Department of Exercise Science, University of Massachusetts, Amherst, Massachusetts 01003
Submitted 16 December 2003
; accepted in final form 11 May 2004
It has been suggested that the effects of old ...age on the ability to resist fatigue may be task dependent. To test one aspect of this hypothesis, we compared the neuromuscular responses of nine young (26 ± 4 yr, mean ± SD) and nine older (72 ± 4 yr) healthy, relatively sedentary men to intermittent isometric (3 min, 5 s contract/5 s rest) and dynamic (90 at 90°/s) maximum voluntary contractions (MVC) of the ankle dorsiflexor muscles. To assess the mechanisms of fatigue (defined as the ratio of postexercise MVC to preexercise MVC), we also measured isometric central activation ratios (CAR), tetanic torque, contractile properties, and compound muscle action potentials before and immediately after exercise. Because dynamic contractions are more neurally complex and metabolically demanding than isometric contractions, we expected an age-related fatigue resistance observed during isometric exercise to be absent during dynamic exercise. In contrast, older men (O) fatigued less than young (Y) during both isometric (O = 0.77 ± 0.07, Y = 0.66 ± 0.02, mean ± SE; P < 0.01) and dynamic (O = 0.45 ± 0.07, Y = 0.27 ± 0.02; P = 0.04) contractions (ratio of postexercise to preexercise MVC), with no evidence of peripheral activation failure in either group. We observed no obvious limitations in central activation in either group, as assessed using isometric CAR methods, after both isometric and dynamic contractions. Preexercise half-time of tetanic torque relaxation, which was longer in O compared with Y, was linearly associated with fatigue resistance during both protocols ( r = 0.62 and 0.66, P 0.004, n = 18). These results suggest that relative fatigue resistance is enhanced in older adults during both isometric and isokinetic contractions and that age-related changes in fatigue may be due largely to differences within the muscle itself.
ankle dorsiflexors; compound muscle action potential; contractile properties; central activation
Address for reprint requests and other correspondence: J. A. Kent-Braun, Dept. of Exercise Science, Totman Bldg. 108, University of Massachusetts, Amherst, MA 01003 (E-mail: janekb{at}excsci.umass.edu ).
Magnetic Resonance Unit, Department of Radiology, University of
California, San Francisco, California 94121
To examine the influences of age, gender, and habitual
physical activity level on human ...skeletal muscle composition, we
developed a relatively simple magnetic resonance imaging method for the quantitation of leg anterior compartment contractile and noncontractile content. We studied 23 young (11 women and 12 men, 26-44 yr old) and 21 older (10 women and 11 men, 65-83 yr old) healthy adults. Analysis was by two-factor (age, gender) ANOVA. Physical activity, quantitated by three-dimensional accelerometer worn about the waist for
1 wk, was not different between groups. Men had larger contractile and
noncontractile cross-sectional areas
(cm 2 ) than women, with no gender
effect on percent noncontractile area. Young subjects had larger
contractile areas and smaller absolute
(cm 2 ) and relative (percent
total) noncontractile areas than older subjects. There was a
significant linear relationship between physical activity and percent
noncontractile area in older ( r = 0.68, P = 0.002) but not young
subjects. These data demonstrate a more than twofold increase in the
noncontractile content of locomotor muscles in older adults and provide
novel support for physical activity as a modulator of this age-related
change in muscle composition.
