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Fatigability increases while the capacity for mitochondrial energy production tends to decrease significantly with age. Thus, diminished mitochondrial function may contribute to higher levels of fatigability in older adults. The relationship between fatigability and skeletal muscle mitochondrial function was examined in 30 participants aged 78.5 ± 5.0 years (47% female, 93% white), with a body mass index of 25.9 ± 2.7 kg/m(2) and usual gait-speed of 1.2 ± 0.2 m/s. Fatigability was defined using rating of perceived exertion (6-20 point Borg scale) after a 5-minute treadmill walk at 0.72 m/s. Phosphocreatine recovery in the quadriceps was measured using (31)P magnetic resonance spectroscopy and images of the quadriceps were captured to calculate quadriceps volume. ATPmax (mM ATP/s) and oxidative capacity of the quadriceps (ATPmax·Quadriceps volume) were calculated. Peak aerobic capacity (VO2peak) was measured using a modified Balke protocol. ATPmax·Quadriceps volume was associated with VO2peak and was 162.61mM ATP·mL/s lower (p = .03) in those with high (rating of perceived exertion ≥10) versus low (rating of perceived exertion ≤9) fatigability. Participants with high fatigability required a significantly higher proportion of VO2peak to walk at 0.72 m/s compared with those with low fatigability (58.7 ± 19.4% vs 44.9 ± 13.2%, p < .05). After adjustment for age and sex, higher ATPmax was associated with lower odds of having high fatigability (odds ratio: 0.34, 95% CI: 0.11-1.01, p = .05). Lower capacity for oxidative phosphorylation in the quadriceps, perhaps by contributing to lower VO2peak, is associated with higher fatigability in older adults.

Increased maximal oxygen uptake (V(O(2)max)), mitochondrial capacity and energy coupling efficiency are reported after endurance training (ET) in adult subjects. Here we test whether leg exercise performance (power output of the legs, P(max), at V(O(2)max)) reflects these improvements with ET in the elderly. Fifteen male and female subjects were endurance trained for a 6 month programme, with 13 subjects (69.5 ± 1.2 years old, range 65-80 years old; n = 7 males; n = 6 females) completing the study. This training significantly improved P(max) (Δ17%; P = 0.003), V(O(2)max) (Δ5.4%; P = 0.021) and the increment in oxygen uptake (V(O(2))) above resting (ΔV(O(2)m-r) = V(O(2)max) - V(O(2)rest; Δ9%; P < 0.02). In addition, evidence of improved energy coupling came from elevated leg power output per unit V(O(2))at the aerobic capacity [Δ(P(max)/ΔV(O(2)m-r)); P = 0.02] and during submaximal exercise in the ramp test as measured by delta efficiency (ΔP(ex)/ΔV(O(2)); P = 0.04). No change was found in blood lactate, muscle glycolysis or fibre type. The rise in P(max) paralleled the improvement in muscle oxidative phosphorylation capacity (ATP(max)) in these subjects. In addition, the greater exercise energy coupling [Δ(P(max)/ΔV(O(2)m-r)) and delta efficiency] was accompanied by increased mitochondrial energy coupling as measured by elevated ATP production per unit mitochondrial content in these subjects. These results suggest that leg exercise performance benefits from elevations in energy coupling and oxidative phosphorylation capacity at both the whole-body and muscle levels that accompany endurance training in the elderly.

Chronic kidney disease (CKD) leads to the retention of uremic solutes that disrupt skeletal muscle function1 leading to reduced physical performance and mobility limitation2. Disruption of muscle mitochondrial energetics in CKD may precede the onset of detectable functional limitations. In particular, reduced coupling of ATP production to oxygen consumption (P/O ratio) within mitochondria of skeletal muscle indicates oxidative stress, impaired metabolism, and reduced exercise efficiency3–6. Comprehensive analysis of skeletal muscle mitochondrial energetics in CKD patients has been limited by lack of precise, real-time, non-invasive techniques.

