WORK CAPACITY AND METABOLIC AND MORPHOLOGIC CHARACTERISTICS OF THE HUMAN QUADRICEPS MUSCLE IN RESPONSE TO UNLOADING

The response of skeletal muscle to unweighting was studied in six heal thy males who were subjected to four weeks of lower limb suspension. T hey performed three bouts of 30 consecutive maximal concentric knee ex tensions, before unloading and the day after (POST 1), 4 days after (P OST 2) and 7 weeks after (REC) resumed weight-bearing. Peak torque of each contraction was recorded and work was calculated as the mean of t he average peak torque for the three bouts and fatigability was measur ed as the decline in average peak torque over bouts. Needle biopsies w ere obtained from m. vastus lateralis of each limb before and at POST 1. Muscle fibre type composition and area. capillarity and the enzyme activities of citrate synthase (CS) and phosphofructokinase (PFK) were subsequently analyses. Mean average peak torque for the three bouts a t POST1, POST2 and REC was reduced (P<0.05) by 17,13 and 7%, respectiv ely. Fatigability was greater (P<0.05) at POST2 than before unloading. Type I, IIA and IIB percentage, Type I and II area and capillaries pe r fibre of Type I and II did not change (P>0.05) in response to unload ing. The activity of CS, but not PFK, decreased (P<0.05) after unloadi ng. The weight-bearing limb showed no changes in the variables measure d. The results of this study suggest that this human lower limb suspen sion model produces substantial impairments of work and oxidative capa city of skeletal muscle. The performance decrements are most likely in duced by lack of weight-bearing.