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Influence of long-term bed rest on muscle architecture and tendon mechanical properties

Reeves, Neil D. and Maganaris, Constantinos N. and Ferretti, Gianluigi and Narici, Marco V. (2002) Influence of long-term bed rest on muscle architecture and tendon mechanical properties. Physiological Society.

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Abstract

Muscle weakness and atrophy are well-known consequences of microgravity exposure. However, little is known about how this condition affects the internal structure of the muscle, properties of the tendons and their functional consequences. Therefore, to simulate the effects of microgravity, six males (age 32.7 ± 3.6 years; height 1.73 ± 0.04 m; body mass 71.5 ± 6.3 kg, means ± S.D.) underwent a period of long-term bed rest (LTBR) lasting 90 days. This study (LTBR-2001) was organised by ESA together with NASDA and CNES and performed in Toulouse, France. Ethical committee approval was gained from the host institution. Gastrocnemius medialis (GM) muscle architecture (GM cross-sectional area, fibre fascicle length and pennation angle) and tendon dimensions and mechanical properties were studied in vivo using real-time ultrasonography and magnetic resonance imaging, before and after LTBR. Subjects were seated, the right foot securely positioned in a dynamometer and the knee flexed at 90 deg. Changes in muscle architecture were assessed during graded isometric contraction at 40, 60 and 100 % of the maximal voluntary plantarflexion (MVP), at 20 deg of dorsiflexion. Changes in tendon stiffness were assessed during a MVP after normalisation for tendon dimensions (Young's module, i.e. stress/strain). Paired-samples Student's t test was used to detect changes pre- to post-LTBR; level of significance was set at P < 0.05. After LTBR (1) plantarflexion force was reduced by 55 ± 19 % (P < 0.01); (2) fascicle length and pennation angle were reduced by 9 ± 3 % (P < 0.01) and 14 ± 5 % (P < 0.01), respectively; (3) cross-sectional areas of the GM muscle at mid-calf level, were reduced by 32 ± 2 % (P < 0.001), and (4) the GM tendon Young's modulus at MVP was reduced by 32 ± 5 % (P < 0.05). These findings are in agreement with previous research in disuse conditions (Narici et al. 1998) and strongly suggest a loss of both in-series and in-parallel sarcomeres induced by the prolonged unloading during bed rest. After LTBR percentage fascicle shortening during MVP was reduced by 35 ± 18 (P < 0.01). This is probably due to a loss of sarcomeres in-series limiting the extent of absolute fascicle shortening. Hence after LTBR, the same tendon force can be developed by a reduced fascicle shortening; this may be advantageous to remain on the same portion of the sarcomere lengthÐtension relationship. Therefore the muscle atrophy associated with prolonged disuse involves significant changes in muscle architecture and of tendon mechanical properties. These structural and mechanical alterations are likely to have an effect on the lengthÐforce relationship of this muscle group and on its rate of force development.

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