than the metabolic pathways. At higher exercise intensities, early blood lactate accumulation suggests that the coupling of respiration to metabolism is transiently broken, to be re-established when, at steady state, blood lactate stabilizes at higher levels than resting. In the severe exercise domain, coupling cannot be re-established, so that anaerobic lactic metabolism also contributes to sustain energy demand, lactate concentration goes up and arterial pH falls continuously. The $${\dot{V}}_{A}/{\dot{V}}_{R}{CO}_{2}$$ V ˙ A / V ˙ R CO 2 decreases below − 21.6, because of ensuing hyperventilation, while lactate keeps being accumulated, so that exercise is rapidly interrupted. The most extreme rupture of the homeostatic equilibrium occurs during breath-holding, because oxygen flow from ambient air to mitochondria is interrupted. No coupling at all is possible between respiration and metabolism in this case.

Theoretical 100-m performance times (t100-m) of a top athlete at Mexico-City (2250 m a.s.l.), Alto-Irpavi (Bolivia) (3340 m a.s.l.) and in a science-fiction scenario “in vacuo” were estimated assuming that at the onset of the run: (i) the velocity (v) increases exponentially with time; hence (ii) the forward acceleration (af) decreases linearly with v, iii) its time constant (τ) being the ratio between vmax (for af = 0) and af max (for v = 0). The overall forward force per unit of mass (Ftot), sum of af and of the air resistance (Fa = k v2, where k = 0.0037 J·s2·kg−1·m−3), was estimated from the relationship between af and v during Usain Bolt’s extant world record. Assuming that Ftot is unchanged since the decrease of k at altitude is known, the relationships between af and v were obtained subtracting the appropriate Fa values from Ftot, thus allowing us to estimate in the three conditions considered vmax, τ, and t100-m. These were also obtained from the relationship between mechanical power and speed, assuming an unchanged mechanical power at the end of the run (when af ≈ 0), regardless of altitude. The resulting t100-m amounted to 9.515, 9.474, and 9.114 s, and to 9.474, 9.410, and 8.981 s, respectively, as compared to 9.612 s at sea level. Neglecting science-fiction scenarios, t100-m of a world-class athlete can be expected to undergo a reduction of 1.01 to 1.44% at Mexico-City and of 1.44 to 2.10%, at Alto-Irpavi.

New findings What is the topic of this review? This narrative review explores past and recent findings on the mechanical determinants of energy cost during human locomotion, obtained by using a mechanical approach based on Konig's theorem (Fenn's approach). What advances does it highlight? Recent developments in analytical methods and of their applications allow a better understanding of the mechanical-bioenergetic interaction. Advances are: 1) the determination of "frictional" internal work; 2) the association between tendon work and apparent efficiency; 3) a better understanding of the role of energy recovery and internal work in pathological gait (amputees, stroke and obese); 4) a comprehensive analysis of human locomotion in (simulated) low gravity conditions. Abstract During locomotion, muscles use metabolic energy to produce mechanical work (in a more or less efficient way); as a matter of fact, energetics and mechanics can be considered as two sides of the same coin, the latter being investigated to understand the former. A mechanical approach based on Konig's theorem (Fenn's approach) has proved to be a useful tool to elucidate the determinants of the energy cost of locomotion (e. g. the pendulum-like model of walking and the bouncing model of running) and has resulted in many advances in this field. During the last 60 years, this approach has been refined and applied to explore the determinants of energy cost and efficiency in a variety of conditions (e.g. low gravity, un-steady speed). This narrative review aims to summarise current knowledge about the role that mechanical work has played in our understanding of energy cost to date, and to underline how recent developments in analytical methods and their applications in specific locomotion modalities (on a gradient, at low gravity and in un-steady conditions) and in pathological gaits (asymmetric gait pathologies, obese subjects and in the elderly) could continue to push this understanding further. The recent in vivo quantification of new aspects that should be included in the assessment of mechanical work (e.g. frictional internal work and elastic contribution) deserve future research and would improve our knowledge on the mechanical-bioenergetic interaction during human locomotion, as well as in sport science and space exploration. This article is protected by copyright. All rights reserved.

Comfortable walking speed and energy cost of walking are physiological markers of metabolic activity during gait. People with multiple sclerosis are characterized by altered gait biomechanics and energetics, related to the degree of disability and spasticity, which lead to an increased energy cost of walking. Several studies concerning the energy cost of walking in multiple sclerosis have been published. Nevertheless, differences in protocols and characteristics of the sample have led to different outcomes. The aim of the present meta-analysis is to summarize results from studies with specific inclusion characteristics, and to present data about the comfortable walking speed and the energy cost of walking at that speed. Moreover, a detailed discussion of the potential mechanisms involved in the altered metabolic activity during exercise was included. A total of 19 studies were considered, 12 of which were also part of the quantitative analysis. Despite the strict selection process, high between-group heterogeneity was found for both outcomes. Nevertheless, the overall results suggest a pooled mean comfortable walking speed of 1.12 m/s (95% CI 1.05–1.18) and energy cost of 0.19 mLO2/kg/m (95% CI 0.17–0.21). These findings support the results of previous studies suggesting that energy cost of walking may be increased by 2–3 times compared to healthy controls (HC), and encourage the use of this marker in association with other parameters of the disease.

Assessing football players’ sprint mechanical outputs is key to the performance management process (e.g. talent identification, training, monitoring, return-to-sport). This is possible using linear sprint testing to derive force–velocity-power outputs (in laboratory or field settings), but testing requires specific efforts and the movement assessed is not specific to the football playing tasks. This proof-of-concept short communication presents a method to derive the players’ individual acceleration-speed (AS) profile in-situ, i.e. from global positioning system data collected over several football sessions (without running specific tests). Briefly, raw speed data collected in 16 professional male football players over several training sessions were plotted, and for each 0.2 m/s increment in speed from 3 m/s up to the individual top-speed reached, maximal acceleration output was retained to generate a linear AS profile. Results showed highly linear AS profiles for all players (all R2 > 0.984) which allowed to extrapolate the theoretical maximal speed and accelerations as the individual’s sprint maximal capacities. Good reliability was observed between AS profiles determined 2 weeks apart for the players tested, and further research should focus on deepening our understanding of these methodological features. Despite the need for further explorations (e.g. comparison with conceptually close force–velocity assessments that require, isolated and not football-specific linear sprint tests), this in-situ approach is promising and allows direct assessment of football players within their specific acceleration-speed tasks. This opens several perspectives in the performance and injury prevention fields, in football and likely other sprint-based team sports, and the possibility to “test players without testing them”.