Aims Pioglitazone (Pio) is known to improve insulin sensitivity in skeletal muscle. However, the role of Pio in skeletal muscle lipid metabolism and skeletal muscle oxidative capacity is not clear. The aim of this study was to determine the effects of chronic Pio treatment on skeletal muscle mitochondrial activity in individuals with type 2 diabetes (T2D). Materials and Methods Twenty-four participants with T2D (13M/11F 53.38±2.1years; BMI 36.47±1.1kg/m 2 ) were randomized to either a placebo (CON, n=8) or a pioglitazone (PIO, n=16) group. Following 12weeks of treatment, we measured insulin sensitivity by hyperinsulinemic–euglycemic clamp (clamp), metabolic flexibility by calculating the change in respiratory quotient (ΔRQ) during the steady state of the clamp, intra- and extra-myocellular lipid content (IMCL and EMCL, respectively) by 1 H magnetic resonance spectroscopy ( 1 H-MRS) and muscle maximal ATP synthetic capacity (ATPmax) by 31 P-MRS. Results Following 12weeks of PIO treatment, insulin sensitivity ( p p Conclusions These results suggest that 12weeks of pioglitazone treatment improves insulin sensitivity, metabolic flexibility and myocellular lipid distribution without any effect on maximal ATP synthetic capacity in skeletal muscle. Consequently, pioglitazone-induced enhancements in insulin responsiveness and fuel utilization are independent of mitochondrial function.

A reduction in exercise efficiency accompanies ageing in humans. Here we evaluated the impact of changes in the contractile-coupling and mitochondrial-coupling efficiencies on the reduction in exercise efficiency in the elderly. Nine adult (mean, 38.8 years old) and 40 elderly subjects (mean, 68.8 years old) performed a cycle ergometer test to measure O2 uptake and leg power output up to the aerobic limit ( ). Reduced leg power output per unit O2 uptake was reflected in a drop in delta efficiency (eD) from 0.27 ± 0.01 (mean ± SEM) in adults to 0.22 ± 0.01 in the elderly group. Similar declines with age were apparent for both the leg power output at and the ATP generation capacity (ATPmax) determined in vivo using (31)P magnetic resonance spectroscopy. These similar declines resulted in unchanged contractile-coupling efficiency values (eC) in the adult (0.50 ± 0.05) versus the elderly group (0.58 ± 0.04) and agreed with independent measures of muscle contractile-coupling efficiency in human quadriceps (0.5). The mitochondrial-coupling efficiency calculated from the ratio of delta to contractile-coupling efficiencies in the adults (eD/eC = 0.58 ± 0.08) corresponded to values for well-coupled mitochondria (0.6); however, eD/eC was significantly lower in the elderly subjects (0.44 ± 0.03). Conversion of ATPmax per mitochondrial volume (ATPmax/Vv[mt,f]) reported in these groups into thermodynamic units confirmed this drop in mitochondrial-coupling efficiency from 0.57 ± 0.08 in adults to 0.41 ± 0.03 in elderly subjects. Thus, two independent methods revealed that reduced mitochondrial-coupling efficiency was a key part of the drop in exercise efficiency in these elderly subjects and may be an important part of the loss of exercise performance with age.

The objective of this study was to determine the role of maximum mitochondrial capacity on the variation in insulin sensitivity within a population of patients with type 2 diabetes mellitus (T2DM).Fifty-eight participants enrolled in a cross-sectional design: eight active controls [maximum aerobic capacity (VO(2max))40 ml/kg · min], 17 healthy sedentary controls without a family history (FH-) and seven with a family history (FH+) of diabetes, four obese participants, and 21 patients with T2DM. Mitochondrial capacity was measured noninvasively using (31)P magnetic resonance spectroscopy of the vastus lateralis. Maximal ATP synthetic rate (ATP(max)) was determined from the rate of phosphocreatine (PCr) recovery after short-term isometric exercise.ATP(max) was lower (P0.001) in T2DM and higher (P0.001) in active as compared with healthy sedentary FH- (active, 1.01 ± 0.2; FH-, 0.7 ± 0.2; FH+, 0.6 ± 0.1; obese, 0.6 ± 0.1; T2DM, 0.5 ± 0.2 mm ATP/sec; ANOVA P0.0001). Insulin sensitivity, measured by euglycemic-hyperinsulinemic (80 mIU/m(2) · min) clamp was also reduced in T2DM (P0.001) (active, 12.0 ± 3.2; FH-, 7.8 ± 2.2; FH+, 6.8 ± 3.5; obese, 3.1 ± 1.0; T2DM, 3.4 ± 1.6; mg/kg estimated metabolic body size · min; ANOVA P0.0001). Unexpectedly, there was a broad range of ATP(max) within the T2DM population where 52% of subjects with T2DM had ATP(max) values that were within the range observed in healthy sedentary controls. In addition, 24% of the T2DM subjects overlapped with the active control group (range, 0.65-1.27 mm ATP/sec). In contrast to the positive correlation between ATP(max) and M-value in the whole population (r(2) = 0.35; P0.0001), there was no correlation between ATP(max) and M-value in the patients with T2DM (r(2) = 0.004; P = 0.79).Mitochondrial capacity is not associated with insulin action in T2DM.

