Search

Purpose: Several papers (algorithm papers) describe computational algorithms that assess alveolar breath-by-breath gas exchange by accounting for changes in lung gas stores. It is unclear, however, if the effects of the latter are actually considered in literature. We evaluated dissemination of algorithm papers and the relevant provided information.Methods: The list of documents investigating exercise transients (in 1998-2017) was extracted from Scopus database. Documents citing the algorithm papers in the same period were analyzed in full text to check consistency of the relevant information provided.Results: Less than 8% (121/1522) of documents dealing with exercise transients cited at least one algorithm paper; the paper of Beaver et al. (J Appl Physiol 51:1662-1675, 1981) was cited most often, with others being cited tenfold less. Among the documents citing the algorithm paper of Beaver et al. (J Appl Physiol 51:1662-1675, 1981) (N = 251), only 176 cited it for the application of their algorithm/s; in turn, 61% (107/176) of them stated the alveolar breath-by-breath gas exchange measurement, but only 1% (1/107) of the latter also reported the assessment of volunteers' functional residual capacity, a crucial parameter for the application of the algorithm. Information related to gas exchange was provided consistently in the methods and in the results in 1 of the 107 documents.Conclusion: Dissemination of algorithm papers in literature investigating exercise transients is by far narrower than expected. The information provided about the actual application of gas exchange algorithms is often inadequate and/or ambiguous. Some guidelines are provided that can help to improve the quality of future publications in the field. [ABSTRACT FROM AUTHOR]

In the mid-nineteenth century, the concept of muscle behaving like a stretched spring was developed. This elastic model of contraction predicted that the energy available to perform work was established at the start of a contraction. Despite several studies showing evidence inconsistent with the elastic model, it persisted into the twentieth century. In 1923, W. O. Fenn published a paper in which he presented evidence that appeared to clearly refute the elastic model. Fenn showed that when a muscle performs work it produces more heat than when contracting isometrically. He proposed that energy for performing work was only made available in a muscle as and when that work was performed. However, his ideas were not adopted and it was only after 15 years of technical developments that in 1938 A. V. Hill performed experiments that conclusively disproved the elastic model and supported Fenn's conclusions. Hill showed that the rate of heat production increased as a muscle made the transition from isometric to working contraction. Understanding the basis of the phenomenon observed by Fenn and Hill required another 40 years in which the processes that generate force and work in muscle and the associated scheme of biochemical reactions were established. Demonstration of the biochemical equivalent of Hill's observations—changes in rate of ATP splitting when performing work—in 1999 was possible through further technical advances. The concept that the energy, from ATP splitting, required to perform work is dynamically modulated in accord with the loads a muscle encounters when contracting is key to understanding muscle energetics. [ABSTRACT FROM AUTHOR]

Diabetes alters numerous physiological functions and can lead to disastrous consequences in the long term. Neuromuscular function is particularly affected and is impacted early, offering an opportunity to detect the onset of diabetes-related dysfunctions and follow the advancement of the disease. The role of physical training for counteracting the deleterious effects of diabetes is well accepted but at the same time, it appears difficult to reliably assess the effects of exercise on functional capacity in patients with diabetic peripheral neuropathy (DPN). In this paper, we will review the specific characteristics of various neuromuscular dysfunctions associated with diabetes according to the DPN presence or not, and their changes over time. We present several propositions regarding the onset of neuromuscular alterations in people with diabetes compared to people with DPN. It appears that motor unit loss and neuromuscular transmission impairment are among the main mechanisms explaining the considerable degradation of neuromuscular function in the transition from a diabetic to neuropathic state. Rate of force development and contractile properties could start to decrease with the onset of preferential type II fiber atrophy, commonly reported in people with DPN. Finally, Mmax amplitude could decrease with neuromuscular fatigue only in people with DPN, reflecting the fatigue-related neuromuscular transmission impairment reported in people with DPN. In this review, we show that the different neuromuscular parameters are altered at different stages of diabetes, according to the presence of DPN or not. The precise evaluation of these parameters might participate in adapting the physical training prescription. [ABSTRACT FROM AUTHOR]

