Volume 90, Issue 1 , Pages 178-180, January 2009
Restoration of Voluntary Muscle Strength After 3 Weeks of Cast Immobilization is Suppressed in Women Compared With Men
Article Outline
Abstract
Clark BC, Manini TM, Hoffman RL, Russ DW. Restoration of voluntary muscle strength after 3 weeks of cast immobilization is suppressed in women compared with men.
Objective
To investigate sex-related differences in the loss and recovery of voluntary muscle strength after immobilization.
Design
Longitudinal, repeated measures.
Setting
Research laboratory.
Participants
Healthy men (n=5) and healthy women (n=5).
Intervention
Three weeks of forearm immobilization.
Main Outcome Measures
Voluntary wrist flexion muscle strength was assessed at baseline and weekly during the immobilization protocol and 1 week after cast removal. Central activation was assessed before and after immobilization and after 1 week of recovery to determine what percentage of the muscle could be activated voluntarily.
Results
Men and women lost voluntary strength at a similar rate during immobilization. However, after 1 week of recovery voluntary strength had returned to within 1% of baseline in the men, but remained approximately 30% less than baseline in the women (P=0.03). Both sexes displayed reduced central activation after immobilization (P=0.02), but the decrease was similar in both sexes (P=0.82).
Conclusions
These findings suggest sex-dependent adaptations to and recovery from limb immobilization, with voluntary strength recovering slower in women. As such, sex-specific rehabilitation protocols may be warranted, with women requiring additional or more intensive rehabilitation programs after periods of disuse. Future work is needed to determine the extent and mechanisms of these differences.
Key Words: Electromyography, Muscle, Rehabilitation, Sex
List of Abbreviations: ANOVA, analysis of variance, MVC, maximal voluntary contraction, PAP, post activation potentiation
INDIVIDUALS FREQUENTLY experience injury or illness that requires periods of reduced muscle activity, leading to loss of voluntary muscle strength that can result in severe negative consequences, such as decreased functional capacity and quality of life.1 Women are 4 times more likely than men to experience forearm fractures requiring immobilization,2 yet only 2 reports are known to have evaluated sex-specific differences in loss of voluntary strength after disuse in humans. One study suggests that women experience a greater loss of voluntary strength,3 whereas the other indicates no difference.4 This brief report details our findings of sex-dependent adaptations during and after 3 weeks of forearm cast immobilization and further examines the restoration of voluntary strength after a 1-week period of recovery. We report that while women and men lose voluntary strength at a proportional rate, the rate of recovery of voluntary muscle strength is depressed in women when compared with muscle-strength recovery men.
Methods
Ten healthy subjects completed 3 weeks of forearm immobilization (5 men, 5 women; age, 18–29y). The study was approved by the Ohio University institutional review board. After a baseline testing session, the immobilization subjects were fitted with a wrist-hand splinta (immobilization of the wrist in a neutral position with the fingers extended) on the nondominant arm. Voluntary wrist flexion strength was assessed weekly during immobilization and 7 days after cast removal. Additionally, the ability to voluntarily activate the wrist flexor muscles (central activation) was determined before immobilization, on cast removal, and after 1 week of recovery.
The setup for assessing wrist flexion strength and central activation is shown in figure 1A. Exerted forces were recorded from a Biodex System 4b and sampled at 625Hz.c To determine what percentage of the force-generating capacity of the muscles can be produced voluntarily, a combination of voluntary and electrically stimulated contractions were used to perform the interpolated doublet technique as previously described.5 Briefly, a 100Hz doublet was delivered to the median nerve in the bicipital groove while each subject performed an MVC. Supramaximality of the stimulus was determined as previously reported.5 Any increase in force evoked during the MVC was expressed relative to the PAP force (fig 1B), and the percentage of the muscle voluntarily activated was calculated: % central activation=[1−(evoked force during MVC/PAP evoked force)]×100.
Fig 1.
(A) The experimental setup for assessing voluntary wrist flexion strength and central activation. (B) Force trace of a maximal voluntary contraction coupled with superimposed electric stimulation to assess central activation. (C) No sex differences were observed at baseline for central activation, nor were immobilization-induced sex-specific effects observed. (D) Temporal changes in voluntary wrist flexion strength by sex during 3 weeks of immobilization and after 1 week of recovery.
*Indicates significantly less than baseline when data are pooled across sex (P≤.05).
†The recovery of voluntary muscle strength was depressed in women when compared with men (P=.03, η2=.46).
Repeated measures ANOVAs were performed to detect sex differences over time for the outcome measures of voluntary strength and central activation, with Sidak post hoc tests when needed. Nonparametic analyses were also performed (Wilcoxon signed-rank test and Mann-Whitney U test), but these findings corroborated the ANOVA results and are thus not reported. Effect sizes (η2) between the sexes for each time point in comparison to baseline data are reported to aid in the interpretation of the magnitude of effects.