Mitochondrial changes are at the centre of a wide range of maladies, including diabetes, neurodegeneration and ageing-related dysfunctions. Here we describe innovative optical and magnetic resonance spectroscopic methods that non-invasively measure key mitochondrial fluxes, ATP synthesis and O(2) uptake, to permit the determination of mitochondrial coupling efficiency in vivo (P/O: half the ratio of ATP flux to O(2) uptake). Three new insights result. First, mitochondrial coupling can be measured in vivo with the rigor of a biochemical determination and provides a gold standard to define well-coupled mitochondria (P/O approximately 2.5). Second, mitochondrial coupling differs substantially among muscles in healthy adults, from values reflective of well-coupled oxidative phosphorylation in a hand muscle (P/O = 2.7) to mild uncoupling in a leg muscle (P/O = 2.0). Third, these coupling differences have an important impact on cell ageing. We found substantial uncoupling and loss of cellular [ATP] in a hand muscle indicative of mitochondrial dysfunction with age. In contrast, stable mitochondrial function was found in a leg muscle, which supports the notion that mild uncoupling is protective against mitochondrial damage with age. Thus, greater mitochondrial dysfunction is evident in muscles with higher type II muscle fibre content, which may be at the root of the preferential loss of type II fibres found in the elderly. Our results demonstrate that mitochondrial function and the tempo of ageing varies among human muscles in the same individual. These technical advances, in combination with the range of mitochondrial properties available in human muscles, provide an ideal system for studying mitochondrial function in normal tissue and the link between mitochondrial defects and cell pathology in disease.

This study tested the hypothesis that acidic pH inhibits oxidative ATP supply during exercise in hand (first dorsal interosseus, FDI) and lower limb (leg anterior compartment, LEG) muscles. We measured oxidative flux and estimated mitochondrial capacity using the changes in creatine phosphate concentration ([PCr]) and pH as detected by 31P magnetic resonance (MR) spectroscopy during isometric exercise and recovery. The highest oxidative ATP flux in sustained exercise was about half the estimated mitochondrial capacity in the LEG (0.38 +/- 0.06 vs. 0.90 +/- 0.14 mM ATP s(-1), respectively), but at the estimated capacity in the FDI (0.61 +/- 0.05 vs. 0.61 +/- 0.09 mM ATP s(-1), respectively). During sustained exercise at a higher contraction rate, intracellular acidosis (pH6.88) prevented a rise in oxidative flux in the LEG and FDI despite significantly increased [ADP]. We tested whether oxidative flux could increase above that achieved in sustained exercise by raising [ADP] (0.24 mM) and avoiding acidosis using burst exercise. This exercise raised oxidative flux (0.69 +/- 0.05 mM ATP s(-1)) to nearly twice that found with sustained exercise in the LEG and matched (0.65 +/- 0.11 mM ATP s(-1)) the near maximal flux seen during sustained exercise in the FDI. Thus both muscles reached their highest oxidative fluxes in the absence of acidosis. These results show that acidosis inhibits oxidative phosphorylation in vivo and can limit ATP supply in exercising muscle to below the mitochondrial capacity.

This paper asks how the decline in maximal O(2) uptake rate (VO(2),max) with age is related to the properties of a key muscle group involved in physical activity - the quadriceps muscles. Maximal oxygen consumption on a cycle ergometer was examined in nine adult (mean age 38.8 years) and 39 elderly subjects (mean age 68.8 years) and compared with the oxidative capacity and volume of the quadriceps. VO(2),max declined with age between 25 and 80 years and the increment in oxygen consumption from unloaded cycling to VO(2),max (delta VO(2)) in the elderly was 45 % of the adult value. The cross-sectional areas of the primary muscles involved in cycling - the hamstrings, gluteus maximus and quadriceps - were all lower in the elderly group. The quadriceps volume was reduced in the elderly to 67 % of the adult value. Oxidative capacity per quadriceps volume was reduced to 53 % of the adult value. The product of oxidative capacity and muscle volume - the quadriceps oxidative capacity - was 36 % of the adult value in the elderly. Quadriceps oxidative capacity was linearly correlated with delta VO(2) among the subjects with the slope indicating that the quadriceps represented 36 % of the VO(2) increase during cycling. The decline in quadriceps oxidative capacity with age resulted from reductions in both muscle volume and oxidative capacity per volume in the elderly and appears to be an important determinant of the age-related reduction in delta VO(2) and VO(2),max found in this study.