A previous approach to estimate the time course of instantaneous metabolic power and O2 consumption in team sports has been updated to assess also energy expenditure against air resistance and to identify walking and running separately. Whole match energy expenditure turned out ≈14% smaller than previously obtained, the fraction against the air resistance amounting to ≈2% of the total. Estimated net O2 consumption and overall energy expenditure are fairly close to those measured by means of a portable metabolic cart; the average difference, after a 45 min exercise period of variable intensity and mode, amounting to ≈10%. Aerobic and anaerobic energy yields, metabolic power, energy expenditure and duration of High (HI) and Low (LI) intensity bouts can also be estimated. Indeed, data on 497 soccer players during the 2014/2015 Italian "Serie A" show that the number of HI efforts decreased from the first to the last 15-min periods of the match, without substantial changes in mean metabolic power (≈22 W·kg−1) and duration (≈6.5 s). On the contrary, mean metabolic power of the LI decreased (5.8 to 4.8 W·kg−1), mainly because of a longer duration thereof, thus underscoring the need for longer recovery periods between HI.

After a brief overview of the principles underlying the assessment of the energy cost of accelerated/decelerated running on flat terrain, as obtained from the biomechanical equivalence between this last and uphill/downhill running at constant speed, metabolic power and actual O2 consumption (\({\dot{\text{V}}}{\text{O}_2}\)) will be estimated during a typical training drill and during a brief period of a soccer match. This exercise will show that: (i) \({\dot{\text{V}}}{\text{O}_2}\), as estimated from metabolic power assuming mono-exponential \({\dot{\text{V}}}{\text{O}_2}\) on/off responses with a time constant of 20 s at the muscle level, is essentially equal to that directly measured by means of a metabolic cart. Hence, (ii) the fractions of the overall energy expenditure derived from aerobic and anaerobic sources can also be estimated, provided that the individual \({\dot{\text{V}}}{\text{O}_2}\)max is also known. Furthermore, the metabolic power approach will be updated to make it possible to estimate energy cost and metabolic power also during walking episodes. Indeed, whereas for any given incline the energy cost of running is independent of the speed, the energy cost of walking reaches a minimum at an optimal speed above and below which it increases. With increasing the incline of the terrain, the curve relating energy cost to walking speed retains this general U shape, but is shifted to a higher level, the optimum speed decreasing towards lower values. In addition, for any given incline there exists a “transition speed” at which the energy cost of walking becomes greater than that of running, and which corresponds to the speed at which the subject spontaneously adopts the running gait. Hence the updated algorithms detect this transition speed, as a function of the equivalent slope, and utilise the appropriate energy cost of walking or running in the subsequent calculations. The concluding paragraphs are devoted to a critical discussion of the principal assumptions underlying the metabolic power approach. Specifically, we will stress that the approach as such can be meaningfully applied only to forward walking or running. Thus, whereas other specific sport activities such as jumping, moving backwards, laterally, or with the ball, may be taken into account, at least in a broad sense, assuming appropriate correction factors (possibly on the bases of actual experimental data), other activities, such as those occurring in rugby, of American football, need entirely different approaches, a fact that, all too often, does not seem to be appropriately considered.

The time course of metabolic power during 100–400 m top running performances in world class athletes was estimated assuming that accelerated running on flat terrain is biomechanically equivalent to uphill running at constant speed, the slope being dictated by the forward acceleration. Hence, since the energy cost of running uphill is known, energy cost and metabolic power of accelerated running can be obtained, provided that the time course of the speed is determined. Peak metabolic power during the 100 and 200 m current world records (9.58 and 19.19 s) and during a 400 m top performance (44.06 s) amounted to 163, 99 and 75 W kg−1, respectively. Average metabolic power and overall energy expenditure during 100–5000 m current world records in running were also estimated as follows. The energy spent in the acceleration phase, as calculated from mechanical kinetic energy (obtained from average speed) and assuming 25% efficiency for the transformation of metabolic into mechanical energy, was added to the energy spent for constant speed running (air resistance included). In turn, this was estimated as: (3.8 + k′ v2) · d, where 3.8 J kg−1 m−1 is the energy cost of treadmill running, k′ = 0.01 J s2 kg−1 m−3, v is the average speed (m s−1) and d (m) the overall distance. Average metabolic power decreased from 73.8 to 28.1 W kg−1 with increasing distance from 100 to 5000 m. For the three shorter distances (100, 200 and 400 m), this approach yielded results rather close to mean metabolic power values obtained from the more refined analysis described above. For distances between 1000 and 5000 m the overall energy expenditure increases linearly with the corresponding world record time. The slope and intercept of the regression are assumed to yield maximal aerobic power and maximal amount of energy derived from anaerobic stores in current world records holders; they amount to 27 W kg−1 (corresponding to a maximal O2 consumption of 77.5 ml O2 kg−1 min−1 above resting) and 1.6 kJ kg−1 (76.5 ml O2 kg−1). This last value is on the same order of the maximal amount of energy that can be derived from complete utilisation of phosphocreatine in the active muscle mass and from maximal tolerable blood lactate accumulation. The anaerobic energy yield has also been estimated, throughout the overall set of distances (100–5000 m), assuming that, at work onset, the rate of O2 consumption increases with a time constant of 20 s tending to the appropriate metabolic power, but stops increasing once the maximal O2 consumption is attained. Hence the overall energy expenditure can be partitioned into its aerobic and anaerobic components. This last increases from about 0.6 kJ kg−1 for the shortest distance (100 m) to a maximum close to that estimated above (1.6 kJ kg−1) for distances of 1500 m or longer.