The consequence of using HR to control the exercise intensity is that target HR zones of exercise intervals (i.e., 85-95% of HRreserve) and recovery pauses (i.e., 60-70% or 20% of HR reserve depending on exercise designs) were far from being achieved during the whole duration of intervals and pauses. Therefore, we support that when studies aim at comparing the effects of varied exercises, focusing on exercise designs, they should pay more attention to the control of the exercise variables they manipulate, and to the exercise doses these exercises represent. First of all, in the present paper, exercise intensity was monitored by heart rate (HR) which is a real gamble because this parameter is known to be ineffective in controlling exercise intensity in HIIE (Guiraud et al. [3]). [Extracted from the article]

Purpose: The goal of this paper was to determine if sports participation influences torque control differently for adolescent boys and young men during a slow ramp task. Methods: Twenty-one adolescent boys (11 athletes) and 31 young men (16 athletes) performed a slow ramp increase in plantar flexion torque from 0 to maximum. We quantified torque control as the coefficient of variation (CV) of torque during the ramp and quantified the Achilles tendon mechanical properties using ultrasonography. Results: Relative to adolescent boys, young men were taller, heavier, stronger, and had a longer and stiffer Achilles tendon. However, these characteristics were not different between athletes and non-athletes in adolescent boys. For the CV of torque, there was a significant interaction with sports participation, indicating that only adolescent boys who were non-athletes had greater variability than young men. The CV of torque of all participants was predicted from the maximum torque and torque oscillations from 1 to 2 Hz, whereas the CV of torque for adolescent boys was predicted only from torque oscillations from 1 to 2 Hz. Conclusion: These findings suggested that adolescent boys who participate in sports exhibited lower torque variability during a slow ramp task, which was not explained by differences in Achilles tendon properties or strength. [ABSTRACT FROM AUTHOR]

Blood flow restriction pressures are typically set as a percentage of the arterial occlusion pressure. For those who do not have the ability to measure the arterial occlusion pressure, estimation equations are available. However, notable considerations are needed when estimating pressure with a narrow cuff (5 cm) in the lower body. A previously published equation in this journal was developed but was created only using 55% of the sample because the arterial occlusion of the others could not be obtained within the manufacturer's pressure limit. The purpose of this article was twofold: (1) to investigate how previous studies have implemented the equation and (2) to highlight potential concerns of using this equation. Two databases were used to locate articles that used the equation from Loenneke et al. (2015). We found that this equation had been cited 10 times to estimate arterial occlusion pressure with some notable concerns. Some did not use a 5 cm wide cuff, while others used it for participants who had arterial occlusion pressures exceeding 300 mmHg. To highlight the latter, we also applied the Loenneke et al. (2015) lower body equation to participants with arterial occlusion pressures known to exceed 300 mmHg to demonstrate potential concerns. This retrospective analysis found that 52% of the sample with known pressures over 300 mmHg (40 out of 77) would be estimated below 300 mmHg. This paper highlighted important considerations for those trying to estimate arterial occlusion pressure in the lower body with a narrow cuff (5 cm). [ABSTRACT FROM AUTHOR]

