Fatigue of elbow flexor muscles in younger and older adults

Voluntary activation during maximal contraction with advancing age: a brief review

It is well established that the loss of muscle mass (i.e. sarcopenia) is the primary factor contributing to the reduction in muscle force with ageing. Based on the observation that force declines at a faster rate than muscle mass, neural alterations are also thought to contribute to muscle weakness by reducing central drive to the agonist muscles and by increasing coactivation of the antagonist muscles. Researchers have attempted to quantify the contribution of impaired voluntary drive to the decline in muscle force using superimposed electrical stimulation during maximal voluntary contractions (MVCs) and by recording surface electromyographic (EMG) activity. Although reduced voluntary activation of agonist muscles and increased coactivation of antagonist muscles during a MVC have been reported with advancing age, such changes are not supported by all studies. These discrepancies may be explained by differences in sensitivity between the methods used to assess voluntary activation, as well as differences between the characteristics of the study population, the muscle group that is tested, and the type of contraction that is performed. The objective of this review is to summarize current knowledge regarding the activation of agonist and antagonist muscles during MVC in elderly and to try to clarify the disparities in literature concerning the influence of a possible deficit in voluntary activation on the maximal force capacity of muscles in elderly adults.

Supraspinal fatigue does not explain the sex difference in muscle fatigue of maximal contractions

Young women are less fatigable than young men for maximal and submaximal contractions, but the contribution of supraspinal fatigue to the sex difference is not known. This study used cortical stimulation to compare the magnitude of supraspinal fatigue during sustained isometric maximal voluntary contractions (MVCs) performed with the elbow flexor muscles of young men and women. Eight women (25.6 +/- 3.6 yr, mean +/- SD) and 9 men (25.4 +/- 3.8 yr) performed six sustained MVCs (22-s duration each, separated by 10 s). Before the fatiguing contractions, the men were stronger than the women (75.9 +/- 9.2 vs. 42.7 +/- 8.0 N.m; P < 0.05) in control MVCs. Voluntary activation measured with cortical stimulation before fatigue was similar for the men and women during the final control MVC (95.7 +/- 3.0 vs. 93.3 +/- 3.6%; P > 0.05) and at the start of the fatiguing task (P > 0.05). By the end of the six sustained fatiguing MVCs, the men exhibited greater absolute and relative reductions in torque (65 +/- 3% of initial MVC) than the women (52 +/- 9%; P < 0.05). The increments in torque (superimposed twitch) generated by motor cortex stimulation during each 22-s maximal effort increased with fatigue (P < 0.05). Superimposed twitches were similar for men and women throughout the fatiguing task (5.5 +/- 4.1 vs. 7.3 +/- 4.7%; P > 0.05), as well as in the last sustained contraction (7.8 +/- 5.9 vs. 10.5 +/- 5.5%) and in brief recovery MVCs. Voluntary activation determined using an estimated control twitch was similar for the men and women at the start of the sustained maximal contractions (91.4 +/- 7.4 vs. 90.4 +/- 6.8%, n = 13) and end of the sixth contraction (77.2 +/- 13.3% vs. 73.1 +/- 19.6%, n = 10). The increase in the area of the motor-evoked potential and duration of the silent period did not differ for men and women during the fatiguing task. However, estimated resting twitch amplitude and the peak rates of muscle relaxation showed greater relative reductions at the end of the fatiguing task for the men than the women. These results indicate that the sex difference in fatigue of the elbow flexor muscles is not explained by a difference in supraspinal fatigue in men and women but is largely due to a sex difference of mechanisms located within the elbow flexor muscles.

Decreases in motor unit firing rate during sustained maximal-effort contractions in young and older adults

Previous studies have suggested that older adults may be more resistant to muscular fatigue than young adults. We sought to determine whether motor unit firing rate might be a factor that determines the response to fatiguing exercise in young and older subjects. Motor unit recordings and muscular forces were obtained from the tibialis anterior (TA) muscle of 11 young and 8 older individuals. Maximal voluntary force was first measured during maximal-effort dorsiflexion contractions. Each subject then performed a series of 15 maximal isometric contractions, with each contraction lasting 30s. A 10-s rest period separated the fatiguing contractions. As a result of the fatiguing exercise, both subject groups demonstrated a significant loss in maximal force. The force decline was less in the older adults (20.4%) than in the young adults (33.8%). As expected, prior to muscle fatigue, maximal firing rates in the TA muscle were greater in the young (28.1+/-5.8 imp/s) than in the older adults (22.3+/-4.8 imp/s). The decrease in motor unit firing rate with fatigue was also greater in the young adults (34.9%), than in the older adults (22.0%). These results suggest that the greater fatigue-resistance exhibited by older individuals might be explained by the fact that the decline in motor unit firing rate during fatigue is greater in young persons than it is in older adults.

