EFFECTS OF A 14-DAY BED REST ON LOWER EXTREMITY NEUROMUSCULAR FUNCTION987

Abstract

Previous studies suggest that extended duration unloading (≥30 d) induces marked decrements in maximal voluntary isometric contraction (MVC) of the lower limb antigravity extensors but not the antagonistic flexors. However, the time course of these changes and its relation to changes in neural activation are not well defined. The purposes of this study were to determine if: (1) short duration (14 d) unloading significantly altered MVC;(2) the response differed among extensor and flexor muscle groups; and (3) neural activation of prime movers and coactivation of antagonists were altered. In addition, the rate of tension development (RTD) and electromechanical delay (EMD) were measured. Five healthy men (mean±SD: 33±7 yr, 176±6 cm, 67±6 kg) underwent 14 d of 6° head-down bed rest after giving written informed consent. Knee (K) and ankle(A) extensor/flexor (E/F) MVCs were tested at 60° of knee flexion and 10° of plantar flexion, respectively. Surface electromyography (EMG) was recorded from m. vastus medialis, m. rectus femoris, m. vastus lateralis, m. semitendinosus, m. biceps femoris, m. medial gastrocnemius, m. soleus (SL), and m. tibialis anterior. MVCs were reduced 10% and 12%, respectively, for KE(238±10 to 214±11 N·m) and AE (119±5 to 105±3 N·m) (p<0.05), while KF and AF MVCs did not change. Only SL showed a change in maximal root mean square EMG (29% increase, p<0.05) or EMG/MVC ratio (50% increase, p<0.05). Coactivation among antagonists was minimal and did not change. Instantaneous RTD(N·m·s-1) and EMD were not altered in any muscle group. These findings indicate: (1) lower extremity extensor rather than flexor strength decrement with unloading is rapid, (2) neither decreased neural activation of agonists nor increased coactivation of antagonists appear to account for the reduced MVC.