Effect of aging on human muscle architecture

Ageing and physical activity: evidence to develop exercise recommendations for older adultsThis article is part of a supplement entitled Advancing physical activity measurement and guidelines in Canada: a scientific review and evidence-based foundation for the future of Canadian physical activity guidelines co-published by Applied Physiology, Nutrition, and Metabolism and the Canadian Journal of Public Health. It may be cited as Appl. Physiol. Nutr. Metab. 32(Suppl. 2E) or as Can. J. Public Health 98(Suppl. 2)

An abundance of epidemiological research confirms the benefits of physical activity in reducing risk of various age-related morbidities and all-cause mortality. Analysis of the literature focusing on key exercise variables (e.g., intensity, type, and volume) suggests that the requisite beneficial amount of activity is that which engenders improved cardiorespiratory fitness, strength, power, and, indirectly, balance. Age-related declines in these components are such that physical limitations impinge on functional activities of daily living. However, an exercise programme can minimize declines, thus preventing older adults (age 65+ years) from crossing functional thresholds of inability. Cross-sectional and longitudinal data demonstrate that cardiorespiratory fitness is associated with functional capacity and independence; strength and, importantly, power are related to performance and activities of daily living; and balance-mobility in combination with power are important factors in preventing falls. Exercise interventions have documented that older adults can adapt physiologically to exercise training, with gains in functional capacities. The few studies that have explored minimal or optimal activity requirements suggest that a threshold (intensity) within the moderately vigorous domain is needed to achieve and preserve related health benefits. Thus, physical activity and (or) exercise prescriptions should emphasize activities of the specificity and type to improve components related to the maintenance of functional capacity and independence; these will also delay morbidity and mortality. An appropriate recommendation for older adults includes moderately vigorous cardiorespiratory activities (e.g., brisk walking), strength and (or) power training for maintenance of muscle mass and specific muscle-group performance, as well as “balance-mobility practice” and flexibility (stretching) exercise as needed.

Plasticity of the muscle-tendon complex with disuse and aging

Muscles and tendons are highly adaptive tissues in response to chronic changes in loading and to aging. A remarkable reorganization in muscle architecture occurs in both conditions together with significant alterations in tendon mechanical properties. This review discusses the possible mechanisms underlying these myotendinous changes and the influence thereof on the behavior of the muscle-tendon complex as a whole.

Alterations in IGF-I affect elderly: role of physical activity

The growth hormone–insulin-like growth factor I (IGF-I) axis is an important physiological regulator muscle for development. Although there is evidence that aging muscle retains the ability to synthesize IGF-I, there is also evidence that aging may be associated with attenuation of the ability of exercise to induce an isoform of IGF-I that promotes satellite cell proliferation. However, it is clear that overexpression of IGF-I in the muscle can protect against age-related sarcopenia. Strength training appears to be the intervention of choice for the prevention and treatment of sarcopenia. IGF-I has been implicated in the loss of the muscle with age, and IGF-I expression levels change as a consequence of strength training in older adults. However, it seems that advancing age, rather than declining serum levels of IGF-I, appears to be a major determinant of life-time changes in body composition in women and men. We concluded that resistive exercise is a significant determinant of muscle mass and function. Elevated levels of IGF-I have been found in physically active compared to sedentary individuals. Recent work suggests that IGF-I as a mediator plays an important role in muscle hypertrophy and angiogenesis, both of which characterize the anabolic adaptation of muscles to exercise.

Influence of concentric and eccentric resistance training on architectural adaptation in human quadriceps muscles

Studies using animal models have been unable to determine the mechanical stimuli that most influence muscle architectural adaptation. We examined the influence of contraction mode on muscle architectural change in humans, while also describing the time course of its adaptation through training and detraining. Twenty-one men and women performed slow-speed (30 degrees /s) concentric-only (Con) or eccentric-only (Ecc) isokinetic knee extensor training for 10 wk before completing a 3-mo detraining period. Fascicle length of the vastus lateralis (VL), measured by ultrasonography, increased similarly in both groups after 5 wk (Delta(Con) = +6.3 +/- 3.0%, Delta(Ecc) = +3.1 +/- 1.6%, mean = +4.7 +/- 1.7%; P < 0.05). No further increase was found at 10 wk, although a small increase (mean approximately 2.5%; not significant) was evident after detraining. Fascicle angle increased in both groups at 5 wk (Delta(Con) = +11.1 +/- 4.0%, Delta(Ecc) = +11.9 +/- 5.4%, mean = 11.5 +/- 3.2%; P < 0.05) and 10 wk (Delta(Con) = +13.3 +/- 3.0%, Delta(Ecc) = +21.4 +/- 6.9%, mean = 17.9 +/- 3.7%; P < 0.01) in VL only and remained above baseline after detraining (mean = 13.2%); smaller changes in vastus medialis did not reach significance. The similar increase in fascicle length observed between the training groups mitigates against contraction mode being the predominant stimulus. Our data are also strongly indicative of 1) a close association between VL fascicle length and shifts in the torque-angle relationship through training and detraining and 2) changes in fascicle angle being driven by space constraints in the hypertrophying muscle. Thus muscle architectural adaptations occur rapidly in response to resistance training but are strongly influenced by factors other than contraction mode.