Central fatigue in continuous and intermittent contractions of triceps brachii

Conflicting results have been found across studies concerning the effect of rest periods on the development of central fatigue during prolonged muscle activity. Thus, the aim of the present study was to assess differences in the development of central fatigue between continuous and intermittent elbow extension fatigue tasks in the same subjects. Force and electromyographic data were collected on eight healthy volunteers. The ability to maximally activate the triceps brachii muscle was assessed by delivering trains of electrical stimulation during maximal voluntary efforts. This was done before, during, and after three fatigue tasks involving a maximal contraction in elbow extension. One short-duration ( approximately 55-s) and two long-duration (3-min) fatigue tasks were performed by all subjects on separate sessions. One 3-min task was intermittent (5-s rests every 30 s) and the other was continuous. The main findings were that the development and extent of central fatigue were task-dependent, with a greater decrease in the ability to maximally activate triceps brachii observed for the 3-min continuous task. Also, the voluntary activation (VA) ratio was found to be a more sensitive index of central fatigue than the central activation ratio (CAR). These results suggest that, when assessing central fatigue in patients, conclusions may vary depending on the continuous/intermittent nature of the task performed and the estimate of voluntary activation used.

Influence of time of day and partial sleep loss on muscle strength in eumenorrheic females

Disrupted sleep is the most common form of sleep deprivation in travellers, shift workers, athletes the night before important competitions and among parents of infants. The influence of partial sleep loss on muscle strength might differ according to the time of testing on the following day. This study was therefore designed to assess the interaction between the effects of partial sleep loss and time of day on muscle strength in females. Eight sedentary eumenorrheic females (mean +/- SD; age 30 +/- 6 years, height 1.62 +/- 0.06 m and body mass 67 +/- 5.0 kg) took part in the study, in a counterbalanced design. Measurements of muscle strength were carried out at 06:00 and 18:00 hours after the one control night (no sleep loss) and the one night of partial sleep loss, during menses. Muscle strength measures included isokinetic (at 1.05, 3.14 rad s(-1); 90 degrees range of motion) and isometric peak torque (at 60 degrees of knee flexion) of knee extensors and flexors (dominant leg). In addition, isometric force of knee extensors with super-imposed electrical twitches (50 Hz, 250 V, 200 mus pulse width) was measured using the same procedure in order to control for motivation. Rectal temperature was measured during the 30 min before muscle strength measurements. Partial sleep loss consisted of allowing 2.5 h sleep (between 03:00 and 05:30 h), whilst in the control condition (no sleep loss) subjects retired between 22:30 and 23:30 h, rising at 05:30 hours. All measurements were conducted at just one phase of the menstrual cycle (menses) to prevent any masking effect due to different phases of the menstrual cycle. In both conditions (with and without partial sleep loss) a diurnal variation was observed in peak torque of knee flexors at 1.05 (F(1,7) = 5.5, p < 0.05) and 3.14 rad s(-1) (F(1,7) = 8.0, p < 0.05); values at 18:00 hours were 4.5 and 5.9% higher than at 06:00 hours, respectively. No significant diurnal variation was observed for the other muscle strength measures. No significant effect of partial sleep loss or interaction effect (sleep x time of day) was observed for muscle strength measures. However, the performance rhythms were in phase with the circadian rhythm in rectal temperature. Partial sleep deprivation over one night did not have any adverse effect on maximal muscle strength, nor on diurnal variations of muscle strength indices. As the effect of time of day was observed with some of the muscle strength measures, it is suggested that, in designing future studies using females, the control of time of day is essential.

