Age-related changes in motor unit function

Invited review: Aging and sarcopenia

Aging is associated with progressive loss of neuromuscular function that often leads to progressive disability and loss of independence. The term sarcopenia is now commonly used to describe the loss of skeletal muscle mass and strength that occurs in concert with biological aging. By the seventh and eighth decade of life, maximal voluntary contractile strength is decreased, on average, by 20-40% for both men and women in proximal and distal muscles. Although age-associated decreases in strength per unit muscle mass, or muscle quality, may play a role, the majority of strength loss can be accounted for by decreased muscle mass. Multiple factors lead to the development of sarcopenia and the associated impact on function. Loss of skeletal muscle fibers secondary to decreased numbers of motoneurons appears to be a major contributing influence, but other factors, including decreased physical activity, altered hormonal status, decreased total caloric and protein intake, inflammatory mediators, and factors leading to altered protein synthesis, must also be considered. The prevalence of sarcopenia, which may be as high as 30% for those >/=60 yr, will increase as the percentage of the very old continues to grow in our populations. The link between sarcopenia and disability among elderly men and women highlights the need for continued research into the development of the most effective interventions to prevent or at least partially reverse sarcopenia, including the role of resistance exercise and other novel pharmacological and nutritional interventions.

Morphological characteristics of skeletal muscles in relation to gender

BACKGROUND AND AIMS

The aim of this study was to ascertain whether there are gender-related differences in the morphological characteristics of the soleus and tibialis anterior muscles in young adult and old Fischer 344/Brown Norway F1 rats.

METHODS

We tested 1) whether there was a gender-related difference between the fiber type composition of these muscles, and 2) whether the cross-sectional area of individual muscle fibers demonstrated gender-associated differences, fibers from males being larger than fibers from females.

RESULTS

Gender differences were not found in the fiber type composition of the soleus and tibialis anterior muscles, but were present in the single skeletal fiber cross-sectional area of the tibialis anterior muscle. The cross-sectional area of type I fibers in females was greater than that in males at both 12 (16%) and 30 (5%) months of age. In contrast, the cross-sectional area of type Ila fibers of 12-month-old males was larger than that of 12-month-old females. No significant differences between genders were found for the cross-sectional area of type Ilb fibers in either age group. In the soleus muscle, 30-month-old males had larger single fiber cross-sectional areas of both fiber types I and lIa. At 12 months of age, type I fibers from females were larger than those from males.

CONCLUSIONS

Our findings indicate that gender-related differences exist in the size of individual skeletal fibers from the soleus and tibialis anterior muscles and that they may influence metabolism and the adaptive response to rehabilitation programs.

Effect of ageing on the electrical and mechanical properties of human soleus motor units activated by the H reflex and M wave

This study was designed to investigate the effect of ageing on the mechanical and electromyographic (EMG) characteristics of the soleus motor units (MUs) activated by the maximal Hoffmann reflex (Hmax) and by the direct muscle compound action potential (Mmax). Eleven young (mean age 25 +/- 4 years) and ten elderly (mean age 73 +/- 5 years) males took part in this investigation. The senior group presented lower amplitudes of Mmax (-57 %, P < 0.001) and Hmax (-68 %, P < 0.001) waves compared to the younger population. These were associated with a depression of relative twitch torque of the plantar flexors. The average values of the Hmax/Mmax ratio did not statistically differ between the two populations, despite a tendency for lower values (~23 %) in the senior group. However, the older adults showed a greater relative amplitude of the sub-maximal M wave evoked at Hmax (MatHmax) than did the younger males (young 5 % vs. elderly 29 % of the Mmax, P < 0.01). This finding suggests an increased homogeneity between the excitability threshold of sensory and motor axons. The twitch torque at Hmax (PtH-M) was subsequently calculated by subtraction from the total twitch torque of the mechanical contamination associated with MatHmax. The resulting PtH-M was significantly lower in the elderly (-59 %, P < 0.001). Despite a discrepancy of 20 % between the two groups, the mechanical ratio (PtH-M/PtM; PtM, twitch tension related to the Mmax compound action potential), like the EMG ratio, did not statistically differ between the young and older individuals. Nevertheless, the senior subjects exhibited a higher twitch/EMG ratio for the reflexively activated MUs (PtH-M/Hmax) than the younger individuals (+40 %, P < 0.05). This finding suggests an on-going neuromuscular remodelling, resulting in an increased innervation ratio. The neural rearrangement may be viewed as a compensatory adaptation of the motor system to preserve the mechanical efficiency of the surviving MUs, despite the age-related impairment of the segmental reflex system. This phenomenon is confirmed by the maintenance, with senescence, of the approximately constant values of the twitch/EMG ratio for the entire motor pool (PtM/Mmax).