Age-related neuromuscular function during drop jumps

Muscle- and movement-specific fascicle-tendon interaction affects the performance of the neuromuscular system. This interaction is unknown among elderly and consequently contributes to the lack of understanding the age-related problems on neuromuscular control. The present experiment studied the age specificity of fascicle-tendon interaction of the gastrocnemius medialis (GM) muscle in drop jump (DJ) exercises. Twelve young and thirteen elderly subjects performed maximal squat jumps and DJs with maximal rebound effort on a sledge apparatus. Ankle and knee joint angles, reaction force, and electromyography (EMG) from the soleus (Sol), GM, and tibialis anterior (TA) muscles were measured together with the GM fascicle length by ultrasonography. The results showed that the measured ankle joint stiffness (AJS) during the braking phase correlated positively with the rebound speed in both age groups and that both parameters were significantly lower in the elderly than in young subjects. In both groups, the AJS correlated positively with averaged EMG (aEMG) in Sol during the braking phase and was further associated with GM activation (r = 0.55, P < 0.01) and TA coactivation (TA/GM r = -0.4 P < 0.05) in the elderly subjects. In addition, compared with the young subjects, the elderly subjects showed significantly lower GM aEMG in the braking phase and higher aEMG in the push-off phase, indicating less utilization of tendinous tissue (TT) elasticity. These different activation patterns are in line with the mechanical behavior of GM showing significantly less fascicle shortening and relative TT stretching in the braking phase in the elderly than in the young subjects. These results suggest that age-specific muscle activation patterns as well as mechanical behaviors exist during DJs.

Muscle - tendon unit mechanical and morphological properties and sprint performance

The objective of this study was to determine whether sprint performance is related to the mechanical (elongation - force relationship of the tendon and aponeurosis, muscle strength) and morphological (fascicle length, pennation angle, muscle thickness) properties of the quadriceps femoris and triceps surae muscle - tendon units. Two groups of sprinters (slow, n = 11; fast, n = 17) performed maximal isometric knee extension and plantar flexion contractions on a dynamometer at 11 different muscle - tendon unit lengths. Elongation of the tendon and aponeurosis of the gastrocnemius medialis and the vastus lateralis was measured using ultrasonography. We observed no significant differences in maximal joint moments at the ankle and knee joints or morphological properties of the gastrocnemius medialis and vastus lateralis between groups (P > 0.05). The fast group exhibited greater elongation of the vastus lateralis tendon and aponeurosis at a given tendon force, and greater maximal elongation of the vastus lateralis tendon and aponeurosis during maximum voluntary contraction (P < 0.05). Furthermore, maximal elongation of the vastus lateralis tendon and aponeurosis showed a significant correlation with 100-m sprint times (r = -0.567, P = 0.003). For the elongation - force relationship at the gastrocnemius medialis tendon and aponeurosis, the two groups recorded similar values. It is suggested that the greater elongation of the vastus lateralis tendon and aponeurosis of the fast group benefits energy storage and return as well as the shortening velocity of the muscle - tendon unit.

Effects of joint angle and age on ankle dorsi- and plantar-flexor strength

This study aimed at examining the effects of joint angle and age on the maximal voluntary contraction (MVC) torque, for the agonist and antagonist muscle groups around the ankle, i.e., the dorsi- and plantar-flexors. To this aim, neural and muscular factors were investigated in two groups of healthy men: 11 young (mean age, 24 years) and 18 older (mean age, 78 years). Plantar-flexion (PF) and dorsiflexion (DF) isometric MVC torques were measured in three different ankle joint angles and surface electromyographic activities of the triceps surae and of the tibialis anterior muscles were recorded. The main findings were that the DF-to-PF MVC torque ratio varied with joint angle and age, indicating that aging affected at different rates the two muscle groups: this ratio was always higher in older adults because of the PF strength decline with aging. Furthermore, the DF MVC torque-angle relationship appeared to be especially explained by neural factors, whereas the relationship in PF seemed to be mainly due to muscular parameters. These relationships would not be a discriminating factor between the two age groups. As a consequence, measurements at one ankle joint angle, whatever the angle, are thus enough to examine the differences within age groups and to perform a rapid assessment of the imbalance at the ankle joint.

Influence of muscle architecture on the torque and power-velocity characteristics of young and elderly men

This study investigated the contribution of muscle architecture to the differences in the torque-velocity and power-velocity relationships between older (OM n = 9, aged 69-82 years) and younger men (YM n = 15, aged 19-35 years). Plantarflexors' (PF) maximal isometric and concentric torques were recorded at 0.87, 1.75, 2.62, 3.49 and 4.36 rad s(-1). Physiological cross-sectional area (PCSA) was calculated as the ratio of muscle volume (determined by magnetic resonance imaging) to muscle fascicle length (Lf, measured by ultrasonography). GM PCSA and Lf of the OM were, respectively, 14.3% (P < 0.05) and 19.3% (P < 0.05) smaller than of the YM. In the OM, GM maximum isometric torque and maximum contraction velocity (Vmax), estimated from Hill's equation were, respectively, 48.5 and 38.2% lower (P < 0.001) than in the YM. At all contraction velocities, the OM produced less torque than the YM (46.3% of YM at 0.87 rad s(-1) to 14.7% at 4.36 rad s(-1), P < 0.001). Peak power (PP) of the OM was 80% lower than that of the YM and normalisation of PP to muscle volume only reduced this difference by 10%. Normalisation of torque to PCSA reduced, but did not eliminate, differences in torque between YM and OM (9.6%) and differences in torque/PCSA increased with contraction velocity (P < 0.05). After normalisation of velocity to Lf, the difference in Vmax between the OM and the YM was reduced to 15.9%. Thus, although muscle architecture contributes significantly to the differences in the torque- and power-velocity properties of OM and YM, other contractile factors, intrinsic to the muscle, seem to play a role. It is noteworthy that the deficit in PP between OM and YM is far greater than that of muscle torque, even after normalisation of PP to muscle volume. This finding likely plays an important role in the loss of mobility in old age.

