DAILY DURATIONS OF SPONTANEOUS ACTIVITY IN CATS ANKLE MUSCLES

For an understanding of how various degrees of altered use (training, disuse) affect the properties of skeletal muscles, it is important to know how much they are used normally. The main aim of the present proj ect was to produce such background knowledge for hindlimb muscles of t he cat. In four adult female cats, each one being studied in several e xperimental sessions, ankle muscles were chronically implanted with el ectrodes for electromyographic (EMG) recording. The muscles recorded f rom were: extensor digitorum longus (EDL), peroneus longus (PL), tibia lis anterior (TA), lateral gastrocnemius (LG) and soleus (SOL). For FL , TA and LG, there were anterior as well as posterior recording sites. During 24-h experimental sessions, the studied animal stayed, togethe r with another cat, in a box large enough for playing and walking arou nd. Using telemetric techniques, samples of EMG signals were recorded on tape for 4 min every 30 min. In an off-line analysis, measurements were made of the total accumulated duration of activity from each one of the studied muscle regions. These ''duty times'' were expressed as a percentage of total sampling duration. When averaged over the whole 24-h experimental period, the mean duty times per muscle region varied from 1.9% for EDL up to about 13.9% for SOL. Also, among predominantl y fast muscles of mixed-fibre composition (i.e. all studied muscles ex cept SOL), marked and statistically significant differences in duty ti me were found, mean values varying fivefold from 1.9% (EDL) to 9.5% (P L, posterior site). For all three muscles with simultaneous recordings from different sites, consistent and statistically significant differ ences in daily duty time were found between anterior and posterior reg ions (anterior less than posterior for TA and FL; anterior more than p osterior for LG). We also measured the extent to which each 4-min samp ling period was filled with activity (if any). As compared to muscles with a low mean 24-h duty time, those with high duty times were not ac tive during more sampling periods per day, but, whenever being used, t heir activity lasted relatively longer. Such results were consistent w ith the view that differences in mean 24-h duty time might largely ref lect differences in the extent to which the various muscles and muscle regions were used for long-lasting stabilizing contractions.