Introduction: Although neuromuscular function varies significantly between strength and endurance-trained adult athletes, it has yet to be ascertained whether such differences manifest by early adolescence. The aim of the present study was to compare knee extensor neuromuscular characteristics between adolescent athletes who are representative of strength (wrestling) or endurance (triathlon) sports., Methods: Twenty-three triathletes (TRI), 12 wrestlers (WRE) and 12 untrained (CON) male adolescents aged 13 to 15 years participated in the present study. Maximal voluntary isometric contraction (MVIC) knee extensor (KE) torque was measured, and 100-Hz magnetic doublets were delivered to the femoral nerve during and after KE MVIC to quantify the voluntary activation level (%VA). The doublet peak torque (T 100Hz ) and normalized vastus lateralis (VL) and rectus femoris (RF) EMG (EMG/M-wave) activities were quantified. VL and RF muscle architecture was also assessed at rest using ultrasound., Results: Absolute and relative (to body mass) KE MVIC torques were significantly higher in WRE than TRI and CON (p < 0.05), but comparable between TRI and CON. No significant differences were observed between groups for %VA, T 100Hz or either VL or RF muscle thickness. However, VL EMG/M-wave was higher, RF fascicle length longer, and pennation angle smaller in WRE than TRI and CON (all p < 0.05)., Conclusion: The wrestlers were stronger than triathletes and controls, potentially as a result of muscle architectural differences and a greater neural activation. Neuromuscular differences can already be detected by early adolescence in males between predominantly endurance and strength sports, which may result from selection bias and/or physical training., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Purpose: The purpose was to examine the effects of 8-weeks (3 days/week) of linear periodization resistance exercise training (RET) on neuromuscular function in prepubescent youth., Methods: Twenty-five healthy prepubescent youth (11 males, 14 females, age = 9.1 ± 0.8 years) completed the RET (n = 17) or served as controls (CON, n = 8). Isometric maximal voluntary contractions (MVCs) and trapezoidal submaximal contractions at 35 and 60% MVC of the right leg extensors were performed with surface electromyography (EMG) recorded from the leg extensors [vastus lateralis (VL), rectus femoris, and vastus medialis] and flexors (biceps femoris and semitendinosus). EMG amplitude of the leg extensors and flexors were calculated during the MVCs. Motor unit (MU) action potential trains were decomposed from the surface EMG of the VL for the 35 and 60% MVCs. MU firing rates and action potential amplitudes were regressed against recruitment threshold with the y-intercepts and slopes calculated for each contraction. Total leg extensor muscle cross-sectional area (CSA) was collected using ultrasound images. ANOVA models were used to examine potential differences., Results: Isometric strength increased post-RET (P = 0.006) with no changes in leg extensor and flexor EMG amplitude. Furthermore, there were no changes in total CSA or the MU action potential amplitude vs. recruitment threshold relationships. However, there were increases in the firing rates of the higher-threshold MUs post-RET as indicated with greater y-intercepts (P = 0.003) from the 60% MVC and less negative slope (P = 0.004) of the firing rates vs. recruitment threshold relationships at 35% MVC., Conclusions: MU adaptations contribute to strength increases following RET in prepubescent youth., (© 2024. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Pressor responses can be elicited from forelimb muscle afferents (Shea et al. [3]) and thus, local spinal reflex responses based on exercise modality may facilitate HR responses and increased VO SB 2max sb observed. This paper assessed the effects of a 12-week upper body rowing (UBROW) regimen and its effects on cardiorespiratory fitness, cardio-metabolic risk factors, and changes in brachial artery diameter in individuals with spinal cord injury (SCI). Overall, a 12-week rowing exercise modality proves beneficial in individuals of varying SCI and severity for VO SB 2max sb and resting brachial artery dimensions. [Extracted from the article]

For approximately 50 years, hepatic clearance of indocyanine green (ICG) has been used to assess liver function. Steady-state infusion of ICG with simultaneous measurement of arterial and hepatic venous ICG concentrations provides unambiguous measures of the extraction ratio for ICG and the hepatic blood flow rate, but also requires cannulation of a hepatic vein. Transient clearance following injection of a single bolus of ICG, which typically involves only measurement of arterial ICG concentration, is a more commonly used procedure. Since drawing blood from a hepatic vein is often impossible, and, in any event can be difficult, there has been considerable interest in the claim by Grainger et al. (Clin Sci 64:207-212, 1983) that a single-bolus, two-compartment model "enabled the hepatic extraction ratio (ER(ss)) of dye to be determined solely from the plasma disappearance curve". The principal purpose of this paper is to show that the claim by Grainger et al. is not valid because it ignores the fact that a finite fraction of ICG entering the liver passes directly into hepatic veins without being sequestered in the liver. A valid relationship between ER(ss) and parameters determined from single-bolus clearance data is derived in this paper. For individuals with normally functioning livers, the single-bolus method of Grainger et al. yields an extraction ratio approximately 20% too large, but in cirrhotic patients with extensive intrahepatic shunting, the extraction ratio evaluated using the single-bolus method of Grainger et al. may be too large by a factor of two. [ABSTRACT FROM AUTHOR]

Purpose: Sensory electrical stimulation (SES)—i.e., low-intensity electrical currents below, at, or just above the sensory threshold but below the motor threshold—is mainly used to restore/improve postural balance in pathological and healthy subjects. However, the ins and outs of its application as well as the neurophysiological effects induced are not yet well known. Hence, the aim of this paper was to address the effects of SES on postural balance based on these considerations. Method: The immediate/concurrent effects (SES applied during postural balance measurements), the acute effects (SES durably applied before measuring postural balance) and the chronic effects (SES included in training/rehabilitation programs, i.e., measurements performed before and after the programs) were analysed with a comprehensive review. Result: SES can lead to the improvement of postural balance using any of the three applications (immediate/concurrent, acute and chronic), notably in pathological subjects. The beneficial effects of SES can take place at the peripheral (sensory receptors sensitivity), spinal (spinal motoneural excitablity) and supra-spinal (cortex reorganisation or adaptation) levels. In healthy subjects, SES appears interesting, but too few studies have been conducted with this population to report clear results. Moreover, the literature is relatively devoid of comparative studies about the characteristics of the stimulation current (e.g., location, current parameters, duration). Conclusion: In practice, SES appears to be particularly useful to reinforce or restore the postural function in the immediate/concurrent, acute or chronic application in pathlogical populations while its effects should be confirmed in healthy sujects by future studies. Moreover, future research should focus on the different characteristics of stimulation. [ABSTRACT FROM AUTHOR]

