THE EFFECTS OF SHORT‐TERM VOLUNTARY IMMOBILIZATION ON THE CONTRACTILE PROPERTIES OF THE HUMAN TRICEPS SURAE

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Abstract
In four male subjects one leg was immobilized in a full leg cast for 2 weeks. Measurements of the contractile properties of both triceps surae were made before, during and after immobilization. There was a significant increase in the mean twitch time to peak tension (Tc) after 1 week of immobilization from 126 ± 10 ms to 143 ± 20 ms (P 〈 0·05) and in the half‐relaxation time (T½r) from 88 ± 7 ms to 107 ± 12 ms (P 〈 0·01). There were no further significant (P 〈 0·1) changes in the twitch characteristics during the second week of immobilization. The strength of a maximal voluntary contraction (m.v.c.) declined by 11% (P 〈 0·05) after 1 week and 24% (P 〈 0·05) after 2 weeks. Twitch tension, tetanic tension and muscle fatigability were not significantly altered during immobilization. Muscle (plus bone) calf cross‐sectional area (A) was reduced by 5% (P 〈 0·01) after 1 week and a further 3% (P 〈 0·05) after 2 weeks. The Tc and T½r of the twitch, m.v.c. and A returned to control values within 2, 7, 4 and 14 d respectively. No significant (P 〉 0·01) changes were observed in the mechanical properties of the triceps surae of the contralateral limb during the period of immobilization. The results suggest that short‐term voluntary immobilization is associated with atrophy and a diminished capacity of the muscle to develop maximal voluntary force. The increased contraction time of the twitch may indicate a change in the excitation‐contraction coupling mechanism of the muscle and the disproportionate decrease in m.v.c. in relation to A and tetanic tension a reduction in the central neural drive.