Bilateral deficit (BLD) occurs when the force (or power) generated by both limbs together is smaller than the sum of the forces (or powers) developed separately by the two limbs. The amount of BLD can be altered by neural adaptations brought about by the repetitive execution of specific motor tasks (training). Prolonged disuse also leads to relevant neural adaptations; however, its effects on BLD are still unknown. Thus, the aim of this study was to investigate the effects of a 35-day bed rest on the BLD of the lower limbs. Ten young healthy volunteers performed maximal explosive efforts on a sledge ergometer with both lower limbs or with the right and the left limb separately. Electromyography (EMG) of vastus lateralis, rectus femoris, biceps femoris and gastrocnemius medialis was also measured. Before bed rest, maximal explosive power and peak force were significantly higher in monolateral than bilateral efforts (+18.7 and +31.0 %, respectively). Conversely, peak velocity was 11.9 % greater in bilateral than monolateral efforts. BLD attained a value of 18.1 % and was accompanied by lower EMG amplitude of knee extensors (−17.0 %) and gastrocnemius medialis (−11.7 %) during bilateral efforts. Bed rest led to a ~28.0 % loss in both bilateral and monolateral maximal explosive power. Neither BLD magnitude nor the difference in EMG amplitudes as well as in peak force and velocity between bilateral and monolateral efforts were affected by bed rest. These results suggest that the neuromuscular factors underlying BLD are unaltered after prolonged disuse.

In the 2012 Paralympic 100 m and 200 m finals, 86% of athletes with a unilateral amputation placed their unaffected leg on the front starting block. Can this preference be explained biomechanically? We measured the biomechanical effects of starting block configuration for seven nonamputee sprinters and nine athletes with a unilateral amputation. Each subject performed six starts, alternating between their usual and unusual starting block configurations. When sprinters with an amputation placed their unaffected leg on the front block, they developed 6% greater mean resultant combined force compared with the opposite configuration (1.38 ± 0.06 vs 1.30 ± 0.11 BW, P = .015). However, because of a more vertical push angle, horizontal acceleration performance was equivalent between starting block configurations. We then used force data from each sprinter with an amputation to calculate the hypothetical starting mechanics for a virtual nonamputee (two unaffected legs) and a virtual bilateral amputee (two affected legs). Accelerations of virtual bilateral amputees were 15% slower compared with athletes with a unilateral amputation, which in turn were 11% slower than virtual nonamputees. Our biomechanical data do not explain the starting block configuration preference but they do explain the starting performance differences observed between nonamputee athletes and those with leg amputations.

The objective of this study is to quantify the contribution of the force-velocity (F-v) properties to bilateral force deficit (BLD) in ballistic lower limb push-off and to relate it to individual F-v mechanical properties of the lower limbs.The F-v relation was individually assessed from mechanical measurements for 14 subjects during maximal ballistic lower limb push-offs; its contribution to BLD was then investigated using a theoretical macroscopic approach, considering both the mechanical constraints of movement dynamics and the maximal external capabilities of the lower limb neuromuscular system.During ballistic lower limb push-off, the maximum force each lower limb can produce was lower during bilateral than unilateral actions, thus leading to a BLD of 36.7% ± 5.7%. The decrease in force due to the F-v mechanical properties amounted to 19.9% ± 3.6% of the force developed during BL push-offs, which represents a nonneural contribution to BLD of 43.5% ± 9.1%. This contribution to BLD that cannot be attributed to changes in neural features was negatively correlated to the maximum unloaded extension velocity of the lower limb (r = -0.977, P0.001).During ballistic lower limb push-off, BLD is due to both neural alterations and F-v mechanical properties, the latter being associated with the change in movement velocity between bilateral and unilateral actions. The level of the contribution of the F-v properties depends on the individual F-v mechanical profile of the entire lower limb neuromuscular system: the more the F-v profile is oriented toward velocity capabilities, the lower the loss of force from unilateral to bilateral push-offs due to changes in movement velocity.

We tested the linear critical power ( $$\dot{w}_{\text{cr}}$$ ) model for discrete incremental ramp exercise implying recovery intervals at the end of each step. Seven subjects performed incremental (power increment 25 W) stepwise ramps to subject’s exhaustion, with recovery intervals at the end of each step. Ramps’ slopes (S) were 0.83, 0.42, 0.28, 0.21, and 0.08 W s−1; recovery durations (t r) were 0 (continuous stepwise ramps), 60, and 180 s (discontinuous stepwise ramps). We determined the energy store component (W′), the peak power ( $$\dot{w}_{\text{peak}}$$ ), and $$\dot{w}_{\text{cr}}$$ . When t r = 0 s, $$\dot{w}_{\text{cr}}$$ and W′ were 187 ± 26 W and 14.5 ± 5.8 kJ, respectively. When t r = 60 or 180 s, the model for ramp exercise provided inconsistent $$\dot{w}_{\text{cr}}$$ values. A more general model, implying a quadratic $$\dot{w}_{\text{peak}}$$ versus $$\sqrt S$$ relationship, was developed. This model yielded, for t r = 60 s, $$\dot{w}_{\text{cr}}$$ = 189 ± 48 W and W′ = 18.6 ± 17.8 kJ, and for t r = 180 s, $$\dot{w}_{\text{cr}}$$ = 190 ± 34 W, and W′ = 16.4 ± 16.7 kJ. These $$\dot{w}_{\text{cr}}$$ and W′ did not differ from the corresponding values for t r = 0 s. Nevertheless, the overall amount of energy sustaining work above $$\dot{w}_{\text{cr}}$$ , due to energy store reconstitution during recovery intervals, was higher the longer t r, whence higher $$\dot{w}_{\text{peak}}$$ values. The linear $$\dot{w}_{\text{cr}}$$ model for ramp exercise represents a particular case (for t r = 0 s) of a more general model, accounting for energy resynthesis following oxygen deficit payment during recovery.

Purpose: The study�s purpose was to determine the respective influences of the maximal power ( Equation (Uncited) Image Tools ) and the force�velocity (F�v) mechanical profile of the lower limb neuromuscular system on performance in ballistic movements. Methods: A theoretical integrative approach was proposed to express ballistic performance as a mathematical function of Equation (Uncited) Image Tools and F�v profile. This equation was (i) validated from experimental data obtained on 14 subjects during lower limb ballistic inclined push-offs and (ii) simulated to quantify the respective influence of Equation (Uncited) Image Tools and F�v profile on performance. Results: The bias between performances predicted and obtained from experimental measurements was 4%�7%, confirming the validity of the proposed theoretical approach. Simulations showed that ballistic performance was mostly influenced not only by Equation (Uncited) Image Tools but also by the balance between force and velocity capabilities as described by the F�v profile. For each individual, there is an optimal F�v profile that maximizes performance, whereas unfavorable F�v balances lead to differences in performance up to 30% for a given Equation (Uncited) Image Tools . This optimal F�v profile, which can be accurately determined, depends on some individual characteristics (limb extension range, Equation (Uncited) Image Tools ) and on the afterload involved in the movement (inertia, inclination). The lower the afterload, the more the optimal F�v profile is oriented toward velocity capabilities and the greater the limitation of performance imposed by the maximal velocity of lower limb extension. Conclusions: High ballistic performances are determined by both maximization of the power output capabilities and optimization of the F�v mechanical profile of the lower limb neuromuscular system.

