Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC203
Loss of skeletal muscle power in older men is due to loss of muscle mass, specific force and velocity of contraction
J. S. McPhee1, T. M. Maden-Wilkinson1, M. Narici1, H. Degens1, D. A. Jones1
1. Institute for Biomedical Research into Human Movement and Health, Manchester Metropolitan University, Manchester, United Kingdom.
Skeletal muscle size and function deteriorate after the 4th decade of life and symptomatic of these changes is loss of muscle power (Watts): the product of force * velocity of contraction. We investigated loss of muscle strength, size and velocity of contraction as determinants of power in 15 older men (O: age 72±1 yr) compared with 16 young men (Y: 23±1 yr) using Student’s t-tests and regression analysis. Data are mean ± SEM. Subjects completed a whole-body DXA scan and maximal instantaneous muscle power was measured using countermovement jumps on a force platform. The leg lean mass-to-body mass ratio was 22% lower in O than Y. Absolute power was 36% lower in O than Y (2185±138 vs 3435±140 W) and 33% lower when normalised to leg lean mass (all P<0.001). Ground reaction force was similar in Y and O, but velocity of take-off was 31% slower in O (1.4±0.1 vs 2.0±0.2 m.s-1, p<0.001). The hypothesis was tested that loss of power in O was due to a mechanical disadvantage consequent to the reduced leg lean mass-to-body mass ratio, which, according to the force-velocity relationship, would cause the muscles to contract slower to produce force for take-off. Eight young men jumped with an additional 16 kg external load (giving similar leg lean mass-to-total mass of O). Although the velocity of take-off was reduced by 9%, the Y were able to increase the ground reaction force by 13% compared with the unloaded trial. Consequently, absolute power was not changed. The loss of power at old age is thus not solely attributable to lower leg lean mass-to-body mass ratio. A reduced force generating capacity of muscle and slower contractile properties might also cause a slower take-off as the muscle will work on a slower part of the force-velocity relationship. We therefore tested the hypothesis that lower muscle specific force and rate of force development contribute to reduced muscle power in O. Specific force was calculated as the maximal knee extensor isometric strength divided by quadriceps physiological cross sectional area (PCSA: quadriceps volume measured by MRI / fascicle length measured by ultrasonography). O had 38% lower strength (166±9 vs 266±10 Nm), 21% lower PCSA, 14% lower specific force and 38% slower rate of force development compared with Y (all P<0.001). Multiple regression analysis showed that these four variables accounted for 84% of the variability in absolute power (R2=0.842, P<0.001). These results suggest that loss of muscle mass, specific force and slower rate of force development contribute to loss of muscle power in old age. The cause of these changes to skeletal muscles have not been examined here, but might include motorneurone cell death, decreased myosin content, post-translational modifications of the myosin molecules and/or increased intramuscular adipose and connective tissue.
Where applicable, experiments conform with Society ethical requirements