Changes in muscle stiffness produced by motor units of different types in peroneus longus muscle of cat

Abstract

1. The effects of maximal tetanic contractions of varying numbers of motor units of the same type [slow (S), fast fatigue-resistant (FR), or fast fatigable (FF)] on the mechanical responses to muscle stretch were studied in the peroneus longus muscle of anesthetized cats. 2. Two types of stiffness measurements were made: 1) an average stiffness, defined as the tension change from the beginning to end of a 0.5-mm ramp stretch; and 2) a dynamic stiffness, defined as the ratio of peak-to-peak tension to amplitude of a maintained 85-microns sinusoidal stretch at frequencies of 10-80 Hz. 3. Contractions of slow and fast units elicited different increases in average stiffness. Type S units, although developing much smaller tetanic tensions than fast ones, produced a resistance to stretch comparable with or greater than that of fast units developing much higher tensions. 4. For comparable tetanic tensions, slow units also elicited a greater dynamic stiffness than fast units. During sinusoidal stretch, changes in muscle tension led changes in muscle length during contraction of S units, but the reverse was observed for frequencies 30-50 Hz during contraction of FF units. This suggests that the latter perform oscillatory work on the driving apparatus. 5. Type S units, whose low-threshold motoneurons are the first to be recruited, appear well adapted to play a role in posture and in slow movements because of the resistance they offer to forces tending to change joint position or to oppose the progression of slow movements.

Evolution of contractile and elastic properties of rat soleus muscle fibres under unloading conditions

“…On the contrary, a relative increase in slow fibres induced a decrease in soleus muscle compliance (Goubel & Marini, 1987). These suggestions have been supported by Petit et al (1990) who found that, for comparable tetanic tensions, slow motor units of cat peroneus longus produced a lower compliance than fast units.…”

Differential adaptations during growth spurt and in young adult rat muscles

“…These studies suggest that slow-twitch fibres and fasttwitch fibres also differ according to their elastic properties. Moreover, it was reported that the slow-twitch motor unit of a cat's peroneus longus had a greater dynamic stiffness than fast-twitch motor units (Petit et al, 1990). Therefore, for soleus muscle, the growth-induced increase in SEC stiffness can be explained at least partly by the fast-to-slow fibre-type transition observed for this muscle.…”

Tension Distribution of Single Motor Units in Multitendoned Muscles: Comparison of a Homologous Digit Muscle in Cats and Monkeys

“…This qualitative method of classification produced groups of MUs the properties of which generally were consistent with those in other studies (see Results and Discussion below). Nevertheless, because each MU was assigned to one of the three physiological types based on our qualitative assessment of three properties (tension, tetanic fusion, and sag), and because we did not confirm these classifications with a fatigue test, some MUs may have been misclassified Botterman et al, 1985;Petit et al, 1990;. Using the quantitative fatigue test, however, Petit et al (1990) found that this qualitative method only occasionally misclassified a fast intermediate (FI) MU as type FF.…”

“…MUs were classified into three physiological types-slow (S); fast fatigue-resistant (FR), and fast fatigable (FF)-by comparing the features of tension records from 20 versus 40 Hz tetani (Petit et al, 1990). As illustrated in Figure 1 A, slow MUs developed slowly rising tension, without sag during the 20 Hz tetanus.…”

“…Instead, assuming that both muscles contain all three types of MUs, we assigned each MU to one of these three physiological types by comparing the features of its tension responses during 20 and 40 Hz tetani. Our method of classifying the physiological type of each MU was similar to that of Petit et al (1990), who found that this qualitative method resulted in few misclassifications when compared to classification based on more extensive examination, including a fatigue test. In this way, cat EDLat MUs in our sample were found to be 22.4% type S, 45.3% FR, and 32.3% FF, similar to the composition of other cat forelimb and hindlimb muscles (Olson and Swett, 1966;Goslow et al, 1972;Dum and Kennedy, 1980;Dum et al, 1982;Botterman et al, 1985;EmonetDenand et al, 1988;.…”