Our objective was to determine the effects of long-lasting endurance events on the energy cost of running (C r), and the role of maximal oxygen uptake (VO2max), its fractional utilisation (F) and C r in determining the performance. Ten healthy runners (age range 26–59 years) participated in an ultra-endurance competition consisting of three running laps of 22, 48 and 20 km on three consecutive days in the North–East of Italy. Anthropometric characteristics and VO2max by a graded exercise test on a treadmill were determined 5 days before and 5 days after the competition. In addition, C r was determined on a treadmill before and after each running lap. Heart rate (HR) was recorded throughout the three laps. Results revealed that mean C r of the individual laps did not increase significantly with lap number (P = 0.200), thus ruling out any chronic lap effect. Even so, however, at the end of lap 3, C r was 18.0% (P

Video match analysis is used for the assessment of physical performances of professional soccer players, particularly for the identification of "high intensities" considered as "high running speeds." However, accelerations are also essential elements setting metabolic loads, even when speed is low. We propose a more detailed assessment of soccer players' metabolic demands by video match analysis with the aim of also taking into account accelerations.A recent study showed that accelerated running on a flat terrain is equivalent to running uphill at constant speed, the incline being dictated by the acceleration. Because the energy cost of running uphill is known, this makes it possible to estimate the instantaneous energy cost of accelerated running, the corresponding instantaneous metabolic power, and the overall energy expenditure, provided that the speed (and acceleration) is known. Furthermore, the introduction of individual parameters makes it possible to customize performance profiles, especially as it concerns energy expenditure derived from anaerobic sources. Data from 399 "Serie-A" players (mean +/- SD; age = 27 +/- 4 yr, mass = 75.8 +/- 5.0 kg, stature = 1.80 +/- 0.06 m) were collected during the 2007-2008 season.Mean match distance was 10,950 +/- 1044 m, and average energy expenditure was 61.12 +/- 6.57 kJ x kg(-1). Total distance covered at high power (20 W x kg(-1)) amounted to 26% and corresponding energy expenditure to approximately 42% of the total. "High intensities" expressed as high-power output are two to three times larger than those based only on running speed.The present approach for the assessment of top-level soccer players match performance through video analysis allowed us to assess instantaneous metabolic power, thus redefining the concept of "high intensity" on the basis of actual metabolic power rather than on speed alone.

We investigated whether bilateral deficit (BLD): (1) is observed during explosive lower limb contractions; (2) can be attributed to a reduction of neural drive and/or (3) to a different muscle coordination, and/or (4) to changes of the muscle force-velocity (F-v) relationship. Ten volunteers performed maximal explosive efforts of approximately 450 ms on a sledge ergometer, with both lower limbs (BL), with the right and left limb separately (ML), against different overloads. Peak-force (F, N), peak-power (w, W), sledge peak-velocity (v, m/s) and electromyography (EMG) of vastus lateralis (VL), rectus femoris (RF), biceps femoris (BF) and gastrocnemius medialis (GM) were recorded. Average values over the six overloads of F and w, developed by right or left limb during BL, were significantly lower (883 +/- 200 and 918 +/- 141 N; 1089 +/- 407 and 1099 +/- 325 W) than those developed during ML contractions (1285 +/- 177 and 1306 +/- 147 N; 1536 +/- 408 and 1497 +/- 392 W). VL and RF iEMGs were lower in BL than in ML (74 +/- 28 vs. 91 +/- 21% MVC and 39 +/- 21 vs. 56 +/- 28% MVC). The coordination among the four muscles, as determined from an analysis of the time course of iEMG, expressed as percentage of that attained at the end of the push, was poorer in BL, as compared to ML. BL F-v curves were different as compared to the ML ones, the force and power developed, at a given v, being significantly larger in ML. It is concluded that BLD occurs also during explosive pushes with the lower limbs, and that it is mainly due to different muscle coordination.

Exposure to microgravity leads to “cardiovascular deconditioning” (CVD), because of the fluids shift toward the thorax. CVD is characterised by: 1) a decrease of plasma and interstitial fluid volumes, 2) a relative increase of the erythrocytes mass, 3) a decrease of arterial diastolic pressure, of the stroke volume, of the end-diastolic volume and of the left ventricular mass. CVD can be expected to occur also in astronauts living permanently on Lunar or Martian bases, since on these celestial bodies the acceleration of gravity is about 0.165 and 0.379 the Earth value. In these conditions, cycling on appropriately constructed tracks may be useful to recreate artificial gravity and to allow the astronauts to perform physical exercise. Indeed, a cyclist riding a bicycle on a circular track, generates an outward acceleration vector which depends on the radius of the track and on the ground speed. The vectorial sum of this last and the acceleration of gravity acts in the head to feet direction, thus increasing the effects of gravity on the cardiovascular system. We propose to construct on a Lunar or Martian base a circular “track tunnel” with a radius of 25 m. We show here that when cycling on this track tunnel at speeds between 10 to 15 m · s − 1, astronauts will generate a g vector acting along the head to feet axis ranging from 0.44 to 0.99 of the Earth value. We suggest that the logistics and feasibility of these track-tunnels should be studied in view of their possible implementation.

At the onset of a square-wave exercise of moderate intensity, in the absence of any detectable lactate production, the hydrolysis of phosphocreatine (PCr) fills the gap between energy requirement and energy yield by oxidative pathways, thus representing a readily available source of energy for the muscle. We verified experimentally the relationships between high-energy phosphates and/or their changes and the time constant of PCr concentration ([PCr]) kinetics in humans (τPCr). High-energy phosphate concentration (by31P-NMR spectroscopy) in the calf muscles were measured during three repetitions of the rest-to-work transition of moderate aerobic square-wave exercise on nine healthy volunteers, while resting [PCr] was estimated from the appropriate spectroscopy data. PCr concentration decreased significantly (22 ± 6%) from rest to steady-state exercise, without differences among the three repetitions. Absolute resting [PCr] and τPCrwere consistent with literature values, amounting to 27.5 ± 2.2 mM and 23.9 ± 2.9 s, respectively. No significant relationships were detected between individual τPCrand mechanical power, fraction or absolute amount of PCr hydrolyzed, or change in ADP concentration. On the contrary, individual τPCr(s) was linearly related to absolute resting [PCr] (mM), the relationship being described by: τPCr= 0.656 + 0.841·[PCr] ( n = 9, R = 0.708, P < 0.05). These data support the view that in humans PCr concentration sets the time course of the oxidative metabolism in skeletal muscle at the start of exercise, being one of the main controllers of oxidative phosphorylation.