Time to task failure differs with load type when old adults perform a submaximal fatiguing contraction

Young adults exhibit a longer time to task failure when performing a submaximal isometric contraction by pushing against a force transducer (force task) than when supporting an equivalent inertial load (position task). The purpose of this study was to compare the time to failure for old adults when they performed a force task and a position task with the elbow flexor muscles. Eighteen old adults (72 +/- 4 years) performed the force and position tasks at 20% maximal voluntary contraction (MVC) force until task failure. The time to task failure was briefer for the position task (10.6 +/- 6.1 min) than the force task (22.8 +/- 9.1 min, P < 0.05). The rate of increase in electromyographic (EMG) bursting activity, ratings of perceived exertion, mean arterial pressure, heart rate, and fluctuations in motor output during the fatiguing contraction were greater for the position task. However, the increase in averaged EMG for the elbow flexor muscles was greater at termination of the force task. The difference in time to failure for the two tasks was due to a higher level of central neural activity during the position task and was similar to that observed for young adults. These findings indicate that the type of load supported influences the mechanisms and time to task failure for sustained contractions in old adults, and have implications for the design of tasks for rehabilitation and for tasks that minimize fatigue.

Muscle endurance is greater for old men compared with strength-matched young men

The purpose was to compare the time to task failure for a sustained isometric contraction performed at a submaximal intensity with the elbow flexor muscles by young and old men who were matched for strength. Eight young men (18-31 yr) and eight old men (67-76 yr) sustained an isometric contraction at 20% of maximal voluntary contraction (MVC) torque until the target torque could no longer be achieved for at least 5 s. The maximal torque exerted at the wrist was similar for the young and old men before the fatiguing task (65.9 +/- 8.0 vs. 65.4 +/- 8.7 N x m; P > 0.05), and they experienced similar reductions in MVC torque after the fatiguing contraction (31.4 +/- 10.6%; P < 0.05). The time to task failure was longer for the old men (22.6 +/- 7.4 min) compared with the strength-matched young men (13.0 +/- 5.2 min; P < 0.05), despite each group sustaining a similar torque during the fatiguing contraction (P > 0.05). The increases in torque fluctuations, electromyographic (EMG) bursting activity, and heart rate were greater for young men compared with the old men, and they were less at task failure for the old men (P < 0.05). Mean arterial pressure increased at a similar rate for both groups of men (P > 0.05), whereas the averaged EMG activity and rating of perceived exertion reached similar values at task failure for the young and old men (P > 0.05). These findings indicate that the longer time to task failure for the old men when performing the submaximal contraction was not due the absolute target torque exerted during the contraction.

Skeletal muscle fatigue, strength, and quality in the elderly: the Health ABC Study

We examined the muscle fatigue characteristics in older men and women and determined whether these were related to the size, strength, or quality of muscle. A total of 1,512 men and women aged 70-79 yr from the Health, Aging, and Body Composition Study participated in this study. Muscle cross-sectional area and attenuation were determined with computed tomography. Skeletal muscle fatigue and strength (peak torque) of the knee extensors and flexors were measured using isokinetic dynamometry. Men were more fatigue resistant than women for both knee extension (fatigue index: 70.4 +/- 15.3 vs. 66.9 +/- 14.3%; P < 0.05) and knee flexion (67.9 +/- 16.4 vs. 64.9 +/- 17.6%; P < 0.05). Peak torque and muscle quality (specific torque) were higher in men than women for knee extension (99.6 +/- 28.2 vs. 63.0 +/- 16.8 N x m and 1.62 +/- 0.43 vs. 1.51 +/- 0.39 N x m/cm2; both P < 0.05) and for knee flexion (74.0 +/- 26.4 vs. 49.6 +/- 15.9 N x m and 2.47 +/- 1.29 vs. 2.22 +/- 0.78 N x m/cm2; both P < 0.05). Total work and power output was greater in men compared with women for both the quadriceps (1,353 +/- 451 vs. 832 +/- 264 J and 87.7 +/- 33.5 vs. 53.3 +/- 19.2 W; both P < 0.05) and the hamstrings (741 +/- 244 vs. 510 +/- 141 J and 35.4 +/- 16.0 vs. 23.7 +/- 10.2 W; both P < 0.05). In both genders, the quadriceps was able to perform more work with greater power compared with the hamstrings. Those who were stronger actually had greater fatigue after adjusting for age, race, physical activity, and total body fat. In conclusion, older men were more fatigue resistant than women, although in both men and women greater fatigue was not related to muscle weakness.