muscle function; aging; physical activity; fat; gender
We have previously observed less muscle fatigue in women than men under conditions of intact circulation, but similar fatigue across the sexes during local ischemia. Thus, we hypothesized that women ...utilize their aerobic metabolic pathways to a greater extent than do men. To test this hypothesis, we examined the extent to which different pathways of intramuscular adenosine triphosphate (ATP) production were utilized by men and women during maximal voluntary isometric contractions. Force production during 15‐s and 60‐s contractions were recorded in parallel sessions. In one session, central activation was assessed with electrical stimulation. In the other, phosphorus magnetic resonance spectroscopy was used to quantify muscle oxidative capacity, and the contributions of glycolysis and oxidative phosphorylation to ATP synthesis during the 60‐s contraction. Fatigue and central activation were similar in men and women during both the 15‐s and 60‐s contractions. The rate constants of phosphocreatine recovery following the 15‐s contraction were similar in men and women, indicating similar oxidative capacities. Men exhibited greater acidosis and peak glycolytic rates compared with women during the 60‐s contraction, with no differences observed in creatine kinase flux or the percent of oxidative capacity utilized. We conclude that men exhibit greater in vivo glycolysis during brief, intense isometric contractions. Although this metabolic difference did not contribute to any observable differences in fatigue in the present study, these results highlight a potentially important mechanism to explain sex‐related differences in muscle function. Muscle Nerve, 2005
Department of Exercise Science, University of Massachusetts, Amherst, Massachusetts 01003
Submitted 6 August 2002
; accepted in final form 7 August 2003
The purpose of this study was to test the ...hypotheses that, under isovelocity conditions, older compared with young humans would 1 ) be slower to reach target velocity and 2 ) exhibit a downward shift in the torque-velocity and power-velocity relationships in the ankle dorsiflexor and knee extensor muscles. We studied 12 young (26 ± 5 yr, 6 men/6 women) and 12 older (72 ± 6 yr, 6 men/6 women) healthy adults during maximal voluntary concentric contractions at preset target velocities (dorsiflexion: 0240°/s; knee extension: 0400°/s) using an isokinetic dynamometer. The time to target velocity was longer in older subjects in the dorsiflexors and knee extensors (both P 0.02). Averaged across all velocities, older subjects produced 26% less concentric torque and power in the dorsiflexors ( P < 0.01) and 32% less in the knee extensors ( P < 0.01). The downward shift in the torque-velocity relationship persisted even when torque was expressed relative to each subject's maximum. In the knee extensors only, the age-related decrement in power increased with increasing velocities, suggesting that this muscle group may be more susceptible to age-related losses of function than the dorsiflexor muscles are. In support of our hypotheses, these results demonstrate an age-related impairment in the dynamic performance of two functionally distinct muscle groups in healthy older adults. With age, the impairment of dynamic performance appears to exceed the loss of isometric performance, particularly in the knee extensor muscles.
ankle dorsiflexors; knee extensors; sarcopenia; dynamic; isometric
Address for reprint requests and other correspondence: J. A. Kent-Braun, Dept. of Exercise Science, Totman Bldg. 108, Univ. of Massachusetts, Amherst, MA 01003.
Magnetic Resonance Unit, Department of Radiology, University of
California, San Francisco, California 94121
The extents to which
decreased muscle size or activation are responsible for the decrease ...in
strength commonly observed with aging remain unclear. Our purpose was
to compare muscle isometric strength maximum voluntary
contraction (MVC), cross-sectional area (CSA), specific strength
(MVC/CSA), and voluntary activation in the ankle dorsiflexor muscles of
24 young (32 ± 1 yr) and 24 elderly (72 ± 1 yr)
healthy men and women of similar physical activity level.
Three measures of voluntary muscle activation were used: the central
activation ratio MVC/(MVC + superimposed force), the
maximal rate of voluntary isometric force development, and foot tap
speed. Men had higher MVC and CSA than did women. Young men had higher
MVC compared with elderly men 262 ± 19 (SE) vs. 197 ± 22 N, respectively, whereas MVC was similar in young and elderly
women (136 ± 15 vs. 149 ± 16 N, respectively). CSA was greater in young compared with elderly subjects. There was no
age-related impairment of specific strength, central activation ratio,
or the rate of voluntary force development. Foot tap speed was reduced
in elderly (34 ± 1 taps/10 s) compared with young subjects (47 ± 1 taps/10 s). These results suggest that isometric specific
strength and the ability to fully and rapidly activate the dorsiflexor
muscles during a single isometric contraction were unimpaired by aging.
However, there was an age-related deficit in the ability to perform
rapid repetitive dynamic contractions.
physical activity; magnetic resonance imaging; aging; gender; estrogen
Validation of questionnaires to estimate physical activity and functioning in end-stage renal disease.