In this third installment of our four-part historical series, we evaluate contributions that shaped our understanding of heat and cold stress during occupational and athletic pursuits. Our first topic concerns how we tolerate, and sometimes fail to tolerate, exercise-heat stress. By 1900, physical activity with clothing- and climate-induced evaporative impediments led to an extraordinarily high incidence of heat stroke within the military. Fortunately, deep-body temperatures > 40 °C were not always fatal. Thirty years later, water immersion and patient treatments mimicking sweat evaporation were found to be effective, with the adage of cool first, transport later being adopted. We gradually acquired an understanding of thermoeffector function during heat storage, and learned about challenges to other regulatory mechanisms. In our second topic, we explore cold tolerance and intolerance. By the 1930s, hypothermia was known to reduce cutaneous circulation, particularly at the extremities, conserving body heat. Cold-induced vasodilatation hindered heat conservation, but it was protective. Increased metabolic heat production followed, driven by shivering and non-shivering thermogenesis, even during exercise and work. Physical endurance and shivering could both be compromised by hypoglycaemia. Later, treatments for hypothermia and cold injuries were refined, and the thermal after-drop was explained. In our final topic, we critique the numerous indices developed in attempts to numerically rate hot and cold stresses. The criteria for an effective thermal stress index were established by the 1930s. However, few indices satisfied those requirements, either then or now, and the surviving indices, including the unvalidated Wet-Bulb Globe-Thermometer index, do not fully predict thermal strain., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Dr. Marocolo et al. recently published an article in the European Journal of Applied Physiology on the ischemic preconditioning (IPC) and exercise performance: shedding light through smallest worthwhile change (Marocolo et al. [5]). i , was cited in this review and meta-analysis (Marocolo et al. [5]), especially because Marocolo et al. did an impressive quality assessment for each selected paper. [Extracted from the article]

Mechanisms accounting for sex-related differences in the sweat response remain to be elucidated. In the present study, we focused on differences in sweat gland cholinergic sensitivity between males and females. Since, males usually possess higher aerobic capacity than females, we investigated sweating in males and females grouped according to aerobic capacity $$ ({\dot{V}} {\text{O}}_{{2{\text{peak}}}})$$. Forty-four subjects were assigned to four groups: males with higher (MH) and lower (ML), and females with higher (FH) and lower (FL) $$ {\dot{V}} {\text{O}}_{{2{\text{peak}}}}.$$ Forearm sweating was induced by iontophoretic administration (1.5 mA, 60 μA cm−2, 5 min) of pure water or varying concentrations of pilocarpine hydrochloride (0.125, 0.250, 0.5, 1.0 and 2.0%). Local sweat rate (absorbent paper) and the number of activated sweat glands (iodine impregnated paper) were computed. Maximal pilocarpine-induced sweat rate (SRmax) and the pilocarpine concentration which elicited 50% of maximal sweating response ( Km) were calculated. Sweat rate and active gland density increased in response to greater doses of pilocarpine ( p < 0.05). Inter-group differences were evident: SRmax was greatest for MH and lowest for FL ( p < 0.05), but no significant differences were observed between ML and FH ( p = 0.24). Higher SRmax were observed, within-sex, for those with greater aerobic capacity ( p < 0.05). Furthermore, males’ Km values were higher than females’, indicating greater sweat gland affinity for pilocarpine even for groups having similar aerobic capacity ( p < 0.05). In summary, we confirmed that the human sudomotor response is affected by aerobic capacity but, also, that sex-related differences in sweat gland cholinergic sensitivity exist and are not necessarily associated with the typical differences in $$ {\dot{V}} {\text{O}}_{{2{\text{peak}}}}$$ observed between sexes. [ABSTRACT FROM AUTHOR]

