Circadian Force and EMG Activity in Hindlimb Muscles of Rhesus Monkeys

1 Department of Physiological Science, 2 Brain Research Institute, and 3 Department of Neurobiology, University of California, Los Angeles, California 90095; 4 Department of Psychology, Georgia State University, Atlanta, Georgia 30303; 5 Oregon Regional Primate Research Center, Beaverton, Oregon 97006; and 6 Institute of Biomedical Problems, Moscow 123007, Russia Hodgson, J. A., S. Wichayanuparp, M. R. Recktenwald, R. R. Roy, G. McCall, M. K. Day, D. Washburn, J. W. Fanton, I. Kozlovskaya, and V. R. Edgerton. Circadian Force and EMG Activity in Hindlimb Muscles of Rhesus Monkeys. J. Neurophysiol. 86: 1430-1444, 2001. Continuous intramuscular electromyograms (EMGs) were recorded from the soleus (Sol), medial gastrocnemius (MG), tibialis anterior (TA), and vastus lateralis (VL) muscles of Rhesus during normal cage activity throughout 24-h periods and also during treadmill locomotion. Daily levels of MG tendon force and EMG activity were obtained from five monkeys with partial datasets from three other animals. Activity levels correlated with the light-dark cycle with peak activities in most muscles occurring between 08:00 and 10:00. The lowest levels of activity generally occurred between 22:00 and 02:00. Daily EMG integrals ranged from 19 mV/s in one TA muscle to 3339 mV/s in one Sol muscle: average values were 1245 (Sol), 90 (MG), 65 (TA), and 209 (VL) mV/s. The average Sol EMG amplitude per 24-h period was 14 µV, compared with 246 µV for a short burst of locomotion. Mean EMG amplitudes for the Sol, MG, TA, and VL during active periods were 102, 18, 20, and 33 µV, respectively. EMG amplitudes that approximated recruitment of all fibers within a muscle occurred for 5-40 s/day in all muscles. The duration of daily activation was greatest in the Sol [151 ± 45 (SE) min] and shortest in the TA (61 ± 19 min). The results show that even a "postural" muscle such as the Sol was active for only ~9% of the day, whereas less active muscles were active for ~4% of the day. MG tendon forces were generally very low, consistent with the MG EMG data but occasionally reached levels close to estimates of the maximum force generating potential of the muscle. The Sol and TA activities were mutually exclusive, except at very low levels, suggesting very little coactivation of these antagonistic muscles. In contrast, the MG activity usually accompanied Sol activity suggesting that the MG was rarely used in the absence of Sol activation. The results clearly demonstr