Effects of aging on muscle fibre type and size

Aging has been associated with a loss of muscle mass that is referred to as 'sarcopenia'. This decrease in muscle tissue begins around the age of 50 years, but becomes more dramatic beyond the 60th year of life. Loss of muscle mass among the aged directly results in diminished muscle function. Decreased strength and power contribute to the high incidence of accidental falls observed among the elderly and can compromise quality of life. Moreover, sarcopenia has been linked to several chronic afflictions that are common among the aged, including osteoporosis, insulin resistance and arthritis. Loss of muscle fibre number is the principal cause of sarcopenia, although fibre atrophy--particularly among type II fibres--is also involved. Several physiological mechanisms have been implicated in the development of sarcopenia. Denervation results in the loss of motor units and thus, muscle fibres. A decrease in the production of anabolic hormones such as testosterone, growth hormone and insulin-like growth factor-1 impairs the capacity of skeletal muscle to incorporate amino acids and synthesise proteins. An increase in the release of catabolic agents, specifically interleukin-6, amplifies the rate of muscle wasting among the elderly. Given the demographic trends evident in most western societies, i.e. increased number of those considered aged, management interventions for sarcopenia must become a major goal of the healthcare profession.

Differences in the force/endurance relationship between young and older men

The aim of the present study was to ascertain if in six young (23-35 years) and in six older (70-72 years) healthy men matched for comparable absolute and specific maximal force of the dominant elbow flexors, differences in isometric endurance, myoelectrical fatigability, and shortening velocity are still recognizable. To assess the specific force, the muscle cross sectional area (CSA) was determined from magnetic resonance imaging (MRI) scans. The performance of the elbow flexors was studied by assessing the isometric endurance times (ET) at different percentages of maximal isometric contraction (MVC), the average muscle fibre conduction velocity of action potentials (CV), and the median frequency (MDF) of the surface electromyogram (sEMG) of the biceps brachii. Finally, the torque-velocity curve was assessed by means of maximal isokinetic contractions at six fixed angular velocities. All data were expressed as the mean (SD). The results showed that: (1) the ET was longer in the older subjects at the highest levels of isometric contraction, independently from the absolute force; (2) the modifications of muscle fibre CV during isometric effort progressed less rapidly in the older than the younger groups, as did those of MDF; and (3) at the same angular velocity, the older subjects exerted less absolute force than the younger subjects. These results suggest an impairment of the neuromuscular system of older men, which is less powerful and less fatigable than that of young men.

Differences between young and older women in maximal force, force fluctuations, and surface emg during isometric knee extension and elbow flexion

The loss of muscle strength with aging appears to be greater in the lower than upper limbs, but strength and its neural control have never been compared in the same population of individuals in both upper and lower limbs. The aim of this study was to investigate differences between eight young (20-31 years) and eight older (68-76 years) healthy women in maximal voluntary contraction (MVC), force fluctuations, median frequency (MDF) of the surface electromyogram (sEMG), and muscle fiber conduction velocity (MFCV) during sustained isometric elbow flexion (EF) and knee extension (KE), performed at moderate to high force intensity. Older women showed larger fluctuations of force with endurance and changes in sEMG pointing to less fatigue, especially at high level of force, with no differences between upper and lower extremities. This may have significant implications in the design of rehabilitation programs directed to this population.