Overall resistances for heat and vapor transport in a multilayer garment depend on the properties of individual layers and the thickness of any air space between layers. Under uncomplicated, steady-state conditions, thermal and mass fluxes are uniform within the garment, and the rate of transport is simply computed as the overall temperature or water concentration difference divided by the appropriate resistance. However, that simple computation is not valid under cool ambient conditions when the vapor permeability of the garment is low, and condensation occurs within the garment. Several recent studies have measured heat and vapor transport when condensation occurs within the garment (Richards et al. in Report on Project ThermProject, Contract No. G6RD-CT-2002-00846, 2002; Havenith et al. in J Appl Physiol 104:142-149, 2008). In addition to measuring cooling rates for ensembles when the skin was either wet or dry, both studies employed a flat-plate apparatus to measure resistances of individual layers. Those data provide information required to define the properties of an ensemble in terms of its individual layers. We have extended the work of previous investigators by developing a rather simple technique for analyzing heat and water vapor transport when condensation occurs within a garment. Computed results agree well with experimental results reported by Richards et al. (Report on Project ThermProject, Contract No. G6RD-CT-2002-00846, 2002) and Havenith et al. (J Appl Physiol 104:142-149, 2008). We discuss application of the method to human subjects for whom the rate of sweat secretion, instead of the partial pressure of water on the skin, is specified. Analysis of a more complicated five-layer system studied by Yoo and Kim (Text Res J 78:189-197, 2008) required an iterative computation based on principles defined in this paper. [ABSTRACT FROM AUTHOR]

In this, the second of four historical reviews on human thermoregulation during exercise, we examine the research techniques developed by our forebears. We emphasise calorimetry and thermometry, and measurements of vasomotor and sudomotor function. Since its first human use (1899), direct calorimetry has provided the foundation for modern respirometric methods for quantifying metabolic rate, and remains the most precise index of whole-body heat exchange and storage. Its alternative, biophysical modelling, relies upon many, often dubious assumptions. Thermometry, used for >300 y to assess deep-body temperatures, provides only an instantaneous snapshot of the thermal status of tissues in contact with any thermometer. Seemingly unbeknownst to some, thermal time delays at some surrogate sites preclude valid measurements during non-steady state conditions. To assess cutaneous blood flow, immersion plethysmography was introduced (1875), followed by strain-gauge plethysmography (1949) and then laser-Doppler velocimetry (1964). Those techniques allow only local flow measurements, which may not reflect whole-body blood flows. Sudomotor function has been estimated from body-mass losses since the 1600s, but using mass losses to assess evaporation rates requires precise measures of non-evaporated sweat, which are rarely obtained. Hygrometric methods provide data for local sweat rates, but not local evaporation rates, and most local sweat rates cannot be extrapolated to reflect whole-body sweating. The objective of these methodological overviews and critiques is to provide a deeper understanding of how modern measurement techniques were developed, their underlying assumptions, and the strengths and weaknesses of the measurements used for humans exercising and working in thermally challenging conditions., (© 2023. The Author(s), under exclusive licence to Springer-Verlag GmbH Germany, part of Springer Nature.)

Purpose: The purpose of this paper is to review the physiological mechanisms determining eccrine sweat composition to assess the utility of sweat as a proxy for blood or as a potential biomarker of human health or nutritional/physiological status.Methods: This narrative review includes the major sweat electrolytes (sodium, chloride, and potassium), other micronutrients (e.g., calcium, magnesium, iron, copper, zinc, vitamins), metabolites (e.g., glucose, lactate, ammonia, urea, bicarbonate, amino acids, ethanol), and other compounds (e.g., cytokines and cortisol).Results: Ion membrane transport mechanisms for sodium and chloride are well established, but the mechanisms of secretion and/or reabsorption for most other sweat solutes are still equivocal. Correlations between sweat and blood have not been established for most constituents, with perhaps the exception of ethanol. With respect to sweat diagnostics, it is well accepted that elevated sweat sodium and chloride is a useful screening tool for cystic fibrosis. However, sweat electrolyte concentrations are not predictive of hydration status or sweating rate. Sweat metabolite concentrations are not a reliable biomarker for exercise intensity or other physiological stressors. To date, glucose, cytokine, and cortisol research is too limited to suggest that sweat is a useful surrogate for blood.Conclusion: Final sweat composition is not only influenced by extracellular solute concentrations, but also mechanisms of secretion and/or reabsorption, sweat flow rate, byproducts of sweat gland metabolism, skin surface contamination, and sebum secretions, among other factors related to methodology. Future research that accounts for these confounding factors is needed to address the existing gaps in the literature. [ABSTRACT FROM AUTHOR]