This investigation aimed to compare the response of young and older adult men to bed rest (BR) and subsequent rehabilitation (R). Sixteen older (OM, age 55-65 yr) and seven young (YM, age 18–30 yr) men were exposed to a 14-day period of BR followed by 14 days of R. Quadriceps muscle volume (QVOL), force (QF), and explosive power (QP) of leg extensors; single-fiber isometric force (Fo); peak aerobic power (V̇o2peak); gait stride length; and three metabolic parameters, Matsuda index of insulin sensitivity, postprandial lipid curve, and homocysteine plasma level, were measured before and after BR and after R. Following BR, QVOL was smaller in OM (−8.3%) than in YM (−5.7%, P = 0.031); QF (−13.2%, P = 0.001), QP (−12.3%, P = 0.001), and gait stride length (−9.9%, P = 0.002) were smaller only in OM. Fo was significantly smaller in both YM (−32.0%) and OM (−16.4%) without significant differences between groups. V̇o2peak decreased more in OM (−15.3%) than in YM (−7.6%, P < 0.001). Instead, the Matsuda index fell to a greater extent in YM than in OM (−46.0% vs. −19.8%, respectively, P = 0.003), whereas increases in postprandial lipid curve (+47.2%, P = 0.013) and homocysteine concentration (+26.3%, P = 0.027) were observed only in YM. Importantly, after R, the recovery of several parameters, among them QVOL, QP, and V̇o2peak, was not complete in OM, whereas Fo did not recover in either age group. The results show that the effect of inactivity on muscle mass and function is greater in OM, whereas metabolic alterations are greater in YM. Furthermore, these findings show that the recovery of preinactivity conditions is slower in OM.

Previous observations have shown that, in isolated perfused dog gastrocnemii in situ, stimulated to aerobic rhythmic isotonic tetanic contractions (at about 40% of maximal isometric force), only about 20% of the overall metabolic power (proportional to the rate of O2 consumption, VO2) was converted into mechanical power (W). Here we report that, in the same preparation, the maximal velocity during the shortening phase of each tetanus (v, mm s−1) increased with the rate of energy dissipation, as given by the difference between VO2 and W (W kg−1). The relationship between these variables was described by: v=2.85(VO2−W)1.24 (R 2=0.85; n=17). A mathematical analysis of this equation shows that the overall mechanical efficiency (η=WVO2 −1) decreased with increasing v (at constant VO2), whereas it increased with increasing VO2 (at constant v). The net effect of this state of affairs was that the decrease of η over the entire range of work intensities was relatively minor (from 0.22 to 0.15), in spite of a large increase of v, (from 40 to 120 mm s−1), thanks to the concomitant increase of VO2 (from 10 to 25 W kg−1). So, under these experimental conditions, the energetics of work performance seems to be governed by two conflicting needs. The need for a sufficiently high shortening speed (and hence power output), itself requiring a sufficiently large energy dissipation rate, which, however, brings about a fall in η. This is counteracted by the increased VO2, which in turn leads to an increased efficiency at the expense of a fall in shortening speed.

In simulated or actual microgravity, human and animal postural muscles undergo substantial atrophy: after about 270 days, the muscle mass attains a constant value of about 70% of the initial one. Most animal studies reported preferential atrophy of slow twitch fibres whose mechanical properties change towards the fast type. However, in humans, at the end of a 42-days bed rest study, a similar atrophy of slow and fast fibres was observed. After microgravity, the maximal force of several muscle groups showed a substantial decrease (6–25% of pre-flight values). The maximal power during very short “explosive” efforts of 0.25–0.30 s showed an even greater fall, being reduced to 65% after 1 month and to 45% (of pre-flight values) after 6 months. The maximal power developed during 6–7 s “all-out” bouts on an isokinetic cycloergometer was reduced to a lesser extent, attaining about 75% of pre-flight values, regardless of the flight duration. In these same subjects, the muscle mass of the lower limbs declined by only 9–13%. Thus, a substantial fraction of the observed decreases of maximal power is probably due to a deterioration of the motor co-ordination brought about by the absence of gravity. To prevent this substantial decay of maximal absolute power, we propose that explosive exercise be added to the daily in-flight training schedule. We also describe a system aimed at reducing cardiovascular deconditioning wherein gravity is simulated by the centrifugal acceleration generated by the motion of two counter rotating bicycles ridden by the astronauts on the inner wall of a cylindrical space module. Finally, cycling on circular or elliptical tracks may be useful to reduce cardiovascular deconditioning in permanently manned lunar bases. Indeed, on the curved parts of the path, a cyclist generates an outward acceleration vector (ac). To counterbalance ac, the cyclist must lean inwards, so that the vectorial sum of ac plus the lunar gravity tends to the acceleration of gravity prevailing on Earth.