We tested the hypothesis that the cytoplasmic control mechanism for glycolysis is affected by the presence of oxygen during exercise. We used a comparison of maximal twitch stimulation under ischaemic and intact circulation in human wrist flexor and ankle dorsiflexor muscles. 31P magnetic resonance spectroscopy followed the phosphocreatine (PCr), Pi and pH dynamics at 6–9 s intervals. Glycolytic PCr synthesis was determined during stimulation from pH and tissue buffer capacity, as well as the oxidative phosphorylation rate. Ischaemic vs. aerobic stimulation resulted in similar glycolytic fluxes in the two muscles. The onset of glycolysis occured after fifty to seventy stimulations and the extent of glycolytic PCr synthesis was directly proportional to the number of stimulations thereafter. Two-fold differences in the putative feedback regulators of glycolysis, [Pi] and [ADP], were found between aerobic and ischaemic stimulation. The similar glycolytic fluxes in the face of these differences in metabolite levels eliminates feedback as a control mechanism in glycolysis. These results demonstrate that glycolytic flux is independent of oxygenation state and metabolic feedback, but proportional to muscle activation. These results show a key role for muscle stimulation in the activation and maintenance of glycolysis. Further, this glycolytic control mechanism is independent of the feedback control mechanism that governs oxidative phosphorylation. Glycolysis is commonly described as an anaerobic process providing ATP in the absence of oxidative phosphorylation. However, glycolytic flux in anoxic resting muscle is typically low and often does not generate sufficient ATP synthesis to meet ATP demands (Blei et al. 1993; Yamada et al. 1993). In contrast, far higher rates of glycolysis are found in aerobic muscle during exercise than in anaerobic muscles at rest. This high flux provides between 50 and 100 % of the substrate needed for oxidative phosphorylation in exercising muscle of humans (Brooks & Mercier 1994; Connett & Sahlin 1996). Clearly, glycolysis is not simply an anaerobic process and the mechanisms responsible for elevating flux under resting conditions cannot account for the high glycolytic flux found under aerobic exercising conditions. Muscle stimulation is a key difference between resting and exercising muscle that is related to the control of glycolysis and may account for the high glycolytic flux during exercise. Glycogenolytic and glycolytic fluxes vary with muscle stimulation rate under ischaemic or anoxic conditions in animal and human studies and are much larger than in unstimulated muscle (Karpatkin et al. 1964; Ozand & Narahara, 1964; Conley et al. 1997). Calcium activation of glycogenolysis is a well-known example of how substrate mobilization is related to muscle activation rate (see Connett & Sahlin, 1996). Similarly, a rate limiting enzyme in glycolysis, phosphofructokinase (PFK), may also undergo a reversible phosphorylation with muscle stimulation which decreases the Km, increases Vmax and facilitates binding to cytoskeletal elements (Luther & Lee, 1986). If Ca2+ activation governs both glycogenolysis and glycolysis, then glycolytic flux should be proportional to stimulation rate independent of oxygenation state or the rate of oxidative phosphorylation. Such an independent control of glycolysis and oxidative phosphorylation would account for the finding of lactate generation in fully aerobic animal and human muscle (Connett & Sahlin, 1996). An important barrier to characterizing the mechanisms governing glycolysis has been quantifying flux during exercise. Muscle biopsies are limited in the number of variables measured, the time resolution and the muscles that can be examined. The advent of non-invasive 31P magnetic resonance spectroscopy has overcome many of these limitations (Kemp et al. 1994). We have previously shown that 31P spectra contain sufficient information to characterize glycogenolytic and glycolytic flux with high time resolution under ischaemic conditions in human muscle in vivo (Conley et al. 1997). In this paper, we show how to extract the same information under aerobic exercise conditions in two muscle groups that are typically not biopsied. The purpose of this study was to test the mechanism of glycolytic control by comparison of glycolytic flux during aerobic and ischaemic stimulation in two muscle groups differing in contractile and oxidative properties. The first muscle group was a set of wrist flexors composed of primarily fast-twitch fibres (Johnson et al. 1973) and our measurements were centred over the flexor digitorum superficialis, as previously described (Blei et al. 1993; Conley et al. 1997). The second muscle group lies in the anterior leg compartment and has predominantly slow-twitch, aerobic fibres (Johnson et al. 1973). Our measurements were centred over the tibialis anterior. These experiments allowed us to determine whether muscle activation controls glycolytic flux independent of the oxygen content of the muscle and of oxidative phosphorylation.

