Circadian force and EMG activity in hindlimb muscles of Rhesus monkeys

Continuous intramuscular electromyograms (EMGs) were recorded from the sole us (Sol), medial gastrocnemius (MG), tibialis anterior (TA), and vastus lat eralis (VL) muscles of Rhesus during normal cage activity throughout 24-h p eriods and also during treadmill locomotion. Daily levels of MG tendon forc e and EMG activity were obtained from five monkeys with partial datasets fr om three other animals. Activity levels correlated with the light-dark cycl e with peak activities in most muscles occurring between 08:00 and 10:00. T he lowest levels of activity generally occurred between 22:00 and 02:00. Da ily 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 muV, compared wi th 246 muV 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 muV, respec tively. EMG amplitudes that approximated recruitment of all fibers within a muscle occurred for 5-40 s/day in all muscles. The duration of daily activ ation 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 similar to9% of the day, whereas less active muscl es were active for similar to4% 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 leve ls, suggesting very little coactivation of these antagonistic muscles. In c ontrast, the MG activity usually accompanied Sol activity suggesting that t he MG was rarely used in the absence of Sol activation. The results clearly demonstrate a wide range of activation levels among muscles of the same an imal as well as among different animals during normal cage activity.