The effects of using different algorithms to estimate the time constant of changes in oxygen uptake at the onset of square-wave 120 W cycloergometric exercise were evaluated in seven subjects. The volume of oxygen taken up at the alveoli (VO(2Ai)) was determined breath-by-breath (BB) from the volume of O(2) transferred at the mouth (VO(2mi)) minus the corresponding volume changes in O(2) stores in the alveoli: VO(2Ai)= VO(2mi)-[V(Ai-1)(FO(2Ai)- FO(2Ai-1))+ FO(2Ai) x Delta V(Ai)], where V(Ai-1) is the alveolar volume at the end of the previous breath, FO(2Ai) and FO(2Ai-1) are estimated from the fractions of end-tidal O(2) in the current and previous breaths, respectively, and Delta V(Ai) is the change in volume during breath i. These quantities can be measured BB, with the exception of V(Ai-1) which must be assumed. The respiratory cycle has been defined as the time elapsing between identical fractions of expiratory gas in two successive breaths. Using this approach, since FO(2Ai)= FO(2Ai-1), any assumption regarding V(Ai-1) becomes unnecessary. In the present study, VO(2Ai) was calculated firstly, by using this approach, and secondly by setting different V(Ai-1) values (from 0 to FRC+0.5 l, where FRC is the functional residual capacity). Values for alveolar O(2) flow (VO(2Ai)), as calculated from the quotient of VO(2Ai) divided by breath duration, were then fitted bi-exponentially. The time constant of the phase II kinetics of VO(2Ai) (tau(2)) was linearly related to V(Ai-1), increasing from 36.6 s (V(Ai-1)=0) to 46.8 s (V(Ai-1)=FRC+0.5 l) while tau(2) estimated using the first approach amounted to 34.3 s. We concluded that, firstly, the first approach allowed us to calculate O(2A) during transitions in step exercise; and secondly, when using methods wherein V(Ai-1) must be assumed, tau(2) depended on V(Ai-1).

To estimate the energetics and biomechanics of accelerated/decelerated running on flat terrain based on its biomechanical similarity to constant speed running up/down an ‘equivalent slope’ dictated by the forward acceleration (a f). Time course of a f allows one to estimate: (1) energy cost of sprint running (C sr), from the known energy cost of uphill/downhill running, and (2) instantaneous (specific) mechanical accelerating power (P sp = a f × speed). In medium-level sprinters (MLS), C sr and metabolic power requirement (P met = C sr × speed) at the onset of a 100-m dash attain ≈50 J kg−1 m−1, as compared to ≈4 for running at constant speed, and ≈90 W kg−1. For Bolt’s current 100-m world record (9.58 s) the corresponding values attain ≈105 J kg−1 m−1 and ≈200 W kg−1. This approach, as applied by Osgnach et al. (Med Sci Sports Exerc 42:170–178, 2010) to data obtained by video-analysis during soccer games, has been implemented in portable GPS devices (GPEXE©), thus yielding P met throughout the match. Actual O2 consumed, estimated from P met assuming a monoexponential VO2 response (Patent Pending, TV2014A000074), was close to that determined by portable metabolic carts. Peak P sp (W kg−1) was 17.5 and 19.6 for MLS and elite soccer players, and 30 for Bolt. The ratio of horizontal to overall ground reaction force (per kg body mass) was ≈20 % larger, and its angle of application in respect to the horizontal ≈10° smaller, for Bolt, as compared to MLS. Finally, we estimated that, on a 10 % down-sloping track Bolt could cover 100 m in 8.2 s. The above approach can yield useful information on the bioenergetics and biomechanics of accelerated/decelerated running.

Purpose:To investigate the effects of an extreme uphill marathon on the mechanical parameters that are likely to affect the energy cost of running (Cr).Methods:Eleven runners (27–59 y) participated in the Etna SuperMarathon (43 km, 0–3063 m above sea level). Anthropometric characteristics, maximal explosive power of the lower limb (Pmax), and maximal oxygen uptake were determined before the competition. In addition, before and immediately after the race, Cr, contact (tc) and aerial (ta) times, step frequency (f), and running velocity were measured at constant self-selected speed. Then, peak vertical ground-reaction force (Fmax), vertical downward displacement of the center of mass (Δz), leg-length change (ΔL), and vertical (kvert) and leg (kleg) stiffness were calculated.Results:A direct relationship between Cr, measured before the race, and race time was shown (r = .61, P < .001). Cr increased significantly at the end of the race by 8.7%. Immediately after the race, the subjects showed significantly lower ta (–58.6%), f (–11.3%), Fmax (–17.6%), kvert (–45.6%), and kleg (–42.3%) and higher tc (+28.6%), Δz (+52.9%), and ΔL (+44.5%) than before the race. The increase of Cr was associated with a decrement in Fmax (r = –.45), kvert (r = –.44), and kleg (r = –.51). Finally, an inverse relationship between Pmax measured before the race and ΔCr during race was found (r = –.52).Conclusions:Lower Cr was related with better performance, and athletes characterized by the greater Pmax showed lower increases in Cr during the race. This suggests that specific power training of the lower limbs may lead to better performance in ultraendurance running competition.

Microgravity leads to a decline of muscle power especially in the postural muscles of the lower limb. Muscle atrophy primarily contributes to this negative adaptation. Nutritional countermeasures during unloading were shown to possibly mitigate the loss of muscle mass and strength. The aim of this study was to investigate the effects of different diet energy intakes during prolonged inactivity on body composition and lower limbs power output. The effects of lower or higher diet energy intake on the decline of maximal explosive power of the lower limbs, as determined on a sledge ergometer before and after 35 days of bed rest, were investigated on two matched groups of young healthy volunteers. Body composition and lean volume of the lower limb were also measured. After bed rest, fat mass increased (+20.5 %) in the higher energy intake group (N = 9), while it decreased (−4.8 %) in the lower energy intake group (N = 10). Also, the loss of body fat-free mass and lean volume of the lower limb was significantly greater in the higher (−4.6 and −10.8 %, respectively) as compared to the lower (−2.4 and −3.7 %, respectively) diet energy intake group. However, the loss of maximal explosive power was similar between the two groups (−25.2 and −29.5 % in the higher and lower energy intake group, respectively; P = 0.440). The mitigation of loss of muscle mass by means of a moderate caloric diet restriction during prolonged inactivity was not sufficient for reducing the loss of maximal explosive power of the lower limbs.