Changes in motor unit discharge rate are not associated with the amount of twitch potentiation in old men

This study examined, in nine old men (82 +/- 4 yr), whether there is an association between the magnitude of change in motor unit discharge rate and the amount of twitch potentiation after a conditioning contraction (CC). The evoked twitch force and motor unit discharge rate during isometric ramp-and-hold contractions (10-18 s) of the triceps brachii muscle at 10, 20, and 30% of the maximal voluntary contraction were determined before and 10 s, 2 min, 6 min, and 11 min after a 5-s CC at 75% maximal voluntary contraction. After the CC, there was a potentiation of twitch force (approximately twofold), and the discharge rate of the 47 sampled motor units declined (P < 0.05) an average of 1 Hz 10 s after the CC, compared with the control condition. The CC had no effect on the variability (coefficient of variation) of both force and discharge rate, as well as the electromyographic activity recorded over the triceps brachii and biceps brachii muscles. In contrast to our earlier study of young men (Klein CS, Ivanova TD, Rice CL, and Garland SJ, Neurosci Lett 316: 153-156, 2001), the magnitude of the reduction in discharge rate after the CC was not associated (r = 0.06) with the amount of twitch potentiation. These findings suggest an age-related alteration in the neural strategy for adjusting motor output to a muscle after a CC.

Age and contraction type influence motor output variability in rapid discrete tasks

The purpose of this study was to examine the ability to control knee-extension force during discrete isometric (IC), concentric (CC), and eccentric contractions (EC) in 24 young (mean age +/- SD = 25.3 +/- 2.8 yr) and 24 old (mean age +/- SD = 73.3 +/- 5.5 yr) healthy and active individuals. Subjects were to match a parabola with a time to peak force of 200 ms during IC, CC, and EC at six target levels of force [20, 35, 50, 65, 80, and 90% of the maximum voluntary contraction (MVC)]. ICs were performed at 90 degrees of knee flexion, whereas CCs and ECs ranged from 90 to 80 degrees of knee flexion (0 degrees is full extension) at a slow velocity (25 degrees /s). Results showed that subjects produced similar MVC forces for the three types of contractions. Young subjects produced greater MVC forces than old subjects, and within each age group, men produced greater force than women. The variability (standard deviation) of peak force and impulse in absolute values was greater for young compared with old subjects. When variability was normalized to the force produced [coefficient of variation (CV)], however, old subjects exhibited greater CV than young subjects for peak force and impulse. Both the standard deviation and CV of time to peak force and impulse duration were greater for the old adults. In general, ECs were more variable than ICs and CCs, and old adults exhibited greater CV compared with young adults during rapid, discrete ICs, CCs, and particularly ECs of the quadriceps.

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.

Plantar flexor activation capacity and H reflex in older adults: adaptations to strength training

The purpose of this study was to investigate whether the voluntary neural drive and the excitability of the reflex arc could be modulated by training, even in old age. To this aim, the effects of a 16-wk strengthening program on plantar flexor voluntary activation (VA) and on the maximum Hoffman reflex (H(max))-to-maximum M wave (M(max)) ratio were investigated in 14 elderly men (65-80 yr). After training, isometric maximum voluntary contraction (MVC) increased by 18% (P < 0.05) and weight-lifting ability by 24% (P < 0.001). Twitch contraction time decreased by 8% (P < 0.01), but no changes in half relaxation time and in peak twitch torque were observed. The VA, assessed by twitch interpolation, increased from 95 to 98% (P < 0.05). Pretraining VA, also evaluated from the expected MVC for total twitch occlusion, was 7% higher (P < 0.01) than MVC. This discrepancy persisted after training. The interpolated twitch torque-voluntary torque relationship was fitted by a nonlinear model and was found to deviate from linearity for torque levels >65% MVC. Compared with younger men (24-35 yr), the H(max)- to M(max) ratio and nerve conduction velocity (H index) of the older group were significantly lower (42%, P < 0.05; and 29%, P < 0.001, respectively) and were not modulated by training. In conclusion, older men seem to preserve a high VA of plantar flexors. However, the impaired functionality of the reflex pathway with aging and the lack of modulation with exercise suggest that the decrease in the H(max)- to M(max) ratio and H index may be related to degenerative phenomena.