Results
We observed a time × sex interaction with regard to the changes in wrist flexion MVC (P=.01, η2=.37) (fig 1D), with further analysis indicating the men's voluntary strength had returned to baseline levels after 1 week of recovery whereas the women's remained significantly decreased (P=.03; η2=.46). All 5 women exhibited at least a 15% reduction in voluntary strength relative to baseline, whereas only 1 man showed a similar deficit. No sex difference was observed at baseline for central activation, and both sexes displayed a reduced central activation with immobilization (time effect P=.02, η2≤.38); however, there was no difference between sexes with immobilization (fig 1C) (time × sex interaction P=.82, (η2≤.03).
Discussion
We evaluated sex-based differences in voluntary strength and central activation during and after limb immobilization, and observed sex-specific adaptations. Most notably, women exhibited a similar rate of decrease, but a markedly impaired restoration of voluntary muscle strength relative to men. To our knowledge, this is the first report of sex differences in voluntary muscle strength restoration after disuse. Although we did not directly measure muscle mass, the absence of observed sex differences in central activation suggests that the voluntary strength deficit observed in women during recovery was a function of intramuscular mechanisms, most likely atrophy. It is unclear if our observation of a slower rate of voluntary strength recovery in women is related to biologic factors or behavioral issues (ie, limb use during recovery). We did not specifically monitor the subjects' muscle or physical activity during the 1-week recovery period, although all subjects were given the same instructions (to return to activities of daily living, but to not begin an exercise program of any type until all testing sessions were completed).
Although we did not directly measure muscle mass, previous findings6 suggest that our subjects likely experienced a 4% to 5% loss of forearm cross-sectional area after 3 weeks of forearm immobilization, as well as the effects of other mechanisms that can contribute to disuse-induced changes in voluntary force output7 (ie, intramuscular calcium handling, muscle fiber type, motor unit recruitment). Given the observed lack of sex differences in central activation, it seems unlikely that differences in neural activation were a factor. It is possible, however, that differences in hormonal profiles affected the recovery of muscle mass. Consistent with such a mechanism are animal data suggesting that ovarian hormones inhibit muscle protein synthesis,8 and the well-known direct anabolic effects of testosterone on skeletal muscle.9 As such, it is possible that the observed differences in voluntary strength restoration may reflect possible sex-specific hormonal influences on rates of muscle regrowth, both independently and through interactions with any differences in limb-use behavior.
In summary, these preliminary data suggest sex-dependent adaptations occur during the recovery from limb immobilization, with women's voluntary muscle strength recovering at a slower rate when compared with men's recovery. Although more work is clearly needed to determine the extent and mechanisms of these differences, the present findings, and those of others,10 suggest that women may require additional, or more intensive, rehabilitation programs after periods of immobilization or bed rest.
Suppliers
References
- Midlife hand grip strength as a predictor of old age disability. JAMA. 1999;281:558–560
- . The burden of musculoskeletal conditions at the start of the new millenium (WHO Technical Report Series). In: 2003;p. 1–218
- . Sex-based differences in skeletal muscle function and morphology with short-term limb immobilization. J Appl Physiol. 2005;99:1085–1092
- . Prior resistance training and sex influence muscle responses to arm suspension. Med Sci Sports Exerc. 2005;37:1983–1989
- . Adaptations in human neuromuscular function following prolonged unweighting: II (Neurological properties and motor imagery efficacy). J Appl Physiol. 2006;101:264–272
- . Muscle function at the wrist following 9 d of immobilization and suspension. Med Sci Sports Exerc. 1994;26:615–623
- . Sarcopenia ≠ Dynapenia. J Gerontology: Medical Sciences. 2008;63A:829–834
- . Gender differences in protein metabolism. Curr Opin Clin Nutr Metab Care. 2001;4:493–498
- . Testosterone action on skeletal muscle. Curr Opin Clin Nutr Metab Care. 2004;7:271–277
- . Gender comparison of knee strength recovery following ACL reconstruction with contralateral patellar tendon graft. Biomed Sci Instrum. 2005;41:323–328
No commercial party having a direct financial interest in the results of the research supporting this article has or will confer a benefit on the authors or on any organization with which the authors are associated.
Reprints are not available from the authors.
PII: S0003-9993(08)01545-1
doi:10.1016/j.apmr.2008.06.032
© 2009 American Congress of Rehabilitation Medicine. Published by Elsevier Inc. All rights reserved.
Volume 90, Issue 1 , Pages 178-180, January 2009
