Proceedings of The Physiological Society

Physiology 2012 (Edinburgh) (2012) Proc Physiol Soc 27, C75

Oral Communications

Causes of the reduced in vivo specific tension of skeletal muscle in old age

J. McPhee1, T. Maden-Wilkinson1, G. de Vito3, M. Narici2,1, H. Degens1, D. A. Jones1

1. Manchester Metropolitan University, Manchester, United Kingdom. 2. University of Nottingham, Derby, United Kingdom. 3. University College Dublin, Dublin, United Kingdom.

  • Figure 1: Quadriceps in vivo specific tension. Individual data points are shown for men (circles) and women (triangles). The mean for young and older groups are shown as black bars.

Skeletal muscle weakness in old age is associated with increased risk of falling, mobility problems and greater mortality risk. Muscle weakness can occur due to reduced number of sarcomeres in parallel, which is represented by the physiological cross sectional area (PCSA); reduced ability to fully activate the muscles and/or reduced strength per unit muscle mass, also known as specific tension. In young (22 ± 1 years) and older (72 ± 1 years) men (n = 36) and women (n = 35) we used magnetic resonance imaging (MRI) and ultrasonography to determine the quadriceps PCSA as: muscle volume / fascicle length. Electrically-induced contractions were used to measure activation capacity during maximal voluntary contractions. In vivo specific tension was calculated as: (external torque / patellar tendon moment arm) / PCSA multiplied by the cosine of the fascicle angle of pennation. Data are reported as Mean ± SEM. Compared with young, the older subjects were 37% weaker (torque corrected for activation capacity was 246.5 ± 11.7 vs 156.2 ± 7.1 Nm in young and older, respectively); PCSA was 27% lower (204.5 ± 7.3 vs 149.6 ± 5.9 cm2 in young and older, respectively), and specific tension was 17% lower (Fig 1; all P<0.0005). These effects were similar in men and women. Muscle biopsy samples showed similar fibre-type composition and type I fibre cross sectional area in young and older, but 26% smaller type II fibres in older subjects (P<0.0005). MRI pixel intensity and collagen stain both indicated higher levels of connective tissue in older subjects. It is concluded that quadriceps in vivo specific tension is lower in older people. If, as reported, type II fibres have a higher specific tension, the atrophy of these fibres might contribute to about half the observed change in specific tension of the whole muscle; the remainder appears to be due to an accumulation of connective tissue.

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