Fibrillation potentials in paraspinal muscles in chronic lumbosacral radiculopathy

To assess the rate of fibrillation and/or positive sharp waves (FPSW) in the paraspinal muscles (PM) of patients with chronic lumbosacral radiculopathy, PM and leg muscles of 179 patients, 111 men and 68 women, aged 24-68 years (mean 49.0 +/- 9.3), were tested. Illness duration ranged from 8 months to 40 years (9.4 +/- 7.5 years). FPSW were detected in 38 patients (21.2%). In 28 of them (15.6%), FPSW were present in the limb muscles only, in eight (4.5%) in both PM and limb muscles, and in two (1.1%) solely in the PM. FPSW were accompanied in all the above patients by chronic neurogenic changes of motor unit potentials. Logistic regression analysis did not show any significant difference between: (i) the patients with and without fibrillations in leg and/or PM muscles, and (ii) between those with FPSW in both the PM and leg muscles and those with FPSW in the leg muscles only, when the effect of age and duration of illness were considered. These suggest that the paucity of FPSW in the PM is rather genuine and does not depend on the timing of the examination. In consequence, PM electromyography seems to be not as valuable in chronic radiculopathy as it is in the acute stage.

Age-related change in motor unit activation strategy in force production: a mechanomyographic investigation

This study was designed to examine the effect of increasing age on the recruitment and activation of motor units (MU) in the biceps brachii muscle, using the mechanomyogram (MMG)/force relationship during isometric ramp contractions. The relationships between the root mean squared amplitude (RMS) and mean power frequency (MPF) of the MMG and relative force (% MVC) in the elderly (male, n = 10, age = 69.8 +/- 4.7 years, mean +/- SD) were markedly different from those in the young group (male, n = 15, age = 22.7 +/- 1.8 years). In elderly individuals, the RMS increased progressively with force up to 57.6 +/- 3.4% MVC, when a brief rapid increase was followed by a stable trend beyond 63.6 +/- 3.7% MVC. The MPF increased slowly up to 59.4 +/- 2.3% MVC; after a temporary reduction from 59.4% to 64.3 +/- 2.0% MVC, it then increased progressively again. In conjunction with absolute force (F(abs)), both the RMS and MPF in the elderly were smaller than those in the young group throughout the submaximal levels of force exerted. The results reflect an alteration in MU activation strategy, with a predominant role for MUs with slow-twitch fibers and an effective fused tetanus induced at lower firing rate of the MUs, resulting from age-related neuromuscular changes.

Modeling variability of force during isometric contractions of the quadriceps femoris

The authors modeled variability of force during continuous isometric contractions of the quadriceps femoris. Twenty adults (aged 25 +/- 6 years old) performed isometric leg extensions. Target forces were 11 percentages of maximum voluntary contraction (%MVC), ranging from 2 to 95%, and 5 absolute levels, from 25 to 225 N. The authors used standard deviation of absolute force, coefficient of variation, and signal-to-noise ratio as measures of variability. The results suggested a nonlinear relationship between variability and level of force, which could best be expressed as %MVC and not as absolute force. Variability for continuous isometric contractions was described best by a sigmoidal logistic function. The sigmoidal pattern of variability as a function of %MVC is consistent with physiological mechanisms.

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.

The influence of aging and sex on skeletal muscle mass and strength

This brief review examines the influence of aging on skeletal muscle mass and strength and specifically highlights sex-related differences. It is well established that aging is associated with a significant decline in muscle strength that becomes functionally important by the seventh decade of life. Age-related strength losses are mainly secondary to decline in skeletal muscle mass in men and women. While women may experience earlier strength losses than men, overall, age associated decreases in strength are similar when controlling for muscle mass. Although men may experience greater losses of total muscle mass, recent evidence, however, points toward greater declines in muscle quality in older women. The implications and potential mechanisms for these differences are discussed.

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.