Peripheral impairments cause a progressive age-related loss of strength and velocity-dependent power in the dorsiflexors

Muscle power is more functionally relevant than static muscle strength, particularly with aging. However, the effect of age on power derived from isotonic contractions has been studied sparingly, and it has not been studied at all in subjects >75 yr of age. Thus the purpose was to investigate the magnitude and causes of age-related losses in isotonic power among 13 young (26 yr), 13 old (65 yr), and 13 very old (84 yr) men. Six different loads were employed to create velocity-torque and power-torque relationships. Dorsiflexor cross-sectional area was assessed via magnetic resonance imaging for the calculation of specific power. Electromyographic signals of the tibialis anterior and soleus muscles were recorded to assess agonist activation and antagonist coactivation, respectively. Despite similar contractile masses and levels of voluntary drive and antagonist co-activation, power was significantly impaired in the old vs. young (∼25%), and in the very old relative to both the young (∼60%) and old (∼40%). The novel results punctuate two important considerations for studies concerned with the effect of age on the neuromuscular system. First, the decreased ability of muscles from old men to produce power in the presence of reasonably well-preserved strength indicates the utility of studying isotonic contractions. Second, the precipitous decline in many measures between the seventh and ninth decades underscores the benefit of testing more than one group of aged subjects to improve our understanding of rates of change in key variables.

Gastrocnemius muscle-tendon behaviour during walking in young and older adults

AIM

Age-related differences in muscle architectural and tendon mechanical properties have been observed in vivo under static conditions and during single joint contractions. The aim of this study was to determine if there are age-related differences in gastrocnemius fascicle-tendon interactions during a fundamental locomotor task - walking.

METHODS

Eight young adults (YA; 27 +/- 4 years) and eight older adults (OA; 77 +/- 4 years) walked on a treadmill at 1.11 m s(-1) whilst length changes in the gastrocnemius lateralis muscle tendon complex (MTC), fascicles and tendinous tissue (TT) were determined from joint angles, ultrasonography and a geometric MTC model (combining MTC and fascicle measurements) respectively.

RESULTS

There was no age-related difference in lengthening of the MTC during stance. However, the fascicle and TT contribution to MTC lengthening was altered; TT lengthening was larger in OA than in YA (P = 0.05) and fascicle lengthening was less in OA than YA (P < 0.05). There were no differences between groups in MTC, fascicle or TT shortening amplitude during push-off.

CONCLUSION

The observations are consistent with previous reports of increased compliance of TT in older adults.

In vivo motion transmission in the inactive gastrocnemius medialis muscle–tendon unit during ankle and knee joint rotation

The purposes of this study were: (a) to quantify the influence of passive ankle and knee joint angular displacement on the estimated mechanical and architectural properties of the gastrocnemius medialis (GM) muscle-tendon unit, and (b) to determine the strain distribution of separate structures (tendon, aponeurosis and fascicle) during passive lengthening of the GM muscle-tendon unit at rest. Ten male subjects participated in the study. The passive ankle and knee joint movements were performed on an isokinetic dynamometer. The kinematics of the left leg were recorded using the Vicon 624 system with 8 cameras. Two ultrasound probes were used to examine the elongation of the tendon, the aponeurosis, the fascicles and the angle of pennation of the GM. To calculate the elongation of the GM muscle-tendon unit the Achilles tendon path was reconstructed using a series of small reflective markers. The results show that the passive ankle joint angular displacement has a considerable influence on the elongation of the tendinous and architectural structures of the GM muscle-tendon unit. In contrast, the influence of knee joint angular displacement on the GM fascicle length and pennation angle becomes relevant only at knee angles greater than 144 degrees . The contribution of the tendon to the elongation of the GM muscle-tendon unit at rest is relevant because of its greater resting length in comparison to the resting length of the GM fascicles. The results indicate the existence of slackness in the inactive GM muscle-tendon unit between 121 degrees and 107 degrees ankle angle and between 65 degrees and 144 degrees knee angle.

Adaptability of elderly human muscles and tendons to increased loading

Senile sarcopenia, the loss of muscle mass associated with aging, is one of the main causes of muscle weakness and reduced locomotor ability in old age. Although this condition is mainly driven by neuropathic processes, nutritional, hormonal and immunological factors, as well as a reduction in physical activity, contribute to this phenomenon. Sarcopenia alone, however, does not fully account for the observed muscle weakness, as the loss of force is greater than that accounted for by the decrease in muscle size. As a consequence, a reduction in the force per unit area, both at single fibre and at whole muscle level, is observed. We recently suggested that at whole muscle level, this reduction in intrinsic force is the result of the combined effect of changes in (1) muscle architecture, (2) tendon mechanical properties, (3) neural drive (reduced agonist and increased antagonist muscle activity) and (4) single fibre-specific tension. Whereas several studies support the role of the last two factors in the loss of intrinsic muscle force with aging, alterations in muscle architecture and in tendon mechanical properties have also been shown to contribute to the above phenomenon. Indeed, sarcopenia of the human plantarflexors, represented by a 25% reduction in muscle volume, was found to be associated with a 10% reduction in fibre fascicle length and 13% reduction in pennation angle. These architectural alterations were accompanied by a 10% decrease in tendon stiffness, attributable to alterations in tendon material properties, as suggested by a 14% decrease in Young's modulus. Most of these changes may be reversed by 14 weeks of resistive training; both fibre fascicle length and tendon stiffness were found to be increased by 10 and 64%, respectively. Surprisingly, however, training had no effect on the estimated relative length-tension properties of the muscle, indicating that the effects of greater tendon stiffness and increased fascicle length cancelled out each other. It seems that natural strategies may be in place to ensure that the relative operating range of muscle remains unaltered by changes in physical activity, in old age.

Musculoskeletal adaptations to resistance training in old age

Muscle weakness experienced in old age has many detrimental consequences for activities of daily life. Given the serious problems presented by weakness in old age, strategies to prevent or mitigate this process are of paramount importance. In recent years resistance training has emerged as an effective method for increasing strength in the elderly. Despite this, little is known regarding the muscular, neural and tendinous adaptations that occur with resistance training in old age. Hence, we have conducted a series of experiments to investigate these adaptations. We have found increases in maximal isometric and concentric torque by 9-37% after resistance training in older people (65-81 years). Associated with these strength gains were increases in agonist muscle neural drive without any change in the co-activation of antagonist muscles. Resistance training can cause increases in muscle size and also adaptations to the internal muscle structure. Tendons of older adults adapt to resistance training by increasing their stiffness and Young's modulus. In conclusion, many of the musculoskeletal factors characterizing ageing can be at least partially mitigated by resistance training.

