Skeletal Muscle Cell Volume In Lower Limb Might Be A Determinant Of Aerobic Work Capacity In Lean Young Women

Abstract

PURPOSE: Skeletal muscle tissue holds a large amount of water, which is partitioned into intracellular water (ICW) and extracellular water (ECW, the sum of interstitial fluid and blood plasma) fractions. Thus, the muscle volume estimated using these imaging methods includes not only the muscle cell mass but also ECW, which is not related to muscle strength. In other words, a cellular approach is required to measure actual skeletal muscle cell mass. Recently, estimating the regional ICW and ECW became possible using segmental multifrequency bioelectrical impedance spectroscopy (S-BIS), and the ICW is proportional to the regional muscle cell mass. The present study examined the hypothesis that the muscle cell mass in the lower extremities might be related to aerobic work capacity in lean young women. METHODS: Thirteen Japanese female college students aged 20-26 years old (height, 161.3 ± 6.5 cm; weight, 53.63 ± 6.17 kg; BMI, 20.56 ± 1.64 kg/m2) were recruited. The ICW and ECW in the upper and lower extremities were measured by S-BIS. Incremental exercise test from 10 w to completely exhaustion was performed on electrically breaked cycle ergometer with metabolic cart to evaluate VO2peak. Lactate threshold (LT) were determined by lactate concentration of the blood obtained from earlap. Muscle strength and power in the lower extremities was measured. Daily physical activities were monitored by a previously validated triaxial accelerometer during two weeks. RESULTS: The ICW in the lower leg was significantly and highly correlated with the work and VO2 at LT, VO2peak, leg muscle strength and power, and the duration of moderate intensity physical activity, but not light or vigorous intensity physical activity. In contrast, the ICW in the upper extremities and the ECW in the upper and lower extremities were not significantly or just weakly correlated with those variables. CONCLUSIONS: The results suggest that the daily moderate intensity physical activity would be important to maintain muscle cell mass in the lower leg, and the muscle cell mass in the lower extremities might be a determinant of aerobic work capacity in lean young women.