Humans produce less muscle force (F) as they age. However, the relationship between decreased force and muscle cross-sectional area (CSA) in older humans is not well documented. We examined changes in F and CSA to determine the relative contributions of muscle atrophy and specific force (F/CSA) to declining force production in aging humans. The proportions of myosin heavy chain (MHC) isoforms were characterized to assess whether this was related to changes in specific force with age. We measured the peak force of isokinetic knee extension in 57 males and females aged 23-80 years, and used magnetic resonance imaging to determine the contractile area of the quadriceps muscle. Analysis of MHC isoforms taken from biopsies of the vastus lateralis muscle showed no relation to specific force. F, CSA, and F/CSA decreased with age. Smaller CSA accounted for only about half of the 39% drop in force that occurred between ages 65-80 years. Specific force dropped about 1.5% per year in this age range, for a total decrease of 21%. Thus, quantitative changes in muscle (atrophy) are not sufficient to explain the strength loss associated with aging.

This study investigated the disparity between muscle metabolic rate and mitochondrial metabolism in human muscle of sedentary vs. active individuals.Chronic activity level was characterized by a physical activity questionnaire and a triaxial accelerometer as well as a maximal oxygen uptake test. The ATP and O(2) fluxes and mitochondrial coupling (ATP/O(2) or P/O) in resting muscle as well as mitochondrial capacity (ATP(max)) were determined in vivo in human vastus lateralis muscle using magnetic resonance and optical spectroscopy on 24 sedentary and seven active subjects. Muscle biopsies were analyzed for electron transport chain content (using complex III as a representative marker) and mitochondrial proteins associated with antioxidant protection.Sedentary muscle had lower electron transport chain complex content (65% of the active group) in proportion to the reduction in ATP(max) (0.69 ± 0.07 vs. 1.07 ± 0.06 mM sec(-1)) as compared with active subjects. This lower ATP(max) paired with an unchanged O(2) flux in resting muscle between groups resulted in a doubling of O(2) flux per ATP(max) (3.3 ± 0.3 vs. 1.7 ± 0.2 μM O(2) per mM ATP) that reflected mitochondrial uncoupling (P/O = 1.41 ± 0.1 vs. 2.1 ± 0.3) and greater UCP3/complex III (6.0 ± 0.7 vs. 3.8 ± 0.3) in sedentary vs. active subjects.A smaller mitochondrial pool serving the same O(2) flux resulted in elevated mitochondrial respiration in sedentary muscle. In addition, uncoupling contributed to this higher mitochondrial respiration. This finding resolves the paradox of stable muscle metabolism but greater mitochondrial respiration in muscle of inactive vs. active subjects.

Aging is associated with reduced mitochondrial capa city (1,2) and loss of muscle mass and strength (3,4), which could potentially predispose individuals to frailty and a slower preferential walking speed (5,6). Preferred walking speed is slower in older adults (7,8), and is a strong, independent predictor of disability, health care utilization, nursing home admission, and mortality (9–11). A number of studies have found a close relationship between VO2 peak and walking speed in older adults (12,13), which suggests that the decline in aerobic capacity contributes to, and may be predictive of, slower walking speed with age. A reduced efficiency of locomotion is also apparent in older adults, which leads to an increased metabolic cost of walking (14). Aging is also associated with declines in both the capacity and efficiency of energy supply in muscle. Several studies using a variety of techniques have reported reduced capacity to generate ATP with age (1,15,16). The age-related changes in skeletal muscle mitochondrial function apparent in these studies are consistent with their likely role in the parallel loss of aerobic capacity (1,16). In addition, reduced mitochondrial efficiency (energy conversion of O2 uptake into ATP generation) has been reported in a variety of tissues in vitro and in vivo (17–19). The importance of mitochondrial efficiency is that it could affect the ability to generate ATP during ambulation (16) as well as movement efficiency (20–22). One study found that walking speed in patients with peripheral arterial disease was related to their capacity for ATP generation assessed by phosphorus magnetic resonance spectroscopy (31P MRS) (23). Although the loss of muscle mitochondrial function has been widely hypothesized to contribute to the decline in VO2 max and slowing of locomotion with age, there currently is insufficient published evidence to show that reductions in available energy results in a decline in customary walking speed with aging and disease (11). Furthermore, the potential role of mitochondrial efficiencey in energy availablity for walking has not been examined. The goal of this study was to test the hypothesis that reduced mitochondrial capacity and efficiency are associated with slower walking speed in older adults. In vitro (respirometry) and in vivo (31P MRS) measurements of mitochondrial function were combined and related to whole-body aerobic capacity (VO2 peak) and preferred walking speed in a group of older men and women. High-resolution respirometry of permeabilized fibers isolated from muscle biopsy specimens yielded mitochondrial oxidative capacity (State 3 or St3 respiration). These measures of mitochondrial capacity at cellular level were extended to the muscle tissue level by assessing quadriceps muscle volume with magnetic resonance imaging (MRI). 31P MRS was used to determine the maximum mitochondrial ATP production (ATPmax) in vivo, which was combined with St3 respiration to yield an index of mitochondrial efficiency (ATPmax/St3). This study tested the paradigm that muscle mitochondrial properties affect walking speed and that mitochondria capacity and efficiency may be associated with the decline in mobility with age (Figure 1). Figure 1. Concept map illustrating age-related changes in muscle physiology and how they contribute to reduced walking speed in older adults. This study examined the relationships between muscle mitochondrial capacity/efficiency, aerobic capacity, and walking speed ...