Gastrocnemius specific force is increased in elderly males following a 12-month physical training programme

The aim of the present investigation was to determine whether muscle force per physiological cross sectional area (PCSA) of the lateral gastrocnemius (GL) of elderly males increased following a 12-month physical training programme. Eleven elderly males were assigned to a 12-month training programme (TRN mean age 72.7 +/- 3.3 years, mean +/- SD) and eight elderly males were allocated to a control group (CTRL, 73.9 +/- 4.0 years) who maintained their habitual physical activity levels. In vivo measurements of muscle architecture, muscle volume (VOL), achilles tendon moment arm length and plantarflexor torque were used to estimate GL PCSA (VOL/fascicle length) and specific force (GL fascicle force/GL PCSA). Maximal GL fascicle force was calculated accounting for agonist muscle activation and antagonist co-activation. Following training GL fascicle force increased by 31% (P < 0.01), which was not entirely accounted for by a 17% increase in PCSA (from 27.2 +/- 5.9 to 31.8 +/- 6.2 cm(2), P < 0.05). Specific force increased significantly from 8.9 +/- 1.9 to 11.2 +/- 3.0 N cm(-2) (P < 0.05). Pennation angle, but not fascicle length, increased by 12% with training (P < 0.05). The CTRL group showed no change in muscle size, strength or architecture over the 12-month period. In conclusion, with the level of agonist and antagonist muscle activity accounted for a 12-month strength training programme resulted in an increase in both PCSA and specific force in elderly males.

Acute changes in knee-extensors torque, fiber pennation, and tendon characteristics

The aim of the current study was to examine the relationships between quadriceps torque, vastus lateralis pennation angle (theta), and patella tendon stiffness (K) at 07:45 and 17:45 h. Using short-duration static contractions, simultaneous recordings were made of vastus lateralis (VL) electromyograph (EMG), theta and patella tendon K. Peak isometric extension torque (Peak torque Ext(corr)) increased by 29.4+/-6.5% at a knee angle of 70 degrees (p=0.03) in the evening compared to the morning. In the contracted muscle, a 35.0+/-11.0% (p=0.02) time-of-day (TOD)-related change in theta (to a greater evening compared to morning theta) was observed. Morning and evening measures of theta were also made, both at rest and at a standardized force level (250 N), to separate architecture change effects from increased torque capacity effects. Significant increments in theta in both the resting muscle (13.0+/-5.1%, p=0.046) and during the standardized exertions (8.0+/-3.1%, p=0.04) were observed in the evening versus the morning. Increases in theta with TOD were significantly correlated with the 40% (p=0.018) decrease in K both during the standardized contractions (r=0.788, p<0.001) and at rest (r=0.77, p=0.026). These data show that TOD affects K and theta and that these two important factors involved in in-vivo muscle torque generation capacity are associated. The data also show that despite the potentially deleterious effects of the direction of the changes in both K and theta with TOD, peak torque Ext(corr) still shows a significant upward shift in the evening relative to the morning.

Myotendinous alterations and effects of resistive loading in old age

The loss of muscle mass associated with ageing only partly explains the observed decline in muscle strength. This paper provides evidence of the contribution of muscular, tendinous and neural alterations to muscle weakness in old age and discusses the complex interplay between the changes of the contractile tissue with those of the tendinous tissue in relation to the mechanical behavior of the muscle as a whole. Despite the considerable structural and functional alterations, the elderly musculoskeletal system displays remarkable adaptability to training in old age and many of these adverse effects may be substantially mitigated, if not reversed, by resistive loading. The interplay between these muscular and tendinous adaptations has an impact both on the length-force and force-velocity relationships of the muscle and is likely to affect the range of motion, rate of force development, maximum force development and speed of movement of the older individual.

Mechanical and morphological properties of human quadriceps femoris and triceps surae muscle-tendon unit in relation to aging and running

The purpose of this study was to examine the effects of aging and endurance running on the mechanical and morphological properties of different muscle-tendon units (MTUs) in vivo. The investigation was conducted on 30 elderly and 19 young adult males. For the analysis of possible MTU adaptation in response to endurance running the subjects were divided into two subgroups: non-active vs. endurance-runners. All subjects performed isometric maximal voluntary plantarflexion and knee extension contractions on a dynamometer. The distal aponeurosis of the gastrocnemius medialis (GM) and vastus lateralis (VL) during plantarflexion and knee extensions and the muscle architecture of the GM and VL were visualized by ultrasonography. The maximal knee and ankle joint moment were higher for the young compared to the elderly population (p<0.05). No identifiable differences in muscle architecture between young and elderly subjects were detected in VL and GM. Aging results in a reduced (p<0.05) normalized stiffness of the quadriceps femoris tendon and aponeurosis, which were not identifiable for the triceps surae. In contrast, the properties of both MTUs showed no major differences between endurance-runners and the non-active group (p>0.05). Only pennation angle at the GM were higher for the runners compared to the non-active group (p<0.05). The present results indicate that tendon changes related to aging do not occur proportionally in different MTUs. Furthermore, it seems that the extra stress and load imposed on high-load-bearing MTUs during endurance running may not be sufficient to produce significant adaptative processes in the mechanical parameters analyzed.

Myotendinous plasticity to ageing and resistance exercise in humans

The age-related loss of muscle mass known as senile sarcopenia is one of the main determinants of frailty in old age. Molecular, cellular, nutritional and hormonal mechanisms are at the basis of sarcopenia and are responsible for a progressive deterioration in skeletal muscle size and function. Both at single-fibre and at whole-muscle level, the loss of force exceeds that predicted by the decrease in muscle size. For single fibres, the loss of intrinsic force is mostly due to a loss in myofibrillar protein content. For whole muscle, in addition to changes in neural drive, alterations in muscle architecture and in tendon mechanical properties, exemplified by a reduction in tendon stiffness, have recently been shown to contribute to this phenomenon. Resistance training can, however, cause substantial gains in muscle mass and strength and provides a protective effect against several of the cellular and molecular changes associated with muscle wasting and weakness. In old age, not only muscles but also tendons are highly responsive to training, since an increase in tendon stiffness has been observed after a period of increased loading. Many of the myotendinous factors characterizing ageing can be at least partly reversed by resistance training.