Patients on dialysis are less physically active than sedentary persons with normal kidney ...function. To assess the consequences of inactivity and the results of efforts to increase activity in the end-stage renal disease (ESRD) population, valid instruments to measure physical activity and physical functioning in this group are needed.
We performed a cross-sectional study to establish the validity in ESRD of several questionnaires designed to measure physical activity or physical functioning in the general population. Questionnaires studied included the Stanford 7-day Physical Activity Recall questionnaire (PAR), the Physical Activity Scale for the Elderly (PASE), the Human Activity Profile (HAP), and the Medical Outcomes Study Short Form 36-item questionnaire (SF-36). Physical activity was measured using three-dimensional activity monitors (accelerometers) over a seven-day period (the “gold standard”). Patients also underwent physical performance tests, including measurement of gait speed, stair climbing time, and chair rising time. Study questionnaires were administered, and questionnaire results were compared with each other and with activity monitor and physical performance test results.
Thirty-nine maintenance hemodialysis patients participated in the study. Dialysis patients scored worse than previously published healthy norms on all tests. All questionnaires correlated with seven-day accelerometry and with at least one measure of physical performance. The HAP correlated best with accelerometry (r = 0.78, P < 0.0001). Seventy-five percent of the variability in physical activity measured by accelerometry could be explained by a model that combined information from the HAP and the PASE. The HAP and the physical functioning scale of the SF-36 were about equally well correlated with physical performance measures.
These questionnaires are valid in patients on hemodialysis and should be used to study the physical activity and rehabilitation efforts in this population further.
1 Department of Medicine, Nephrology Section, San Francisco Veterans Affairs Medical Center, San Francisco; 2 Department of Medicine, University of California, San Francisco; 3 Northern California ...Institute for Research and Education, San Francisco, California; and 4 Exercise Science Department, University of Massachusetts, Amherst, Massachusetts
Submitted 15 March 2005
; accepted in final form 11 May 2005
Dialysis patients have severe exercise limitations related to metabolic disturbances, but muscle fatigue has not been well studied in this population. We investigated the magnitude and mechanisms of fatigue of the ankle dorsiflexor muscles in patients on maintenance hemodialysis. Thirty-three dialysis patients and twelve healthy control subjects performed incremental isometric dorsiflexion exercise, beginning at 10% of their maximal voluntary contraction (MVC) and increasing by 10% every 2 min. Muscle fatigue (fall of MVC), completeness of voluntary activation, and metabolic responses to exercise were measured. Before exercise, dialysis subjects exhibited reduced strength and impaired peripheral activation (lower compound muscle activation potential amplitude) but no metabolic perturbation. During exercise, dialysis subjects demonstrated threefold greater fatigue than controls with evidence of central activation failure but no change in peripheral activation. All metabolic parameters were significantly more perturbed at end exercise in dialysis subjects than in controls, including lower phosphocreatine (PCr) and pH, and higher P i , P i /PCr, and H 2 PO 4 . Oxidative potential was markedly lower in patients than in controls 62.5 (SD 27.2) vs. 134.6 (SD 31.7), P < 0.0001. Muscle fatigue was negatively correlated with oxidative potential among dialysis subjects ( r = 0.52, P = 0.04) but not controls. Changes in central activation ratio were also correlated with muscle fatigue in the dialysis subjects ( r = 0.59, P = 0.001) but not the controls. This study provides new information regarding the excessive muscular fatigue of dialysis patients and demonstrates that the mechanisms of this fatigue include both intramuscular energy metabolism and central activation failure.
oxidative phosphorylation; central fatigue; electromyography; muscle activation; 31 P magnetic resonance spectroscopy
Address for reprint requests and other correspondence: K. L. Johansen, Dialysis Unit, San Francisco Veterans Affairs Medical Center, 4150 Clement St., San Francisco, CA 94121 (e-mail: johanse{at}itsa.ucsf.edu )
Magnetic Resonance Unit, Department of Radiology, University of
California, San Francisco, California 94121
It has been suggested that a decline in skeletal muscle
oxidative capacity is a general ...consequence of aging in humans.