The pedometer has become a popular instrument to measure physical activity in humans. The pedometer records physical activity as a simple, raw measure of ambulatory movement: the number of steps taken. Two fundamental questions of the use of the pedometer in pediatric research are: How many steps do children take in a day? How many steps should children accumulate on a daily basis? This raises the question: is 13,000 steps the same amount of physical activity in a 6-year-old and a 12-year-old, given the difference in body size and stride length? Although previous authors have concluded that the number of steps taken is an equivalent amount of physical activity in children regardless of body size, the conclusion has not been explained by a scientific rationale or on an empirical basis depending on the energetics and mechanics of terrestrial locomotion and body size. One approach to explain this question is allometric scaling, which has long been utilized by comparative physiologists to show the relationships between body size and locomotion in animals from shrews to elephants. The aim of this paper was to use allometric equations, examples from comparative mammalian physiology, and results from previous papers that have examined the cost of locomotion in children to provide a better understanding of the interpretation of the pedometer in pediatric research. We show that empirically derived results confirm to allometric equations that consider the relationship between body size and terrestrial locomotion to cover a given distance at a given speed. Thus, the number of steps taken and the energy expended for a particular task is greater in smaller children. However, if the cost of locomotion is related to one step, animals tend to be equally economical, whether large or small. Therefore, the number of steps taken as measured by a pedometer may be interpreted as an equivalent level of physical activity between a 6-year-old and a 12-year-old, if taken at the same speed. However, the locomotor activities in daily life consist of walking and running up- and downhill, carrying loads, and at varying speeds or work intensities, and this exemplifies the limitations of the pedometer. In conclusion, this paper provides a better understanding of the interpretation of pedometer counts and highlights that the findings depend on the expression of physical activity. [ABSTRACT FROM AUTHOR]

In 1976, Paolo Cerretelli published an article entitled “Limiting factors to oxygen transport on Mount Everest” in the Journal of Applied Physiology . The paper demonstrated the role of cardiovascular oxygen transport in limiting maximal oxygen consumption (VO2max). In agreement with the predominant view of VO2max limitation at that time, however, its results were taken to mean that cardiovascular oxygen transport does not limit VO2max at altitude. So it was argued that the limiting factor could be in the periphery, and muscle blood flow was proposed as a possible candidate. Despite this suggestion, the conclusion generated a series of papers on muscle structural characteristics. These experiments demonstrated a loss of muscle oxidative capacity in chronic hypoxia, and thus provided an unambiguous refutation of the then widespread hypothesis that an increased muscle oxidative capacity is needed at altitude to compensate for the lack of oxygen. This analysis is followed by a short account of Cerretelli’s more recent work, with a special attention to the subject of the so-called “lactate paradox”. [ABSTRACT FROM AUTHOR]

This paper presents current knowledge on low-level, long-lasting work-related muscle activity, focusing on the shoulder and the upper part of the trapezius muscle, and on mental, rather than biomechanical reasons for the muscle activation. The paper identifies three sources of vocational muscle activity: the biomechanical need for force production in order to perform movements or maintain postures against the force of gravity, the biomechanical need to stabilise body parts as a reference for performing movements and securing a stable visual field, and finally, muscle activity without obvious biomechanical purposes. This last category has been labelled non-biomechanical muscle activity in this review. Non-biomechanical muscle activity is related to the mental load, the emotional load and the individual characteristics of the subject, and is identified as having a low-level and a low second-to-second variability, resembling a static muscle contraction. Recent research has indicated that the size principle for motor unit recruitment order puts a strain on a limited number of low-threshold motor units which might be heavily taxed despite the overall low level of this muscle activity. However, the paper also cites a recent report showing that motor unit substitution may occur in prolonged low-level muscle activation (longer than a few minutes). Evaluations of muscle load at work usually omit the possibility of extra muscle activation due to non- biomechanical factors, and thus may often give estimates of the muscle load that are too low, or misinterpret non-biomechanical muscle activity as biomechanical muscle load. [ABSTRACT FROM AUTHOR]