Muscle architecture of the common chimpanzee (Pan troglodytes): perspectives for investigating chimpanzee behavior

Thorpe et al. (Am J Phys Anthropol 110:179-199, 1999) quantified chimpanzee (Pan troglodytes) muscle architecture and joint moment arms to determine whether they functionally compensated for structural differences between chimpanzees and humans. They observed enough distinction to conclude that musculoskeletal properties were not compensatory and suggested that chimpanzees and humans do not exhibit dynamically similar movements. These investigators based their assessment on unilateral limb musculatures from three male chimpanzees, of which they called one non-adult representative. Factors such as age, sex, and behavioral lateralization may be responsible for variation in chimpanzee muscle architecture, but this is presently unknown. While the full extent of variation in chimpanzee muscle architecture due to such factors cannot be evaluated with data presently available, the present study expands the chimpanzee dataset and provides a preliminary glimpse of the potential relevance of these factors. Thirty-seven forelimb and 36 hind limb muscles were assessed in two chimpanzee cadavers: one unilaterally (right limbs), and one bilaterally. Mass, fiber length, and physiological cross-sectional area (PCSA) are reported for individual muscles and muscle groups. The musculature of an adult female is more similar in architectural patterns to a young male chimpanzee than to humans, particularly when comparing muscle groups. Age- and sex-related intraspecific differences do not obscure chimpanzee-human interspecific differences. Side asymmetry in one chimpanzee, despite consistent forelimb directional asymmetry, also does not exceed the magnitude of chimpanzee-human differences. Left forelimb muscles, on average, usually had higher masses and longer fiber lengths than right, while right forelimb muscles, on average, usually had greater PCSAs than left. Most muscle groups from the left forelimb exhibited greater masses than right groups, but group asymmetry was significant only for the manual digital muscles. The hind limb exhibited less asymmetry than the forelimb in most comparisons. Examination of additional chimpanzees would clarify the full range of inter- and intra-individual variation.

Understanding Muscle Architectural Adaptation: Macro- and Micro-Level Research

Recent research using muscle-imaging techniques has revealed a remarkable plasticity of human muscle architecture where significant changes in fascicle lengths and angles have resulted from the chronic performance, or cessation, of strong muscle contractions. However, there is a paucity of data describing architectural adaptations to chronic stretching, disuse and immobilization, illness, and aging, and those data that are available are equivocal. Understanding their impact is important in order that effective interventions for illness/injury management and rehabilitation, and programs to improve the physical capacity of workers, the aged and athletes can be determined. Nonetheless, recent advances in myocellular research could provide a framework allowing the prediction of architectural changes in these understudied areas. Examination of the site-specific response to mechanical stress of calpain-dependent ubiquitin-proteasome proteolysis, or of the cellular response to stress after the knockout (or incapacitation) of sarcomeric and cytoskeletal proteins involved in cellular signal transduction, provides an exciting paradigm by which myocellular adaptation can be described. Such research might contribute to the understanding of macro-level changes in muscle architecture.

Effects of Physical Training and Detraining, Immobilisation, Growth and Aging on Human Fascicle Geometry

In addition to its size and the extent of its neural activation, a muscle's geometry (the angles and lengths of its fibres or fascicles) strongly influences its force production characteristics. As with many other tissues within the body, muscle displays significant plasticity in its geometry. This review summarises geometric differences between various athlete populations and describes research examining the plasticity of muscle geometry with physical training, immobilisation/detraining, growth and aging. Typically, heavy resistance training in young adults has been shown to cause significant increases in fascicle angle of vastus lateralis and triceps brachii as measured by ultrasonography, while high-speed/plyometrics training in the absence of weight training has been associated with increases in fascicle length and a reduction in angles of vastus lateralis fascicles. These changes indicate that differences in geometry between various athletic populations might be at least partly attributable to their differing training regimes. Despite some inter-muscular differences, detraining/unloading is associated with decreases in fascicle angle, although little change was shown in muscles such as vastus lateralis and triceps brachii in studies examining the effects of prolonged bed rest. No research has examined the effects of other interventions such as endurance or chronic stretching training. Few data exist describing geometric adaptation during growth and maturation, although increases in gastrocnemius fascicle angle and length seem to occur until maturation in late adolescence. Although some evidence suggests that a decrease in both fascicle angle and length accompanies the normal aging process, there is a paucity of data examining the issue; heavy weight training might attenuate the decline, at least in fascicle length. A significant research effort is required to more fully understand geometric adaptation in response to physical training, immobilisation/detraining, growth and aging.

Effect of different ankle- and knee-joint positions on gastrocnemius medialis fascicle length and EMG activity during isometric plantar flexion

The purpose of this study was to provide evidence on the fact that the observed decrease in EMG activity of the gastrocnemius medialis (GM) at pronounced knee flexed positions is not only due to GM insufficiency, by examining muscle fascicle lengths during maximal voluntary contractions at different positions. Twenty-two male long distance runners (body mass: 78.5+/-6.7 kg, height: 183+/-6 cm) participated in the study. The subjects performed isometric maximal voluntary plantar flexion contractions (MVC) of their left leg at six ankle-knee angle combinations. To examine the resultant ankle joint moments the kinematics of the left leg were recorded using a Vicon 624 system with 8 cameras operating at 120 Hz. The EMG activity of GM, gastrocnemius lateralis (GL), soleus (SOL) and tibialis anterior (TA) were measured using surface electromyography. Synchronously, fascicle length and pennation angle values of the GM were obtained at rest and at the plateau of the maximal plantar flexion using ultrasonography. The main findings were: (a) identifiable differences in fascicle length of the GM at rest do not necessarily imply that these differences would also exist during a maximal isometric plantar flexion contraction and (b) the EMG activity of the biarticular GM during the MVC decreased at a pronounced flexed knee-joint position (up to 110 degrees ) despite of no differences in GM fascicle length. It is suggested that the decrease in EMG activity of the GM at pronounced knee flexed positions is due to a critical force-length potential of all three muscles of the triceps surae.