Influence of aging on sex differences in muscle fatigability

The purpose of this study was to compare time to task failure for a sustained isometric contraction performed at a submaximal intensity with elbow flexor muscles by young and old men and women. Twenty-seven young (14 men and 13 women, 18-35 yr) and 18 old (10 men and 8 women, 65-80 yr) adults sustained an isometric contraction at 20% of maximal voluntary contraction torque until target torque could no longer be achieved for > or = 5 s. Young adults were stronger than old adults (66.8 +/- 17.9 vs. 47.7 +/- 18.1 N x m, P < 0.05), and men were stronger than women (69.8 +/- 17.9 vs. 47.1 +/- 15.3 N x m, P < 0.05), with no interaction between age and sex (P > 0.05). Time to task failure was longer for old than for young adults (22.8 +/- 9.1 vs. 14.4 +/- 7.6 min, P < 0.05) and for young women than for young men (18.3 +/- 8.0 vs. 10.8 +/- 5.2, P < 0.05), but there was no difference between old women and men (21.3 +/- 10.7 and 24.1 +/- 8.0 min, respectively, P > 0.05) or between young women and old adults (P > 0.05). Mean arterial pressure, heart rate, average electromyographic (EMG) activity, and torque fluctuations of elbow flexor muscles increased during the fatiguing contraction (P < 0.05) for all subjects. Rates of increase in mean arterial pressure, heart rate, and torque fluctuations were greater for young men and old adults, with no differences between old men and women (P > 0.05). Similarly, the rate of increase in EMG activity was greater for young men than for the other three groups. EMG bursts were less frequent for old adults (P < 0.05) at the end of the fatiguing contraction, and this was accompanied by reduced fluctuations in torque. Consequently, time to task failure was related to target torque for young, but not old, adults, and differences in task duration were accompanied by parallel changes in the pressor response.

Effect of aging on human muscle architecture

The effect of aging on human gastrocnemius medialis (GM) muscle architecture was evaluated by comparing morphometric measurements on 14 young (aged 27-42 yr) and on 16 older (aged 70-81 yr) physically active men, matched for height, body mass, and physical activity. GM muscle anatomic cross-sectional area (ACSA) and volume (Vol) were measured by computerized tomography, and GM fascicle length (Lf) and pennation angle (theta) were assessed by ultrasonography. GM physiological cross-sectional area (PCSA) was calculated as the ratio of Vol/Lf. In the elderly, ACSA and Vol were, respectively, 19.1% (P < 0.005) and 25.4% (P < 0.001) smaller than in the young adults. Also, Lf and were found to be smaller in the elderly group by 10.2% (P < 0.01) and 13.2% (P < 0.01), respectively. When the data for the young and elderly adults were pooled together, significantly correlated with ACSA (P < 0.05). Because of the reduced Vol and Lf in the elderly group, the resulting PCSA was found to be 15.2% (P < 0.05) smaller. In conclusion, this study demonstrates that aging significantly affects human skeletal muscle architecture. These structural alterations are expected to have implications for muscle function in old age.

Muscle strength, power and adaptations to resistance training in older people

Muscle strength and, to a greater extent, power inexorably decline with ageing. Quantitative loss of muscle mass, referred to as "sarcopenia", is the most important factor underlying this phenomenon. However, qualitative changes of muscle fibres and tendons, such as selective atrophy of fast-twitch fibres and reduced tendon stiffness, and neural changes, such as lower activation of the agonist muscles and higher coactivation of the antagonist muscles, also account for the age-related decline in muscle function. The selective atrophy of fast-twitch fibres has been ascribed to the progressive loss of motoneurons in the spinal cord with initial denervation of fast-twitch fibres, which is often accompanied by reinnervation of these fibres by axonal sprouting from adjacent slow-twitch motor units (MUs). In addition, single fibres of older muscles containing myosin heavy chains of both type I and II show lower tension and shortening velocity with respect to the fibres of young muscles. Changes in central activation capacity are still controversial. At the peripheral level, the rate of decline in parameters of the surface-electromyogram power spectrum and in the action-potential conduction velocity has been shown to be lower in older muscle. Therefore, the older muscle seems to be more resistant to isometric fatigue (fatigue-paradox), which can be ascribed to the selective atrophy of fast-twitch fibres, slowing in the contractile properties and lower MU firing rates. Finally, specific training programmes can dramatically improve the muscle strength, power and functional abilities of older individuals, which will be examined in the second part of this review.