The use of electrical stimulation to increase quadriceps femoris muscle force in an elderly patient following a total knee arthroplasty

BACKGROUND AND PURPOSE

Persistent residual quadriceps femoris muscle force deficits after total knee arthroplasty (TKA) are commonly reported and can prevent patients from returning quickly and fully to functional activities. Neuromuscular electrical stimulation offers a potentially more effective means of increasing muscle force than current rehabilitation protocols.

CASE DESCRIPTION

The patient was a 66-year-old man. Neuromuscular electrical stimulation for increasing quadriceps femoris muscle force was initiated 3 weeks after TKA for 11 sessions to supplement stretching exercises and a high-intensity volitional strengthening program.

OUTCOME

The patient's isometric quadriceps femoris muscle force increased from 50% (involved/uninvolved) at 3 weeks after surgery to 86% at 8 weeks after surgery. A concurrent increase in his uninvolved quadriceps femoris muscle force concealed the patient's true increase in his involved quadriceps femoris muscle force in a side-to-side comparison. The patient's final involved quadriceps femoris muscle force (10 weeks after surgery) was 93% of the initial uninvolved quadriceps femoris muscle force.

DISCUSSION

Our patient was able to return to independent activities of daily living and recreational activities, with force gains that surpassed those reported in the literature.

Effects of Age on the Spinal Stretch Reflex

The spinal stretch reflex consists of a relatively simple neuronal network. The Ia afferent fiber of the muscle spindle communicates to the alpha motoneuron via a single synapse. This basic pathway has been studied extensively over the past century, yet considerable information continues to emerge concerning the manner in which this pathway adapts to aging. It is well accepted that the amplitude of the spinal stretch reflex declines with normal aging, and it is intuitively agreed that these changes have a detrimental impact on the motor output of aging individuals. Understanding the changes observed in the spinal stretch reflex pathway due to aging requires a recognition of the changes that can occur in each component of this spinal network. This review will address these changes by following the spinal stretch reflex from initiation to completion. The components that result in the sensory input to the motoneuron will be covered first, followed by a review of the physiological changes that can occur to the motoneuron soma that can affect the processing of the sensory input. The output of the motoneuron encompasses the remaining components from the motor axon itself, to the neuromuscular junction, and then to the characteristic changes in the muscle. Finally, the functional effect that these changes have on the reflex as a fundamental motor behavior will be addressed, especially in terms of its impact on posture and balance.

Age-dependent fatigue in single intact fast- and slow fibers from mouse EDL and soleus skeletal muscles

In the present work, we investigate age-dependent changes in isometric endurance in response to repetitive stimulation in single intact fast- and slow-twitch muscle fibers from young and old mice. To examine this issue we performed in vitro experiments in manually dissected EDL and soleus muscle fibers. We examined the force generation capacity of fibers in response to two stimulation protocols characterized by different inter-tetanic intervals, named short (1-s) and long interval (3.65-s). Fatigability was measured according to the fatigue index (FI, ratio between the maximum tension recorded in the last over the first tetanus in a train of pulses), the time course of the FI and sag (gradual decrease in force during a partially fused tetanic contraction). Fibers were classified according to the FI using two different criteria previously used in the literature (first criterion: FI > or = 1, 075-099, 0.5-074 and < 0.5; second criterion: FI > or = 1, 0.75-0.99, 0.25-0.74 and < 0.25). The fatigue index distribution recorded in the population of fibers corresponding to EDL and soleus muscles from young and old mice studied with the short and long interval protocols was not statistically different. In summary, these results support the concept that the decline in mechanical performance with aging is not related with changes in fatigability of individual fast- or slow twitch muscles fibers.

Characterization of the human quadriceps muscle in active elders

OBJECTIVE

To compare muscle activation deficits and muscle physiology in older versus younger adults.

DESIGN

A maximal volitional isometric contraction of the quadriceps muscle with burst-superimposition was used to assess strength and activation. In addition, force-frequency testing during fresh, fatigue, and recovery conditions and electrically elicited fatigue testing were performed.

SETTING

Muscle performance laboratory.

PATIENTS

Healthy, active young (age range, 20-28 yr) and older (age range, 66-83 yr) subjects.

MAIN OUTCOME MEASURES

Torque production, activation of the quadriceps, F50 values from the force-frequency relationships (frequency at which 50% of the maximum normalized force is produced), and the average amount of fatigue.