Aging-sensitive cellular and molecular mechanisms associated with skeletal muscle hypertrophy

Sarcopenia is an age-related loss of muscle mass and strength. The aged can increase various measures of muscle size and strength in response to resistance exercise (RE), but this may not normalize specific tension. In rats, aging reduces the hypertrophy response and impairs regeneration. In this study, we measured cellular and molecular markers, indicative of muscle hypertrophy, that also respond to acute increases in loading. Comparing 6- and 30-mo-old rats, the aims were to 1) determine whether these markers are altered with age and 2) identify age-sensitive responses to acute RE. The muscles of old rats exhibited sarcopenia involving a deficit in contractile proteins and decreased force generation. The RNA-to-protein ratio was higher in the old muscles, suggesting a decrease in translational efficiency. There was evidence of reduced signaling via components downstream from the insulin/insulin-like growth factor (IGF)-I receptors in old muscles. The mRNA levels of myostatin and suppressor of cytokine signaling 2, negative regulators of muscle mass, were lower in old muscles but did not decrease following RE. RE induced increases in the mRNAs for IGF-I, mechano-growth factor, cyclin D1, and suppressor of cytokine signaling 3 were similar in old and young muscles. RE induced phosphorylation of the IGF-I receptor, and Akt increased in young but not old muscles, whereas that of S6K1 was similar for both. The results of this study indicate that a number of components of intracellular signaling pathways are sensitive to age. As a result, key anticatabolic responses appear to be refractory to the stimuli provided by RE.

Triceps surae muscle power, volume, and quality in older versus younger healthy men

This study investigated whether loss of power with aging is fully accounted for by a decrease in muscle volume. Triceps surae power and volume (VOL) were measured in 18 older (OM: 69-82 years) and 12 younger men (YM: 19-35 years). Isokinetic peak torque was measured to determine torque-velocity and power-velocity relationships. Both peak power observed (PP(obs)) and peak power estimated from Hill's equation (PP(est)) were markedly reduced in the OM (PP(obs) was 45% and PP(est) was 43% of those of the YM). VOL was 81% of that of the YM (p <.001). Specific power (PP(est)/VOL) of the OM was 55.2% of that of the YM (p <.001). Torque at PP(est) accounted for a greater proportion of the decline in PP(est) in the OM than did optimum velocity (50% vs 13%, respectively). Hence, the present results showed that only approximately half of the loss in triceps surae peak power in old age is due to decreases in muscle VOL.

Inevitable joint angular rotation affects muscle architecture during isometric contraction

The purpose of this study was to quantify the influence of inevitable ankle joint motion during an isometric contraction on the measured change of the gastrocnemius medialis muscle (GM) architecture in vivo during the loading and the unloading phase. Sitting on a dynamometer subjects performed isometric maximal voluntary contractions as well as contractions induced by electrostimulation. Synchronous joint angular motion, plantarflexion moment, foot's centre of pressure and real-time ultrasonography of muscle architecture changes of the GM were obtained. During the contraction the ankle joint position altered and significantly affected the change in muscle architecture. At maximal tendon force (1094+/-323 N), the measured fascicle length overestimated the change in fascicle length due to the tendon force by 1.53 cm, while the measured pennation angle overestimated the change in pennation angle due to the tendon force by 5.5 degrees . At the same tendon force the measured fascicle length and pennation angle were significantly different between loading and unloading conditions. After correcting the values for the change in ankle joint angle no differences between the loading and the unloading phase at the same tendon force were found. Concerning the estimation of GM fascicle length-force and pennation angle-force curves during the loading and unloading phase of an isometric contraction, these findings indicate that not accounting for ankle joint motion will produce unreliable results.

Regulation of the pituitary growth hormone-releasing hormone receptor in ageing male and female LOU rats: new insights into healthy ageing

Ageing is characterised by a decrease of somatotroph functionality, involving growth hormone-releasing hormone receptor (GHRH-R). The present study was conducted in LOU/C/jall (LOU) rats, a strain described as a model of healthy ageing, which is characterised by a low adiposity and long life expectancy without developing severe pathologies. Effects of age and diet (chow versus self-selection), on levels of anterior pituitary GHRH-R mRNA transcripts, were assessed in male and female LOU rats. The effect of age on pituitary GHRH-R functionality was examined in the anterior pituitary of both males and females fed chow diet. Moreover, serum insulin-like growth factor-I (IGF-I), T4 and leptin were measured because changes in their concentration could affect GHRH-R expression. In the pituitary of 18-month-old male and female LOU/C/jall rats fed standard chow, the level of 2.5-kb GHRH-R mRNA transcript, coding for functional GHRH-R, was significantly decreased. In 24- to 34-month-old males and females, it progressively returned to the level of younger animals, suggesting an enrichment of the group with survivors maintaining functional GHRH-R. In males and females repeatedly submitted to self-selection, this phenomenon was not observed. Studies with the GHRH-R agonist, Fluo-GHRH, revealed that 73% of 16-18-month-old male and female rats studied did not show an increase of fluorescence density characteristic of receptor-mediated internalisation upon incubation at 37 degrees C. In the other 27%, the increase of fluorescence was identical to that observed in pituitaries of young rats, suggesting the presence of an optimal level of functional GHRH-R. Serum levels of leptin, free T4 and total IGF-I decreased more drastically in ageing males and in rats fed a self-selection diet. A positive correlation was demonstrated between leptin and IGF-I levels in ageing males and females fed standard chow and ageing females submitted to a self-selection regimen. In conclusion, healthy ageing in LOU rats fed chow diet appears to be associated with a maintenance of functional pituitary GHRH-R levels found in younger rats but not necessarily with those of serum leptin, T4 and IGF-I.