Perceived exertion is elevated in old age during an isometric fatigue task

The purposes of this study were to compare the ratings of perceived exertion (RPE) of young [25 (1) years] and old men [84 (1) years] during intermittent (3 s on, 2 s off) voluntary isometric contractions of the elbow flexors at 60% of each subject's maximal voluntary contraction (MVC) force, and to determine whether potential differences in RPE were accompanied by altered neuromuscular parameters during fatigue. Subjects performed the fatigue protocol on two different visits. All subjects in both groups reported a maximal (10) perceived exertion using a modified Borg-scale at fatigue, but the old men reported a greater perceived exertion compared to the young men at an initial stage of the protocol during the two visits (P<0.05). The amount of prefatigue MVC force remaining at fatigue was not different between groups (approximately 57%), and voluntary activation, as assessed with twitch interpolation, was not different from the prefatigue value for either group throughout the fatigue protocol (>95%), and did not show any age-related difference. Moreover, there was no significant difference in 60% target electromyography between groups at any time point during the fatigue protocol, and there was no significant effect of age on the decline in 50-Hz tetanic force. Thus, the old men were able to exert themselves to the same relative degree of muscle fatigue as the young men, and despite an elevated perceived exertion compared to the young men during initial stages of the protocol, they did not terminate the protocol prematurely, or demonstrate an inability to sustain a high level of voluntary activation.

Assessment of voluntary activation by stimulation of one muscle or two synergistic muscles

In order to test the hypothesis that suboptimal activation of synergistic muscles explains the nonlinear nature of the interpolated twitch-voluntary torque relationship, we evaluated differences between stimulating a single muscle [biceps brachii (BB)] and two synergists simultaneously [BB and brachioradialis (BR)] on variables related to the estimation of voluntary activation. Ten subjects performed maximal and submaximal voluntary contractions while electrical stimulation was applied either to the BB, or BB and BR simultaneously, to assess voluntary activation. Simultaneous stimulation of the two synergistic muscles produced significantly greater torque at rest and a significantly greater activation index (95% vs. 91.7% for stimulation of BB only). However, simultaneous stimulation of BB and BR did not consistently elicit greater additional torque during contraction, nor did it improve the linearity of the interpolated twitch-voluntary torque relationship. The greater activation index observed for the simultaneous stimulation of BB and BR compared to BB is explained by the greater torque elicited at rest. We conclude that the nonlinear relationship between interpolated and voluntary elbow flexion torque is not explained by suboptimally activated synergistic muscles.

Human skeletal muscle responses vary with age and gender during fatigue due to incremental isometric exercise

The purpose of this study was to compare the magnitude and mechanisms of ankle dorsiflexor muscle fatigue in 20 young (33 +/- 6 yr, mean +/- SD) and 21 older (75 +/- 6 yr) healthy men and women of similar physical activity status. Noninvasive measures of central and peripheral (neuromuscular junction, sarcolemma) muscle activation, muscle contractile function, and intramuscular energy metabolism were made before, during, and after incremental isometric exercise. Older subjects fatigued less than young (P < 0.01); there was no effect of gender on fatigue (P = 0.24). For all subjects combined, fatigue was modestly related to preexercise strength (r = 0.49, P < 0.01). Neither central (central activation ratio) nor peripheral (compound muscle action potential) activation played a significant role in fatigue in any group. During exercise, intracellular concentrations of P(i) and H(2)PO increased more and pH fell more in young compared with older subjects (P < 0.01) and in men compared with women (P < 0.01). These varied metabolic responses to exercise suggest a greater reliance on nonoxidative sources of ATP in young compared with older subjects and in men compared with women. These results suggest that the mechanisms of fatigue vary with age and gender, regardless of whether differences in the magnitude of fatigue are observed.

Voluntary muscle activation varies with age and muscle group

The consistency and the number of attempts required to achieve maximal voluntary muscle activation have not been documented and compared between young and old adults. Furthermore, few studies have contrasted activation between functional pairs of muscle groups, and no study has tested upper limb muscles. The purpose of this study was to measure and compare voluntary muscle activation of the elbow flexors and extensors in young and old men over two separate test sessions. With the method of twitch interpolation to measure activation, six young (24 +/- 1 yr) and six old (83 +/- 4 yr) men performed five maximal voluntary contractions (MVC) during each session for each muscle group. Elbow flexion and extension MVC was less (43 and 47%, respectively) in the old men, yet the best maximal voluntary muscle activation was similar between age groups. However, when all 10 attempts at MVC were compared, the mean activation scores were slightly less (approximately 5%) in the elbow extensors but were approximately 11% less (P < 0.001) in the elbow flexors of old men, compared with young men. During the second session, there was a significant improvement of 13% (P < 0.005) in mean elbow flexor activation in the old men. There were no session differences for either muscle group for the young men. The results indicate that, for aged men, elbow flexor maximal activation is achieved less frequently compared with elbow extensors, and thus mean activation for elbow flexors is less than for elbow extensors. However, if sufficient attempts are provided, the best effort for the old men is not different from that of the young men for either muscle group.