However, previous studies have not always controlled for the effects of varying levels of physical activity on muscle oxidative capacity. To
test the hypothesis that, when matched for comparable habitual physical
activity levels, there would be no age-related decline in the oxidative
capacity of a locomotor muscle, the postexercise recovery time of
phosphocreatine was compared in the tibialis anterior muscle of young
n = 19; 33.8 ± 4.8 (SD) yr and older n = 18; 75.5 ± 4.5 yr healthy women and men
of similar, relatively low, activity levels. The intramuscular
metabolic measurements were accomplished by using phosphorus magnetic
resonance spectroscopy. The results indicate that there was no age
effect on the postexercise recovery time of phosphocreatine recovery,
thus supporting the stated hypothesis. These data suggest that there is
no requisite decline in skeletal muscle oxidative capacity with aging
in humans, at least through the seventh decade.
physical activity; aging; gender; magnetic resonance spectroscopy
1 Department of Exercise Science, University of Massachusetts, Amherst, Massachusetts 01003; and 2 Department of Radiology and Radiological Sciences, Vanderbilt University, Nashville, Tennessee 37235
...Submitted 23 December 2003
; accepted in final form 27 July 2004
Although skeletal muscle perfusion is fundamental to proper muscle function, in vivo measurements are typically limited to those of limb or arterial blood flow, rather than flow within the muscle bed itself. We present a noninvasive functional MRI (fMRI) technique for measuring perfusion-related signal intensity (SI) changes in human skeletal muscle during and after contractions and demonstrate its application to the question of occlusion during a range of contraction intensities. Eight healthy men (aged 2031 yr) performed a series of isometric ankle dorsiflexor contractions from 10 to 100% maximal voluntary contraction. Axial gradient-echo echo-planar images (repetition time = 500 ms, echo time = 18.6 ms) were acquired continuously before, during, and following each 10-s contraction, with 4.5-min rest between contractions. Average SI in the dorsiflexor muscles was calculated for all 240 images in each contraction series. Postcontraction hyperemia for each force level was determined as peak change in SI after contraction, which was then scaled to that obtained following a 5-min cuff occlusion of the thigh (i.e., maximal hyperemia). A subset of subjects ( n = 4) performed parallel studies using venous occlusion plethysmography to measure limb blood flow. Hyperemia measured by fMRI and plethysmography demonstrated good agreement. Postcontraction hyperemia measured by fMRI scaled with contraction intensity up to 60% maximal voluntary contraction. fMRI provides a noninvasive means of quantifying perfusion-related changes during and following skeletal muscle contractions in humans. Temporal changes in perfusion can be observed, as can the heterogeneity of perfusion across the muscle bed.
ankle dorsiflexors; muscle activation; blood flow; occlusion
Address for reprint requests and other correspondence: J. A. Kent-Braun, Dept. of Exercise Science, Totman 108, Univ. of Massachusetts, Amherst, MA 01003 (E-mail: janekb{at}excsci.umass.edu )
In humans, decreases in cardiac output play an important role in the age-related decrease in whole-body oxidative capacity. What remains less clear is whether a decline in skeletal muscle oxidative ...capacity is also an inevitable consequence of aging, as a number of other factors that could affect oxidative capacity also change with age, including: physical activity, health status, fibre-type composition, rates of protein synthesis and muscle blood supply. Both in vitro studies using muscle biopsy tissue and in vivo studies using 31P-magnetic resonance spectroscopy are used to study muscular oxidative capacity. Using these methodologies, researchers have found age-associated reductions in the oxidative capacities of specific muscles. In most cases, however, the influence of physical activity has not been adequately controlled, making it difficult to evaluate the effects of age itself from those of lifestyle changes associated with aging. Upon critical evaluation of the existing literature, the following picture regarding the effect of age on muscle oxidative capacity appears: although the maximum level of muscular oxidative capacity attainable through training may decline with age, much of the age-associated decline in oxidative function is related to the reductions in fitness and/or habitual physical activity that typically occur in this population. Future studies in this area must account for the health and activity status of their study participants.
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