The mechanical power for cycling (P(c)) at constant ground speed (s), in the absence of wind on smooth hard terrain is the sum of the power dissipated against rolling resistance, gravity and air resistance: P(c)=a x s + M x g x s x sin gamma + b x S3, where a and b are constants, M is the mass of the subject plus bike, g is the acceleration of gravity and gamma is the angle of the terrain with the horizontal. The constant b depends upon the drag coefficient (Cd), the overall area projected on the frontal plane (A(f)), and the air density (rhoa): b = 0.5 x C(d) x A(f) x rhoa. In turn, rhoa depends on air pressure (P(B)) and temperature (T): rhoa=rho0 x 0.359 x P(B) x T(-1), where rho0 is the air density at 760 mmHg (101.3 kPa) and 273 K. The metabolic power developed by the cyclist (E(c)) is related to P(c) E(c) = P(c) x eta(-1), where eta is the mechanical efficiency of cycling. The experimental values of a, b and eta are fairly well known so that, if the maximal metabolic power as a function of the performance time is known for a given cyclist, the following set of data can be individually calculated: (1) best performances over any given distance and for any given altitude above sea level, (2) the effects of posture and body size on maximal speeds, and (3) the maximal incline of the terrain that can be overcome at any given speed or coasting speed for any given downslope. The above set of information makes it possible also to calculate the characteristics of a "Twin Bikes System" (TBS) for preventing microgravity deconditioning during long-term space flight. The TBS consists of two bicycles that are mechanically coupled by a differential gearing, which move at the very same speed, but in the opposite sense, along the inner wall of a cylindrically shaped space module. The circular trajectories induce a centrifugal acceleration vector (a(c)) oriented along the head-to-feet direction of each subject: a(c) = v(t)2 x r(-1) where v(t) is the tangential velocity and r the radius of gyration, which is equal to the inner radius of the space module. So, any desired value of a(c) can be achieved by appropriately selecting v(t), wherefrom the mechanical and metabolic powers that the astronauts must generate can be readily calculated. Experiments performed in a ground-based human centrifuge have shown that the discomfort derived from the rotating environment is reasonably low and well tolerated. If the appropriate atmospheric pressure is provided, cycling on circular or elliptical tracks may be useful to reduce cardiovascular deconditioning that occurs due to the reduced gravity in permanently manned lunar bases. Indeed, on the curved parts of the path a cyclist will generate a horizontal outward acceleration: a(c) = s2 x r(-1), where s is the velocity along the track and r is the radius of curvature. To counterbalance a(c), the subject plus bike must lean inwards so that the vectorial sum of a(c) plus the lunar gravity (g(L) = 1.62 m x s(-2)) is applied along a straight line that includes the centre of mass of the system and the point of wheel contact with the ground. For values of s from 10 to 20 m x s(-1) and r from 50 to 200 m, this vectorial sum ranges from 1.05 to 5.03 g(L) (0.17-0.83 g).

Because the cardiocirculatory response of heart transplant recipients (HTR) to exercise is delayed, we hypothesized that their O2 uptake (V˙o 2) kinetics at the onset of subthreshold exercise are slowed because of an impaired early “cardiodynamic” phase 1, rather than an abnormal subsequent “metabolic” phase 2. Thus we compared the V˙o 2 kinetics in 10 HTR submitted to six identical 10-min square-wave exercises set at 75% (36 ± 5 W) of the load at their ventilatory threshold (VT) to those of 10 controls (C) similarly exercising at the same absolute (40 W; C40W group) and relative load (67 ± 14 W; C67W group). Time-averaged heart rate, breath-by-breathV˙o 2, and O2pulse (O2p) data yielded monoexponential time constants of the V˙o 2 (s) and O2p increase. Separating phase 1 and 2 data permitted assessment of the phase 1 duration and phase 2 V˙o 2 time constant ([Formula: see text]). The V˙o 2 time constant was higher in HTR (38.4 ± 7.5) than in C40W (22.9 ± 9.6; P ≤ 0.002) or C67W (30.8 ± 8.2; P ≤ 0.05), as was the O2p time constant, resulting from a lower phase 1V˙o 2 increase (287 ± 59 vs. 349 ± 66 ml/min; P ≤ 0.05), O2p increase (2.8 ± 0.6 vs. 3.6 ± 1.0 ml/beat; P ≤ 0.0001), and a longer phase 1 duration (36.7 ± 12.3 vs. 26.8 ± 6.0 s; P≤ 0.05), whereas the[Formula: see text]was similar in HTR and C (31.4 ± 9.6 vs. 29.9 ± 5.6 s; P = 0.85). Thus the HTR have slower subthresholdV˙o 2 kinetics due to an abnormal phase 1, suggesting that the heart is unable to increase its output abruptly when exercise begins. We expected a faster[Formula: see text]in HTR because of their prolonged phase 1 duration. Because this was not the case, their muscular metabolism may also be impaired at the onset of subthreshold exercise.

The aim of this study was to investigate: (1) the role of ![Graphic][1] , the fraction of ![Graphic][2] ( F ) and the metabolic cost of transport (CoT) in determining performance during an ultra-endurance competition and (2) the effects of the race on several biomechanical and morphological parameters of the lower limbs that are likely to affect CoT. Eleven runners (aged 29–54 years) participated in an ultra-endurance competition consisting of three running stages of 25, 55 and 13 km on three consecutive days. Anthropometric characteristics, body composition, morphological properties of the gastrocnemius medialis, maximal explosive power of the lower limb and ![Graphic][3] were determined before the competition. In addition, biomechanics of running and CoT were determined, before and immediately after each running stage. Performance was directly proportional to ![Graphic][4] ( r =0.77) and F ( r =0.36), and inversely proportional to CoT ( r =−0.30). Low CoT values were significantly related to high maximal power of the lower limbs ( r =−0.74) and vertical stiffness ( r =−0.65) and low footprint index (FPI, r =0.70), step frequency ( r =0.62) and external work ( r =0.60). About 50% of the increase in CoT during the stages of the competition was accounted for by changes in FPI, which represents a global evaluation of medio-lateral displacement of the foot during the whole stance phase, which in turn is associated with the myotendinous characteristics of the lower limb. Thus, lower CoT values were related to greater muscular power and lower FPI, suggesting that a better ankle stability is likely to achieve better performance in an ultra-endurance running competition. [1]: /embed/inline-graphic-1.gif [2]: /embed/inline-graphic-2.gif [3]: /embed/inline-graphic-3.gif [4]: /embed/inline-graphic-4.gif

In type 1 diabetic patients, exercise contributes to enhance insulin sensitivity (1,2) and may help, together with diet and insulin therapy, to achieve and maintain better metabolic control (3,4). However, lack of insulin regulation during exercise leads to frequent metabolic imbalances (5). Fat and carbohydrates are the main substrates for energy production in skeletal muscle during aerobic exercise in well-fed humans (6), with their relative contribution to total energy production being a function of exercise intensity (7–10). Below the anaerobic threshold, both oxygen consumption and heart rate during exercise increase linearly as a function of exercise intensity (6). On the basis of these relationships, the aim of the present study was to verify the possibility of using heart rate to estimate the amount of glucose oxidized during exercise in type 1 diabetic patients as well as in a control group of healthy subjects. Fifteen type 1 diabetic patients (9 men, 6 women, HbA1c [A1C] 6 ± 1.2%, reference values 4–6%) (11) aged 18–45 years (average 38 ± 6) and 15 healthy subjects (9 men, 6 women, 34 ± 7 years) gave their voluntary consent to participate after being informed on the nature, purpose, and possible risks. Duration of diabetes and mean insulin dose were 17 ± 8 years and 0.52 ± 0.1 IU · kg−1 · day−1, respectively. Average body weight and height were 71 ± 9 kg and 175 ± 9 cm in diabetic patients vs. 70 ± 9 kg and …