RESULTS

Older subjects were weaker (574.4 +/- 156 N) than younger subjects (900.9 +/- 295 N) and had significantly greater deficits in central activation in the quadriceps muscles (elderly = 95.5% activation; younger = 98.1% activation). The force-frequency curves for the elderly were to the left of the younger subjects for all 3 testing conditions. Aged muscles fatigued to the same extent as younger muscle (young = 49.8% +/- 2.6%, elderly = 51.1% +/- 2.8%).

CONCLUSIONS

These results can be used to modify high-intensity strength training protocols designed to optimize sustainable strength gains in the elderly during rehabilitation.

Myoelectric manifestations of muscle changes in stroke patients

OBJECTIVE

To evaluate the development of myoelectric fatigue in paretic and healthy tibialis anterior muscles of stroke patients.

DESIGN

Case series.

SETTING

Occupational therapy and clinical neurophysiology unit.

PARTICIPANTS

Eight patients with hemiparesis or hemiplegia 9 months to 10 years poststroke.

MAIN OUTCOME MEASURES

Current pulses of 0.1-ms width and 40-Hz repetition rate were applied for 10 seconds with a monopolar technique; myoelectric signals (M waves) were detected with surface electrodes.

RESULTS

Mean values and initial values of the median frequency (MDF) between paretic and healthy side were statistically different, with the values on the healthy side much higher than the paretic side. Changes of MDF showed a decreasing pattern for both the paretic and the healthy sides, with the downslope of the curve of the healthy side more evident.

CONCLUSIONS

In paretic muscles of stroke patients, the tendency toward atrophy of type II fibers appears to be frequent. Our study suggests this muscle rearrangement uses techniques much less invasive than muscle biopsy, and gives useful information about muscle function. This kind of information can help identify rehabilitation strategies, particularly for chronic stroke survivors.

Ageing is not associated with a decline in neuromuscular innervation or reduced specific force in men aged 20 and 50 years

The exact mechanisms responsible for the decline in strength with age are yet to be completely elucidated. Three proposed mechanisms responsible for the detrimental effect of increasing age on strength include changes in muscle mass, specific force and/or neuromuscular innervation. Thus, the purpose of this investigation was to determine if the age-related reduction in peak isometric strength was primarily associated with changes in muscle cross-sectional area, neuromuscular innervation and/or specific force. The cross-sectional area of the knee extensor muscles (QCSA) was estimated in 13 younger men (YM; 20.8 +/- 1.6 years) and eight middle-aged men (MM; 53.8 +/- 4.2 years) prior to performing a series of four maximal voluntary isometric contractions on an isokinetic dynamometer at an angle of 60 degrees knee flexion. Peak force was determined and surface electromyography was sampled from the rectus femoris muscle during each maximal voluntary contraction. The cross-sectional area of the knee extensor muscles, peak force and integrated electromyography (IEMG) were significantly lower in the MM (P<0.01). However, when peak force and peak IEMG values were corrected for QCSA, there were no significant differences between age groups. These results suggest that the reduction in peak isometric force observed in the MM was primarily associated with quantitative changes in muscle mass, rather than reduced neuromuscular innervation or specific force. Therefore, preserving muscle mass through resistance training may significantly reduce the age-associated differences in peak strength and assist in promoting quality of life and functional independence in older adults.

Age effects on the temporal evolution of isometric and swallowing pressure

BACKGROUND

The tongue plays a key role in bolus propulsion through the oropharyngeal chamber. In this study, possible age effects on the magnitude and timing of lingual pressure generation were analyzed.

METHODS

Oral pressure was measured during isometric and swallowing tasks for 10 elderly (mean age = 81 years) and 10 young (mean age = 51 years) subjects. Three trials each of the isometric task and swallows of three different boluses (3 ml semisolid, 3 ml liquid, and 10 ml liquid) were performed by each subject. The timing and magnitude of isometric and swallowing pressure generation along with the pattern of the swallowing pressure waveform were analyzed.

RESULTS

Whereas maximum lingual isometric pressures decreased with age (p < .001). no significant age difference was found for swallowing pressure. Time taken to reach peak pressure also was reduced with age in both the isometric task and swallows of liquid boluses (p < .05), while no significant age effect was found for semisolid swallows. Finally, only elderly subjects showed a pattern of liquid swallowing pressure generation in which multiple lingual gestures were required to reach peak pressure (termed "pressure building"), a pattern demonstrated by both young and elderly groups for semisolids.