Mechanical and morphological properties of different muscle–tendon units in the lower extremity and running mechanics: effect of aging and physical activity

The objectives of this work were (i) to investigate whether chronic endurance running is a sufficient stimulus to counteract the age-related changes in the mechanical and morphological properties of human triceps surae(TS) and quadriceps femoris (QF) muscle–tendon units (MTUs) by comparing runners and non-active subjects at different ages (young and old), (ii) to identify adaptational phenomena in running mechanics due to age-related changes in the mechanical and morphological properties of the TS and QF MTUs,and finally (iii) to examine whether chronic endurance-running exercise is associated with adaptational effects on running characteristics in old and young adults.

The investigation was conducted on 30 old and 19 young adult males divided into two subgroups according to their running activity: endurance-runners vs non-active. To analyse the properties of the MTUs, all subjects performed isometric maximal voluntary (MVC) ankle plantarflexion and knee extension contractions at 11 different MTU lengths on a dynamometer. The activation of the TS and QF during MVC was estimated by surface electromyography. The gastrocnemius medialis and the vastus lateralis and their distal aponeuroses were visualized by ultrasonography at rest and during MVC, respectively. Ground reaction forces and kinematic data were recorded during running trials at 2.7 m s–1.

The TS and QF MTU capacities were reduced with aging (lower muscle strength and lower tendon stiffness). Runners and non-active subjects had similar MTU properties, suggesting that chronic endurance-running exercise does not counteract the age-related degeneration of the MTUs. Runners showed a higher mechanical advantage for the QF MTU while running (lower gear ratio) compared to non-active subjects, indicating a task-specific adaptation even at old age. Older adults reacted to the reduced capacities of their MTUs by increasing running safety (higher duty factor, lower flight time) and benefitting from a mechanical advantage for the TS MTU, lower rate of force generation and force generation per meter distance. We suggest that the improvement in running mechanics in the older adults happens due to a perceptual motor recalibration and a feed-forward adaptation of the motor task aimed at decreasing the disparity between the reduced capacity of the MTUs and the running effort.

In vivo physiological cross-sectional area and specific force are reduced in the gastrocnemius of elderly men

Sarcopenia and muscle weakness are well-known consequences of aging. The aim of the present study was to ascertain whether a decrease in fascicle force (Ff) could be accounted for entirely by muscle atrophy. In vivo physiological cross-sectional area (PCSA) and specific force (Ff/PCSA) of the lateral head of the gastrocnemius (GL) muscle were assessed in a group of elderly men [EM, aged 73.8 yr (SD 3.5), height 173.4 cm (SD 4.4), weight 78.4 kg (SD 8.3); means (SD)] and for comparison in a group of young men [YM, aged 25.3 yr (SD 4.4), height 176.4 cm (SD 7.7), weight 79.1 kg (SD 11.9)]. GL muscle volume (Vol) and Achilles tendon moment arm length were evaluated using magnetic resonance imaging. Pennation angle and fiber fascicle length (Lf) were measured using B-mode ultrasonography during isometric maximum voluntary contraction of the plantar flexors. PCSA was estimated as Vol/Lf. GL Ff was calculated by dividing Achilles tendon force by the cosine of theta, during the interpolation of a supramaximal doublet, and accounting for antagonist activation level (assessed using EMG), Achilles tendon moment arm length, and the relative PCSA of the GL within the plantar flexor group. Voluntary activation of the plantar flexors was lower in the EM than in the YM (86 vs. 98%, respectively, P < 0.05). Compared with the YM, plantar flexor maximal voluntary contraction torque and Ff of the EM were lower by 47 and 40%, respectively (P < 0.01). Both Vol and PCSA were smaller in the EM by 28% (P < 0.01) and 16% (P < 0.05), respectively. Also, pennation angle was 12% smaller in the EM, whereas there was no significant difference in Lf between the YM and EM. After accounting for differences in agonists and antagonists activation, the Ff/PCSA of the EM was 30% lower than that of the YM (P < 0.01). These findings demonstrate that the loss of muscle strength with aging may be explained not only by a reduction in voluntary drive to the muscle, but mostly by a decrease in intrinsic muscle force. This phenomenon may possibly be due to a reduction in single-fiber specific tension.

Changes in triceps surae muscle architecture with sarcopenia

AIM

To investigate whether sarcopenia was evenly distributed among the three components of the triceps surae (TS) muscle group.

METHODS

Muscle volume (VOL), fibre fascicle length (Lf), pennation angle (theta) and physiological cross-sectional area (PCSA = VOL/Lf) were measured in vivo for the lateral (GL) and medial (GM) heads of the gastrocnemius muscles and for the soleus muscle (SOL), in 15 young males (YM, aged 25.3 +/- 4.5 years) and 12 elderly males (EM aged 73.8 +/- 4.4 years).

RESULTS

In the EM, VOL of all three muscles was significantly smaller than in the YM; differences were: 27% for the GL (P < 0.01), 29% for the GM (P < 0.01) and 17% for the SOL (P < 0.05). In total, TS VOL was 22% smaller in EM than in YM (P < 0.01). In the EM, values of theta were significantly smaller than in the YM; by 15-18% for the GL, GM and SOL (P < 0.05). In the EM, Lf of the GM was 16% smaller than in the YM (P < 0.01); no significant differences were found in the other muscles. PCSA of the GL and GM were both found to be smaller in EM by 19% (P < 0.01) and 14.5% (P < 0.05), respectively. No difference was observed in the SOL PCSA between YM and EM. Interestingly, probably because of the prevalent contribution of the SOL to PCSA distribution of each muscle to the TS PCSA, the relative TS PCSA was not different between YM and EM. Furthermore, the Lf/muscle length ratio did not differ between YM and EM.