SummaryCan human muscle be highly efficient in vivo? Animal muscles typically show contraction-coupling efficiencies

In mammals, nicotinamide phosphoribosyltransferase (NAMPT) is responsible for the first and rate-limiting step in the conversion of nicotinamide to nicotinamide adenine dinucleotide (NAD+). NAD+is an obligate cosubstrate for mammalian sirtuin-1 (SIRT1), a deacetylase that activates peroxisome proliferator-activated receptor-γ coactivator-1α (PGC-1α), which in turn can activate mitochondrial biogenesis. Given that mitochondrial biogenesis is activated by exercise, we hypothesized that exercise would increase NAMPT expression, as a potential mechanism leading to increased mitochondrial content in muscle. A cross-sectional analysis of human subjects showed that athletes had about a twofold higher skeletal muscle NAMPT protein expression compared with sedentary obese, nonobese, and type 2 diabetic subjects ( P < 0.05). NAMPT protein correlated with mitochondrial content as estimated by complex III protein content ( R2= 0.28, P < 0.01), MRS-measured maximal ATP synthesis ( R2= 0.37, P = 0.002), and V̇o2max( R2= 0.63, P < 0.0001). In an exercise intervention study, NAMPT protein increased by 127% in sedentary nonobese subjects after 3 wk of exercise training ( P < 0.01). Treatment of primary human myotubes with forskolin, a cAMP signaling pathway activator, resulted in an ∼2.5-fold increase in NAMPT protein expression, whereas treatment with ionomycin had no effect. Activation of AMPK via AICAR resulted in an ∼3.4-fold increase in NAMPT mRNA ( P < 0.05) as well as modest increases in NAMPT protein ( P < 0.05) and mitochondrial content ( P < 0.05). These results demonstrate that exercise increases skeletal muscle NAMPT expression and that NAMPT correlates with mitochondrial content. Further studies are necessary to elucidate the pathways regulating NAMPT as well as its downstream effects.

It is well documented that repeated voluntary activity or electrical stimulation of skeletal muscle results in a decline in force production or power output. However, the precise physiological causes of “muscle fatigue” are not yet well understood. It is conceivable that the mechanism(s) may lie either in the conduction of action potentials in the central and peripheral nervous systems or in the transformation of the electrical event into mechanical force production by the muscle itself. In fact, none of the components of the electrical pathway from generation of impulses in the brain to their conduction over the neuron and the excitable membranes of the muscle can as yet be ruled out as potential contributors to the fatigue process. Relative to that on conduction failure, more information exists concerning the possibility that a defect in the excitation contraction coupling process in skeletal muscle, e.g., intracellular acidosis, inadequate supply of energy for contraction, or a disruption in Ca 2+ homeostasis may also be significant in compromising force production following sustained activity. Despite this, the amount of conflicting data derived from these experiments has hindered the resolution of this question. In the future more attention must be given to such issues as the type of activity used to elicit fatigue and the fiber composition of the muscles studied. This is imperative as these factors clearly impact the nature of correlations between the biochemical and physiological events in muscle that are required to support prospective fatigue mechanisms.