CONCLUSIONS

Decreased lingual strength with age combined with unchanging swallowing pressure leads to a decreased "pressure reserve," perhaps leaving older individuals more at risk for dysphagia resulting from insults directly or indirectly to the swallowing system. Additionally, swallowing is generally "slowed" with age, apparently due to both central and peripheral factors, and this change may have an impact on bolus flow outcomes.

Effects of isokinetic strength training on concentric and eccentric torque development in the ankle dorsiflexors of older adults

This study quantifies concentric and eccentric dorsiflexor muscle torque at various movement velocities and determines the neuromuscular effects from short-term practice of repeated maximal voluntary contractions using an isokinetic resisted exercise program in older adults. Twenty-eight subjects (76.3+/-4.6 years) trained concentric and eccentric dorsiflexion 3 days per week for 2 weeks at 30 degrees, 90 degrees, and 180 degrees/s through 40 degrees of dorsiflexion range of motion. Peak torque, rate of torque development, surface electromyography, and strength curves at each velocity were compared before and after training. Increases were found for concentric (27%) and eccentric (20%) dorsiflexor peak torque and concentric rate of torque production (20%-53%) across all movement velocities (p < .01). Training also significantly increased dorsiflexor concentric (64%) and eccentric (55%) surface electromyography; taken together, this is evidence of apparent adaptation of neural factors in older adults with short-term training. These findings support that eccentric and concentric strength training enhances the control and production of ankle muscle dorsiflexor torque in older adults, with implications for improving functional mobility of the ankle joint.

Age-related changes in muscle fatigue resistance in humans

OBJECTIVE

The goal of this study was to compare the relative contributions from the muscle and the central nervous system to muscle fatigue resistance in aging.

METHODS

Each subject carried out 90 s of sustained maximal voluntary isometric contraction (MVC) of the thumb using the thenar and forearm thumb muscles. Contractile capacity of the thenar muscles was assessed through tetanic stimulation of the median nerve. Interpolated doublets delivered during an MVC represented the overall voluntary activation level while transcranial cortical stimulation with an electromagnetic stimulator was used to assess motor output upstream from the corticomotoneuronal pathway.

RESULTS

Nine elderly subjects [four females and five males, 70+/-9 years old (mean+/-SD)] and 10 younger subjects (five females and five males, 30+/-6 years old) were tested. After the fatiguing exercise, the elderly group's MVC declined by 29% as opposed to 47% in the younger group (p<0.01). The elderly group's greater fatigue resistance was accounted for by increased fatigue resistance at the muscle level as well as in the central nervous system. At least some of the decline in the central motor drive was upstream from the corticomotoneuronal pathway.

CONCLUSION

The higher muscle fatigue resistance in the elderly group was attributable to differences in both the peripheral and central nervous systems.

Pixel T2 distribution in functional magnetic resonance images of muscle

Increases in skeletal muscle (1)H-NMR transverse relaxation time (T2) observed by magnetic resonance imaging have been used to map whole muscle activity during exercise. Some studies further suggest that intramuscular variations in T2 after exercise can be used to map activity on a pixel-by-pixel basis by defining an active T2 threshold and counting pixels that exceed the threshold as "active muscle." This implies that motor units are nonrandomly distributed across the muscle and, therefore, that the distribution of pixel T2 values ought to be substantially broader after moderate exercise than at rest or after more intense exercise, since moderate-intensity exercise should recruit some motor units, and hence some pixels, but not others. This study examined the distribution of pixel T2 values in three muscles (quadriceps, anterior tibialis, and biceps/brachialis) of healthy subjects (5 men and 2 women, 18-46 yr old) at rest, after exercise to fatigue (50% 1 repetition maximum at 20/min to failure = Max), and at 1/2Max (25% 1 repetition maximum, same number of repetitions as Max). Although for each muscle there was a linear relationship between exercise intensity and mean pixel T2, there was no significant difference in the variance of pixel T2 between 1/2Max and Max exercise. There was a modest (10-43%) increase in variance of pixel T2 after both exercises compared with rest, but this was consistent with a Monte Carlo simulation of muscle activity that assumed a random distribution of motor unit territories across the muscle and a random distribution of muscle cells within each motor unit's territory. In addition, 40% of the pixel-to-pixel muscle T2 variations were shown to be due to imaging noise. The results indicate that magnetic resonance imaging T2 cannot reliably map active muscle on a pixel-by-pixel basis in normal subjects.