CONCLUSION

The present study shows that the relative PCSA composition of the TS is maintained with ageing and that the PCSA is scaled down harmonically with the decrease in muscle volume and fascicle length. Such observation suggests that the relative contribution of the components of the TS muscle to the total force developed by this muscle group is maintained with ageing.

Plasticity of dynamic muscle performance with strength training in elderly humans

Data are scarce relating to the plasticity with strength training of dynamic muscle performance in older humans. Hence, we investigated alterations in the torque-velocity relation with strength training in old age, and their origin. Knee extension and leg-press exercises were performed three times per week for 14 weeks. Maximal isokinetic knee extension torque was assessed during concentric and eccentric muscle actions. Agonist-antagonist muscle activation was assessed using electromyography. Vastus lateralis muscle architecture was examined in vivo using ultrasonography. Training increased concentric torque by 22-37% (P < 0.01), but failed to alter eccentric torque (P > 0.05). Increased agonist muscle activation, increased muscle fascicle lengths, and greater elastic energy recovered from tendinous structures may explain the adaptations during concentric actions, whereas the failure of eccentric torque to increase might be explained by the preservation of eccentric force with aging and an underloading of the eccentric movement phase during training. These findings may have important implications for dynamic muscle performance in old age.

Is muscle power output a key factor in the age-related decline in physical performance? A comparison of muscle cross section, chair-rising test and jumping power

Ageing compromises locomotor capacity and is associated with an increased risk of falls. Several lines of evidence indicate that both changes in muscle mass and performance are causative. Most studies, however, do not discern between effects of ageing, sedentarism and comorbidity. The present study compares the age effects in muscle cross section, force and power in physically competent self-selected subjects of different age groups. A total of 169 women and 89 men between 18 and 88 years, without any disease, impairment or medication affecting the musculoskeletal system were enrolled in this study. Calf muscle cross-sectional area was assessed by computed tomography. Muscle force and power were assessed by jumping mechanography. No significant correlation between muscle cross section and age was found in the men. A weak correlation in the women disappeared after correction for height. Close correlations with age, however, were found for peak force and peak power. Correction for muscle cross section or body weight further increased these correlation coefficients, particularly for peak power specific to body weight (r = 0.81 in women and r = 0.86 in men). The non-sedentarian population investigated here depicted a reduction of >50% between the age of 20 and 80 without a reduction in muscle cross section. This suggests a crucial role for muscular power in the ageing process. Possibly, the jumping mechanography as a measurement of anti-gravitational power output is a promising extension of the chair-rising test, known to be predictive for immobilization and the risk of falls.

In vivo human muscle structure and function: adaptations to resistance training in old age

This study investigated changes in elderly muscle joint angle-torque relation induced by resistance training. Older adults were assigned to either training (n = 9, age 74.3 +/- 3.5 years; mean +/-s.d.) or to control groups (n = 9, age 67.1 +/- 2 years). Leg-extension and leg-press exercises were performed three times per week for 14 weeks. Maximal isometric knee extension torque was measured across the knee joint angle range of movement. Vastus lateralis muscle architecture was examined in vivo using ultrasonography. The vastus lateralis muscle fascicle force was estimated from the measured joint torque, enabling construction of the fascicle length-force relation. Electromyographic (EMG) activity was measured from representative agonist and antagonist muscles. Training altered the angle-torque relation: (a) displacing it by 9-31% towards higher torque values (P < 0.05); and (b) shifting the optimal angle from 70 deg (corresponding torque: 121.4 +/- 61 N m) before to 60 deg (134.2 +/- 57.2 N m; P < 0.05) after training. Training also altered the fascicle length-force relation: (a) displacing it by 11-35% towards higher force values; and (b) shifting the optimal fascicle length from 83.7 +/- 8 mm (corresponding force: 847.9 +/- 365.3 N) before to 93.2 +/- 12.5 mm (939.3 +/- 347.8 N; P < 0.01) after training. The upward displacement of the angle-torque relation was mainly due to a training-induced increase in agonist activation, whilst the shift in the optimal angle was associated with changes in muscle-tendon properties.

Effect of resistance training on skeletal muscle-specific force in elderly humans

This study assessed muscle-specific force in vivo following strength training in old age. Subjects were assigned to training (n = 9, age 74.3 +/- 3.5 yr; mean +/- SD) and control (n = 9, age 67.1 +/- 2 yr) groups. Leg-extension and leg-press exercises (2 sets of 10 repetitions at 80% of the 5 repetition maximum) were performed three times/wk for 14 wk. Vastus lateralis (VL) muscle fascicle force was calculated from maximal isometric voluntary knee extensor torque with superimposed stimuli, accounting for the patella tendon moment arm length, ultrasound-based measurements of muscle architecture, and antagonist cocontraction estimated from electromyographic activity. Physiological cross-sectional area (PCSA) was calculated from the ratio of muscle volume to fascicle length. Specific force was calculated by dividing fascicle force by PCSA. Fascicle force increased by 11%, from 847.9 +/- 365.3 N before to 939.3 +/- 347.8 N after training (P < 0.05). Due to a relatively greater increase in fascicle length (11%) than muscle volume (6%), PCSA remained unchanged (pretraining: 30.4 +/- 8.9 cm(2); posttraining: 29.1 +/- 8.4 cm(2); P > 0.05). Activation capacity and VL muscle root mean square electromyographic activity increased by 5 and 40%, respectively, after training (P < 0.05), indicating increased agonist neural drive, whereas antagonist cocontraction remained unchanged (P > 0.05). The VL muscle-specific force increased by 19%, from 27 +/- 6.3 N/cm(2) before to 32.1 +/- 7.4 N/cm(2) after training (P < 0.01), highlighting the effectiveness of strength training for increasing the intrinsic force-producing capacity of skeletal muscle in old age.