Age-related EMG variables during maximum voluntary contraction

We studied the neuromuscular adaptation that occurs with aging by comparing changes in surface electromyography (EMG) variables from the tibialis anterior muscle in 12 young (21.4 +/- 1.7 yr. old) and 13 older subjects (70.8 +/- 3.1 yr. old). EMG variables such as the muscle fiber conduction velocity, median frequency, and averaged rectified value were calculated during maximum voluntary contraction for 5-sec. isometric contractions. The dorsiflexion force, muscle fiber conduction velocity, median frequency, and average rectified value during maximum voluntary contraction were significantly smaller in the older than in the younger group (p < .05). These results suggested that the neuromuscular system in older subjects is affected by the selective atrophy of fast twitch fibers and differences in motor unit firing statistics. Our results suggest the utility of applying the EMG observed during maximum voluntary contraction to the noninvasive evaluation of neuromuscular function in elderly persons.

Neuromuscular fatigue and aging: central and peripheral factors

A limited number of studies have investigated the effect of old age on neuromuscular fatigue, yet a variety of protocols have been used to compare the fatigability of old and young humans. These include voluntary isometric and isokinetic contraction protocols at maximal and submaximal intensities, and electrical stimulation protocols of continuous or intermittent stimulation at a variety of stimulation frequencies. The results of these studies are summarized in this review. Although it seems reasonable to suggest that age-related changes in muscle morphology and motor unit remodeling, as well as the associated loss of strength and slowed contractile properties, may improve the resistance to neuromuscular fatigue in old humans, the collective results suggest that it is not possible to make this generalization. In fact, it cannot be generalized that the muscles of old humans are either more or less fatigable than young adults because the extent of the difference in fatigability relies strongly on the fatigue task performed (task-dependency). Age-related changes that occur within the neuromuscular system may result in some candidate fatigue sites increasing or decreasing their susceptibility to failure under specific task conditions. These candidate fatigue sites include central drive, muscle membrane excitability, excitation-contraction coupling mechanisms, and metabolic capacities. The effect of old age on these various central and peripheral sites is discussed with respect to their relative contribution during different fatigue tasks. Moreover, the impact of the possible confounding effects of subject habituation, physical activity status, and sex on the fatigability comparison is addressed.

Resistance training alters torque-velocity relation of elbow flexors in elderly men

PURPOSE

The aim of the study was to compare in vivo dynamic muscle characteristics (torque-velocity relation and maximal contraction velocity) of sedentary young subjects with elderly men before and after training.

METHODS

Elbow flexion maximal isometric (Tmax) and dynamic torque at velocities between 100 degrees.s-1 and 600 degrees.s-1 were measured, from which maximal contraction velocity was estimated (eVmax). These parameters were obtained from 18 young sedentary subjects (Y) and from 23 healthy sedentary elderly (mean age 63 yr) men (E) before (PRE), after 13 wk (MID), and 26 wk (POST) of moderate intensity resistance training (30RM) of the elbow flexor muscles.

RESULTS

Y and E before training had similar Tmax, but mean dynamic torque over all velocities was 49.1% higher in Y compared with E. After 13 wk of training TmaxE increased by 8.2% and mean dynamic torque increased by 61.2%, with a velocity dependent gain. For eVmaxE an increase of 21.8% was observed. The second 13-wk training period did not induce further increases.

CONCLUSION

The results of the present study demonstrate that, after moderate intensity resistance training of healthy elderly men, maximal contraction velocity of elbow flexors (eVmax), estimated from the torque-velocity curve, increased to values observed for sedentary young men.