Effects of aging on motor-unit control properties

It was hypothesized that the age-related alterations in the morphological properties of a motor unit would be accompanied by modifications in the control aspects of the motor unit, as either an adaptive or compensatory mechanism to preserve smooth force production. In specific, the objective of the study was to investigate the age-related alterations in the concurrent firing behavior of multiple motor units in the first dorsal interosseous (FDI) muscle in isometric contractions at 20 and 50% of the subject's voluntary contraction level. Analysis of the data collected from 10 young (24-37 yr of age) and 10 elderly (65-88 yr of age) subjects led to three novel observations regarding the firing behavior of aged motor units. 1) Among elderly subjects, there is a decrease in the common fluctuations that are observed among the firing rates of motor units in the young. 2) The relationship observed between the firing rate and recruitment threshold of young subjects is disturbed in the elderly. Although in young subjects, at any point in a given submaximal contraction, earlier recruited motor units have higher firing rates than later-recruited units; in aged subjects this dependency of firing rate on recruitment rank is compromised. 3) The progressive decrease observed in the firing rates of concurrently active motor units in constant-force contractions in the young is not seen in the aged. In addition to these original findings, this study provided support for earlier reports of 1) decreased average firing rates probably reflecting the slowing of the muscle, 2) a shift in recruitment thresholds toward lower force levels in line with the shift toward type I fibers, and 3) multiphasic action potential shapes indicative of the reinnervation process that takes place during aging. Taken as a whole, these findings indicate significant age-related modifications in the control properties of human motor units.

Motor unit firing rates and contractile properties in tibialis anterior of young and old men

The effects of aging on motoneuron firing rates and muscle contractile properties were studied in tibialis anterior muscle by comparing results from six young (20.8 +/- 0.8 yr) and six old men (82.0 +/- 1.7 yr). For each subject, data were collected from repeated tests over a 2-wk period. Contractile tests included maximal voluntary contraction (MVC) with twitch interpolation and stimulated twitch contractions. The old men had 26% lower MVC torque (P < 0.01) than did the young men, but percent activation was not different (99.1 and 99.3%, respectively). Twitch contraction durations were 23% longer (P < 0.01) in the old compared with the young men. During a series of repeated brief steady-state contractions at 10, 25, 50, 75, and 100% MVC, motor unit firing rates were recorded. Results from approximately 950 motor unit trains in each subject group indicated that at all relative torque levels mean firing rates were 30-35% lower (P < 0.01) in the old subjects. Comparisons between young and old subjects' mean firing rates at each of 10%, 50%, and MVC torques and their corresponding mean twitch contraction duration yielded a range of moderate-to-high correlations (r = -0.67 to -0.84). That lower firing rates were matched to longer twitch contraction durations in the muscle of old men, and relatively higher firing rates were matched with shorter contraction times from the young men, indirectly supports the neuromuscular age-related remodeling principle.

Quadriceps muscle strength, contractile properties, and motor unit firing rates in young and old men

Changes with age in the voluntary static and dynamic strength of the quadriceps muscle group have been well characterized, and the importance of the muscle group for locomotion and independent living have been highlighted in both normal human aging and in clinical studies. Surprisingly few studies of this muscle group have described age-related changes in voluntary activation ability using twitch interpolation and changes in stimulated contractile properties, and none have assessed the influence of old age on motor unit firing rates. We compared in 13 young (mean age 26 years) and 12 old (mean age 80 years) men the voluntary isometric strength, stimulated contractile properties, and average steady state motor unit firing rates in the quadriceps muscle. Maximum voluntary contraction (MVC) force and twitch tension were approximately 50% lower in the old men, but contractile speed was only approximately 10% slower than in the young men. There was no difference in the ability of either group to activate the quadriceps to a high degree (94-96%). At all isometric force levels tested (10%, 25%, 50%, 75%, and 100% MVC), there were no differences in mean motor unit firing rates. In both groups, the range of firing rates was similar and not large ( approximately 8 Hz at 10% MVC and 26 Hz at MVC). Thus, the substantial age-related weakness in this muscle does not seem to be related to changes in neural drive.