The aim of this study was to: (1) determine directly the energy cost of shuttle running (C Sh) and (2) compare it to the values indirectly estimated from kinematic data. C Sh over distances of ≈10 or ≈20 m was determined on 65 subjects (group 1) from gas exchange measurements over 155 trials, or indirectly estimated on 10 subjects (group 2) from the time course of the speed as follows. (1) The cost to accelerate from zero to peak speed was estimated assuming a 25 % efficiency and added to that of constant-speed running, as obtained on subjects of group 1. (2) Since (i) accelerated running on flat terrain is equivalent to running uphill at constant speed, on a slope dictated by the forward acceleration (di Prampero et al. in J Exp Biol 208:2809-2816, 2005), and (ii) the energy cost of running uphill is known, C Sh was obtained from the time course of the acceleration. C Sh increased with the average speed, at any given speed being significantly greater for the shorter distances; e.g., at ≈4 m/s over 10 m, it amounted to ≈14 J/(kg m), i.e., 3.5-fold larger than that at constant speed. The two indirect methods yielded results not significantly different from C Sh over the longer (≈20 m), but underestimated it over the shorter (≈10 m) distances. From our results, over ≈20 m C Sh can be obtained with sufficient accuracy from actual measurements of peak speed alone, thus, greatly simplifying the experimental procedure. The so-obtained data can then be utilized to assess the athletic status of any subject, as well as to plan appropriate training strategies.

We investigated the relationship between mechanical and energy cost of transport and body mass in running humans. Ten severely obese (body mass ranging from 108.5 to 172.0 kg) and 15 normal-weighted (52.0-89.0 kg) boys and men, aged 16.0-45.8 years, participated in this study. The rate of O(2) consumption was measured and the subjects were filmed with four cameras for kinematic analysis, while running on a treadmill at 8 km h(-1). Mass specific energy cost (C (r)) and external mechanical work (W (ext)) per unit distance were calculated and expressed in joules per kilogram per meter, efficiency (η) was then calculated as W (ext) × C (r) (-1) × 100. Both mass-specific C (r) and W (ext) were found to be independent of body mass (M) (C (r) = 0.002 M + 3.729, n = 25, R (2) = 0.05; W (ext) = -0.001 M + 1.963, n = 25, R (2) = 0.01). It necessarily follows that the efficiency is also independent of M (η = -0.062 M + 53.3298, n = 25, R (2) = 0.05). The results strongly suggest that the elastic tissues of obese subjects can adapt (e.g., thickening) to the increased mass of the body thus maintaining their ability to store elastic energy, at least at 8 km h(-1) speed, at the same level as the normal-weighted subjects.

The study's purpose was to determine the respective influences of the maximal power (Pmax) and the force-velocity (F-v) mechanical profile of the lower limb neuromuscular system on performance in ballistic movements.A theoretical integrative approach was proposed to express ballistic performance as a mathematical function of Pmax and F-v profile. This equation was (i) validated from experimental data obtained on 14 subjects during lower limb ballistic inclined push-offs and (ii) simulated to quantify the respective influence of Pmax and F-v profile on performance.The bias between performances predicted and obtained from experimental measurements was 4%-7%, confirming the validity of the proposed theoretical approach. Simulations showed that ballistic performance was mostly influenced not only by Pmax but also by the balance between force and velocity capabilities as described by the F-v profile. For each individual, there is an optimal F-v profile that maximizes performance, whereas unfavorable F-v balances lead to differences in performance up to 30% for a given Pmax. This optimal F-v profile, which can be accurately determined, depends on some individual characteristics (limb extension range, Pmax) and on the afterload involved in the movement (inertia, inclination). The lower the afterload, the more the optimal F-v profile is oriented toward velocity capabilities and the greater the limitation of performance imposed by the maximal velocity of lower limb extension.High ballistic performances are determined by both maximization of the power output capabilities and optimization of the F-v mechanical profile of the lower limb neuromuscular system.

A functional evaluation of skeletal muscle oxidative metabolism during dynamic knee extension (KE) incremental exercises was carried out following a 35-day bed rest (BR) (Valdoltra 2008 BR campaign). Nine young male volunteers (age: 23.5 ± 2.2 yr; mean ± SD) were evaluated. Pulmonary gas exchange, heart rate and cardiac output (by impedance cardiography), skeletal muscle (vastus lateralis) fractional O2extraction, and brain (frontal cortex) oxygenation (by near-infrared spectroscopy) were determined during incremental KE. Values at exhaustion were considered “peak”. Peak heart rate (147 ± 18 beats/min before vs. 146 ± 17 beats/min after BR) and peak cardiac output (17.8 ± 3.3 l/min before vs. 16.1 ± 1.8 l/min after BR) were unaffected by BR. As expected, brain oxygenation did not decrease during KE. Peak O2uptake was lower after vs. before BR, both when expressed as liters per minute (0.99 ± 0.17 vs. 1.26 ± 0.27) and when normalized per unit of quadriceps muscle mass (46.5 ± 6.4 vs. 56.9 ± 11.0 ml·min−1·100 g−1). Skeletal muscle peak fractional O2extraction, expressed as a percentage of the maximal values obtained during a transient limb ischemia, was lower after (46.3 ± 12.1%) vs. before BR (66.5 ± 11.2%). After elimination, by the adopted exercise protocol, of constraints related to cardiovascular O2delivery, a decrease in peak O2uptake and muscle peak capacity of fractional O2extraction was found after 35 days of BR. These findings suggest a substantial impairment of oxidative function at the muscle level, “downstream” with respect to bulk blood flow to the exercising muscles, that is possibly at the level of blood flow distribution/O2utilization inside the muscle, peripheral O2diffusion, and intracellular oxidative metabolism.