Effect of age on muscle functions investigated with surface electromyography

The purpose of this study was to investigate changes in the surface electromyographic (EMG) signal as a means of defining age-related central and peripheral mechanisms affecting muscle fatigue. Spectral and temporal variables of the surface EMG signal were studied during voluntary isometric contractions of the dominant biceps brachii muscle in a group of 8 healthy elderly men (age range 67-86 years) and a group of 10 healthy young men (age range 23-34 years). The maximal torque developed and the rate of decrease (slope) of spectral variables and conduction velocity (CV) were statistically higher in the young subjects than in the elderly subjects. Motor unit (MU) CV distribution was also estimated from the surface EMG signal and no statistical difference was observed in its variance between the two groups. These results confirm previous findings from the tibialis anterior muscle. Thus, changes in fiber type distribution and decrease in MU firing rate with aging may be factors determining the decrease in maximal voluntary contraction torque and in myoelectric manifestations of muscle fatigue.

Aging of the human neuromuscular system

Loss of cells from the motor system occurs during the normal aging process, leading to reduction in the complement of motor neurons and muscle fibers. The latter age-related decrease in muscle mass has been termed "sarcopenia" and is often combined with the detrimental effects of a sedentary lifestyle in older adults, leading to a significant reduction in reserve capacity of the neuromuscular system, which is the primary subject of this review. Clear evidence of this aging effect is seen when voluntary or stimulated muscle strength is compared across the adult lifespan, with a steady decline of approximately 1-2% per year occurring after the sixth decade. Interestingly, when compared with isometric contractions, the effect of aging is more pronounced for concentric movements and less for eccentric movements (i.e., muscle shortening versus lengthening). This phenomenon appears to be linked to the stiffer muscle structures and prolonged myosin crossbridge cycles of aged muscles. It is encouraging that the capability of physiological adaptations in the motor pathways remains into very old age--when an appropriate exercise stimulus is given--and long-term prevention strategies are advocated to avoid excessive physical impairments and activity restrictions in this age group.

Effect of aging on fatigue characteristics of elbow flexor muscles during sustained submaximal contraction

The purpose of this study was to compare fatigue-related measures of central and peripheral mechanisms between young and elderly subjects for a task performed with elbow flexor muscles. Ten young and nine elderly subjects performed a sustained submaximal fatigue task at 35% of their maximum voluntary contraction torque. Measures of neuromuscular function, reflecting changes in neuromuscular propagation, voluntary activation, excitation-contraction-relaxation processes, and metabolite buildup, were taken before, during, and after the fatigue task. The main results were the absence of neuromuscular propagation failure in either young or elderly subjects, the presence of central fatigue at the end of the fatigue task in 7 of 9 elderly but only 3 of 10 young subjects, and lesser changes in twitch torque contraction-relaxation variables and electromyographic median frequency in elderly compared with young subjects. The lesser fatigue-related changes in twitch contraction speed and median frequency in elderly compared with young subjects could reflect the increase in type I-to-type II fiber area reported with old age. The presence of significant central fatigue can apparently minimize some of the potential differences present in peripheral fatigue sites.

Incomplete recovery of voluntary isometric force after fatigue is not affected by old age

The 60-min recovery profiles of voluntary and electrically stimulated force, contractile speed, surface electromyography, muscle activation via twitch interpolation, and muscle compound action potentials (M-waves) in the elbow flexors of seven young men (24 +/- 2 years) and seven men over 80 years of age (84 +/- 2 years) were compared following intermittent (3 s on, 2 s off) contractions at 60% of each subject's maximum voluntary contraction (MVC) force. There was no age-related difference between groups in the average time to fatigue or the rate of voluntary force loss; both groups lost 40% of their force within approximately 5 min. Despite a rapid increase to approximately 83% of the prefatigue MVC by the third minute of recovery for both groups, MVC force did not return to the prefatigue value within 60 min (94 +/- 4% young, 91 +/- 3% old). These results suggest that the incomplete recovery of voluntary force was likely due to a peripheral limitation in the muscle at the level of excitation--contraction coupling, and was not affected by age. Delayed recovery of voluntary force and a greater degree of low-frequency fatigue in the old men were not observed and there were no age-related impairments in any parameter normalized to the prefatigue value during fatigue or recovery. We suggest that the specific fatigue task may be more important to recovery than proposed alterations in the aged neuromuscular system when normalization and matching of the fatigue task criteria occurs.