Innovative noninvasive methods open a new window on the cell in vivo. This window reveals that the tempo of mitochondrial dysfunction with age varies among muscles and in proportion to Type II muscle fiber content. Exercise training can reverse age-related dysfunction, thereby providing an intervention to slow the pace of aging and disability in the elderly.

This study asked whether the energetic properties of muscles are changed by insulin-dependent diabetes mellitus (or type 1 diabetes), as occurs in obesity and type 2 diabetes. We used 31P magnetic resonance spectroscopy to measure glycolytic flux, oxidative flux, and contractile cost in the ankle dorsiflexor muscles of 10 men with well-managed type 1 diabetes and 10 age- and activity-matched control subjects. Each subject performed sustained isometric muscle contractions lasting 30 and 120 s while attempting to maintain 70–75% of maximal voluntary contraction force. An altered glycolytic flux in type 1 diabetic subjects relative to control subjects was apparent from significant differences in pH in muscle at rest and at the end of the 120-s bout. Glycolytic flux during exercise began earlier and reached a higher peak rate in diabetic patients than in control subjects. A reduced oxidative capacity in the diabetic patients' muscles was evident from a significantly slower phosphocreatine recovery from a 30-s exercise bout. Our findings represent the first characterization of the energetic properties of muscle from type 1 diabetic patients. The observed changes in glycolytic and oxidative fluxes suggest a diabetes-induced shift in the metabolic profile of muscle, consistent with studies of obesity and type 2 diabetes that point to common muscle adaptations in these diseases.

Purpose Fast- and slow-twitch human muscle fibers exhibit large (two- to threefold) differences in metabolic enzyme activities and contractile economy. We asked whether comparable flux differences are evident in the muscles of athletes specializing in extremely different (i.e., sprint and long-distance) running events. Methods We took an in vivo “functional biopsy” of the ankle dorsiflexor muscles of 17 members of a university track team by using 31P magnetic resonance spectroscopy. Ten sprinters (SPR) and seven distance runners (DIS) performed rapid isometric dorsiflexions against the resistance of a plastic foot holder. The contractile cost of exercise and glycolytic flux were calculated from changes in pH, [PCr], and [Pi] during ischemic exercise, and oxidative capacity was calculated from PCr recovery kinetics after aerobic exercise. Results Contractile costs were 47% higher in SPR than in DIS, whereas oxidative capacities were 52% higher in DIS than in SPR. Surprisingly, glycolytic ATP production was similar in the two groups. Conclusion The muscles of SPR and DIS exhibit clear differences in energetic properties, but these differences are smaller than the two- to three-fold variations seen in the properties of individual muscle fibers.

This study determined the cellular energetic and structural adaptations of elderly muscle to exercise training. Forty male and female subjects (69.2 ± 0.6 yr) were assigned to a control group or 6 mo of endurance (ET) or resistance training (RT). We used magnetic resonance spectroscopy and imaging to characterize energetic properties and size of the quadriceps femoris muscle. The phosphocreatine and pH changes during exercise yielded the muscle oxidative properties, glycolytic ATP synthesis, and contractile ATP demand. Muscle biopsies taken from the same site as the magnetic resonance measurements were used to determine myosin heavy chain isoforms, metabolite concentrations, and mitochondrial volume densities. The ET group showed changes in all energetic pathways: oxidative capacity (+31%), contractile ATP demand (−21%), and glycolytic ATP supply (−56%). The RT group had a large increase in oxidative capacity (57%). Only the RT group exhibited change in structural properties: a rise in mitochondrial volume density (31%) and muscle size (10%). These results demonstrate large energetic, but smaller structural, adaptations by elderly muscle with exercise training. The rise in oxidative properties with both ET and RT suggests that the aerobic pathway is particularly sensitive to exercise training in elderly muscle. Thus elderly muscle remains adaptable to chronic exercise, with large energetic changes accompanying both ET and RT.

Elderly humans show large declines in functional abilities and changes in muscle properties compared to young ones. The purpose of this chapter is to relate the age-related changes in muscle to the reductions in maximum force production and in sustained power output (i.e., endurance) found in the elderly. Muscle atrophy accounts for more than half of the decline in muscle force production. The remainder of this decline can be explained by a reduction in the force per cross-sectional area of muscle fibers. Neither impaired neural recruitment of muscles nor a decrease in the proportion of fast muscle fibers appears to contribute significantly to the loss of force with age. Instead, alterations to both excitation–contraction coupling and the contractile elements are possibilities to account for the loss in force per cross-sectional area with age. Nearly equal contributions by the loss of muscle volume and changes in muscle oxidative properties are responsible for the reduction in sustained muscle power output. The reduction in oxidative capacity of elderly muscle stems from lower mitochondrial volume density and lower oxidative capacity per mitochondrial volume. Thus, loss of muscle size and changes in muscle intrinsic properties largely account for the declines in both force production and sustained power production in the elderly. © 2001 Academic Press.

