Relation between fibre composition and daily duration of spontaneous activity in ankle muscles of the cat.

UNLABELLED: This study concerns the relation between use and fibre type composition among limb muscles. The histochemical properties were investigated for ankle muscles from cats that had previously been studied in 24 hr electromyographic (EMG) recordings of daily spontaneous activity. We then reported average daily "duty times" (i.e. the percentage of total sampling time filled with EMG activity) of 1.9% for extensor digitorum longus (EDL), 2.1 and 4.0% for anterior and posterior sites of tibialis anterior (TA), 6.6 and 9.5% for anterior and posterior sites of peroneus longus (PL), and 13.9% for soleus (SOL). In the present experiments, muscles from which these data had been obtained were sectioned in a cryostat and stained for myofibrillar ATPase. Fibres were classified as type I (presumably slow) or II (presumably fast), the latter fibres being further categorized as IIA, IIB and a minor portion of transitional IIAB fibres. As expected, SOL was 100% type I. Among the muscles of mixed fibre-type composition ("mixed muscles"), a statistically significant difference in the mean percentages of type I fibres was found between TA or EDL (2.9-6.0%) vs. PL (11.8-14.6%). For TA the percentage of type I fibres was higher in posterior (deep) than in anterior (superficial) sampling regions; for PL no clear antero-posterior difference was found. A significant correlation was obtained between the percentage of type I fibres and the total duration of daily activity recorded from corresponding mixed muscle sites (5 different recording sites in 4 cats, totally 15 cases of successfully combined physiological and histochemical measurements, r = 0.76, P < 0.001). Similarly, within TA the total duration of daily activity was higher for sites with high (posteriorly) than for those with low (anteriorly) percentages of type I fibres. IN CONCLUSION: a "coarse-grain" relationship was found between fibre type composition and the duration of daily activity among mixed muscles. Possible mechanisms underlying this relationship are discussed.

Circadian and individual variations in duration of spontaneous activity among ankle muscles of the cat.

This article concerns the spontaneous motor behavior of cat hindlimb muscles and muscle regions using 24-h electromyographic (EMG) recordings. Previously, we found marked differences in average daily "duty time" (i.e., the percentage of total sampling time filled with EMG activity) between different muscles, or muscle portions. We have now analyzed systematic differences in duty time between (i) highly active (midday) and relatively inactive (midnight) periods, and (ii) individual cats. Differences between cats seemed to be associated with differences in motor habits. The midnight reduction in activity was particularly striking for muscles with a high midday activity. Quantitative differences in spontaneous activity (duty time), as compared between active and inactive periods of the day or among individual cats, were associated with marked qualitative alterations in the distribution of activity among the sampled muscles, i.e., these quantitative differences could not be described as a simple up- or downscaling of general motor activity.

Use and fibre type composition in limb muscles of cats.

As a background for studies concerning the effects of training on the properties and fibre type composition of skeletal muscle, information is needed concerning the normal duration of muscle use per day. Data of this kind were collected from adult cats, using implanted electrodes for electromyographic recording from hindlimb muscles acting across the ankle joint: extensor digitorum longus (EDL), tibialis anterior (TA), peroneus longus (PL), lateral gastrocnemius (LG) and soleus (SOL). The accumulated duration of any recorded activity was expressed, for each electrode site, as a percentage of total sampling time ("duty time"). As measured intermittently across 24 h periods (4 min sampling per 30 min), these duty times were markedly and significantly different among the various muscles, averages varying from 1.9% for EDL up to 9.5% for posterior PL and 13.9% for SOL. The distribution of activity across the various muscles was markedly different between highly active periods (mid-day) and periods of rest (mid-night). The 24 h duty times were strongly and significantly correlated to duty times obtained for only mid-day activity but not to those for only mid-night activity. Following the end of the physiological measurements, the animals were sacrificed and the muscles were analyzed with regard to fibre type composition (histochemistry). There was a significant positive correlation between the 24 h duty time and the percentage of type I fibres ("slow"). In the Discussion, the present results from cats are briefly compared to previously published data for humans.

Daily durations of spontaneous activity in cat's 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 project was to produce such background knowledge for hindlimb muscles of the cat. In four adult female cats, each one being studied in several experimental sessions, ankle muscles were chronically implanted with electrodes for electromyographic (EMG) recording. The muscles recorded from were: extensor digitorum longus (EDL), peroneus longus (PL), tibialis anterior (TA), lateral gastrocnemius (LG) and soleus (SOL). For PL, TA and LG, there were anterior as well as posterior recording sites. During 24-h experimental sessions, the studied animal stayed, together with another cat, in a box large enough for playing and walking around. 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 predominantly fast muscles of mixed-fibre composition (i.e. all studied muscles except SOL), marked and statistically significant differences in duty time were found, mean values varying fivefold from 1.9% (EDL) to 9.5% (PL, posterior site). For all three muscles with simultaneous recordings from different sites, consistent and statistically significant differences in daily duty time were found between anterior and posterior regions (anterior less than posterior for TA and PL; anterior more than posterior for LG). We also measured the extent to which each 4-min sampling 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 active during more sampling periods per day, but, whenever being used, their activity lasted relatively longer. Such results were consistent with the view that differences in mean 24-h duty time might largely reflect differences in the extent to which the various muscles and muscle regions were used for long-lasting stabilizing contractions.

Task-related differences in distribution of electromyographic activity within peroneus longus muscle of spontaneously moving cats.

In the hindlimb muscle peroneus longus (PerL) of cats, electromyographic activity (EMG) was recorded from anterior and posterior regions during voluntary motor behaviour. In spite of the fact that this muscle is composed of units that all exert their forces in the same direction, the intra-muscular EMG distribution differed in a marked and reproducible way between different types of motor behaviour. Anterior as well as posterior regions were both strongly active in relation to the swing-phase of stepping. In comparison to this stepping-activity, there was a marked predominance of posterior PerL activity during hindlimb standing (or take-off for a jump) and an equally pronounced predominance of anterior PerL activity when the cat was preparing to land from being lifted (or at the end of a jump). It is suggested that these task-associated differences in EMG distribution reflect topographical aspects of the intraspinal organization of motor tasks.