This study determined the decline in oxidative capacity per volume of human vastus lateralis muscle between nine adult (mean age 38.8 years) and 40 elderly (mean age 68.8 years) human subjects (age range 25-80 years). We based our oxidative capacity estimates on the kinetics of changes in creatine phosphate content ([PCr]) during recovery from exercise as measured by (31)P magnetic resonance (MR) spectroscopy. A matched muscle biopsy sample permitted determination of mitochondrial volume density and the contribution of the loss of mitochondrial content to the decline in oxidative capacity with age. The maximal oxidative phosphorylation rate or oxidative capacity was estimated from the PCr recovery rate constant (k(PCr)) and the [PCr] in accordance with a simple electrical circuit model of mitochondrial respiratory control. Oxidative capacity was 50 % lower in the elderly vs. the adult group (0.61 +/- 0.04 vs. 1.16 +/- 0.147 mM ATP s(-1)). Mitochondrial volume density was significantly lower in elderly compared with adult muscle (2.9 +/- 0.15 vs. 3.6 +/- 0.11 %). In addition, the oxidative capacity per mitochondrial volume (0.22 +/- 0.042 vs. 0.32 +/- 0.015 mM ATP (s %)(-1)) was reduced in elderly vs. adult subjects. This study showed that elderly subjects had nearly 50 % lower oxidative capacity per volume of muscle than adult subjects. The cellular basis of this drop was a reduction in mitochondrial content, as well as a lower oxidative capacity of the mitochondria with age.

Clinical diagnostic ultrasound (US) is experiencing an expanding role that is well suited to application on the International Space Station (ISS). Diagnostic US can be used to reduce the risks associated with long duration human space flight by providing a non-invasive tool with head-to-toe diagnostic capability in both biomedical research and crew health care. General health care of the astronauts will be diagnosed with US, e.g., kidney stones, gall bladder disease, appendicitis, etc. Initial studies will focus on detection of “ureteral jets” in the bladder. This is a non-invasive test to rule out obstructive uropathy from kidney stones with minimal requirements for crew training. Biomedical research experiments, focusing on the effects of the microgravity environment, will be performed using both the HHU and the HDI 5000. US will be used to evaluate bone density and muscle mass in this environment. Prolonged or emergency EVAs may occur with the ISS. The hand-held ultrasound unit (HHU) and its telemedici...

Mitochondria are central to normal cell function, and their dysfunction underlies a wide variety of diseases, such as diabetes (insulin resistance), aging, and both cardiovascular and neurodegenerative diseases. Herein we show new approaches and results that help to identify the mitochondrial changes unique to Huntington's disease that occur well before the onset of symptoms. New optical and magnetic resonance spectroscopic techniques provide for non-invasive, in vivo diagnosis of mitochondrial dysfunction in muscle without the need for a surgical biopsy. These methods reveal changes in mitochondria a decade or more prior to the onset of symptoms. We have found a significant decrease in mitochondrial efficiency in presymptomatic subjects (n = 4, mean age 38 years) versus control subjects (n = 11, mean age 38 years). A continued decrease is evident in symptomatic individuals (n = 3, mean age 53 years), but not yet statistically significant relative to the presymptomatic individuals. Accompanying this loss of mitochondrial efficiency is a corresponding increase in the contribution of non-mitochondrial ATP synthesis from 8% to 23% in the HD individuals. Thus, we have found a substantial reorganization of cellular metabolism in subjects presymptomatic for HD, and this reorganization of cellular energetics continues in the symptomatic individuals. These changes are compared to other disease states to identify the biomarkers reflective of mitochondrial changes that are unique to Huntington's disease. For example, the trade-off between mitochondrial inefficiency and increased glycolysis with stable [ATP] are unique to presymptomatic HD. Identification of biomarkers of HD well before the onset of symptoms provides the opportunity to test interventions that may reverse these dysfunctions and forestall irreversible mitochondrial changes. Thus, innovative noninvasive methods provide new insight into the early cellular changes in HD and provide the opportunity for intervention that may stall the onset of symptoms.