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We examined the relation between elbow angle and isometric tension in evoked contractions in 28 normal subjects and carried out particular, detailed, investigations in eight of them. Results from two subjects herein presented give the overall outline of our findings, which show that the human biceps brachii muscle operates through a nearly complete lengthtension relation within the range of movement that it undertakes in co-operation with other muscles.

Changes produced by bromide, nitrate, and iodide in the isometric myogram of the fast-twitch extensor digitorum longus and the slow-twitch soleus muscles of the rat were studied in vitro at 35°C. Replacement of chloride with bromide, nitrate, or iodide produced only small changes in the isometric tetanic tension, but it produced a reversible potentiation of the isometric twitch tension. The order of the potentiating influence was iodide > nitrate > bromide. The tension potentiation was associated with increases in rate of tension rise and the time to peak of the isometric twitch. The results from the two muscles were essentially similar and they were qualitatively similar to those reported from frog twitch muscle. The form of tension enhancement is qualitatively similar to that obtained during summation of mechanical responses, and the results predict with sufficient accuracy the characteristics of the fully summated two-stimuli mechanical response. We suggest that changes in excitation-contraction coupling, similar to those produced by these anions, occur in mammalian skeletal muscle fiber at 35 to 38°C during summation of isometric mechanical responses.

Isometric contractile properties of the fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles of 24 male C57 mice were studied in vitro at 35°C. Ten animals exhibited the hereditary movement disorder known as spasticity. EDL muscles of spastic and normal mice developed similar specific tensions and they had similar twitch tension:tetanic tension ratios. The time-to-peak tension and the time to half relaxation in the twitch of spastic EDL muscles were, on average, slightly longer. SOL muscles of spastic and normal mice also developed similar specific tensions and their twitch contractions had similar times to half-relaxation. The time-to-peak tension in the twitch and the twitch tension: tetanic tension ratio of spastic SOL muscles were, on average, higher than in normal SOL. The significance of our findings remains somewhat uncertain.

Characteristics of summation of isometric mechanical responses were studied in in situ flexor digitorum longus (FDL) (fast-twitch) and soleus (slow-twitch) muscles of the cat at 37°C. The pattern of tension recruitment in short tetanic contractions was examined at different stimulation frequencies. At frequencies leading to the development of maximum tetanic tension, the second stimulus activation made the largest individual contribution to tension development. The tension developed with two stimuli was dependent on the interval separating them. When the first two stimuli were fixed at a short interval such that the muscle developed the highest two-response tension, the tension developed following application of the third stimulus was independent over a wider range of two-to-three stimulus intervals. Application of a stimulus shortly after another, in general, lengthened the contraction. The results from the two muscles were basically similar when allowance was made for the difference in their speed of contraction. The marked tension potentiation seen in the twitch of FDL, with staircase effect and with cooling, was largely absent in the well-fused double response. We suggest that the mechanical activation occurring during the isometric twitch is incomplete in both muscle fiber types and that a second stimulus applied shortly after the first brings about nearly complete mechanical activation.

Characteristics of summation of isometric mechanical responses elicited by two supramaximal nerve stimuli were examined at different temperatures (18 to 38°C) in a fast twitch (flexor digitorum longus) and a slow twitch (soleus) muscle, in situ , in the cat hind leg. At 37 to 38°C, both muscles developed a peak tension that was more than twice the twitch tension when a second stimulus was applied shortly after the first. The rate of rise of tension in such a summated response was also about, or more than, twice that of the twitch tension, and the time to peak tension was about 1.5 times the time to peak of the twitch. The peak tension in summated responses decreased with increasing the stimulus interval so that, when the interval was equal to the time to peak twitch, the tension developed was significantly less. Cooling significantly alters these characteristics in the fast twitch muscle, so that, at 20 to 22°C, the tension, the rate of rise of tension, and the time to peak tension enhancement produced by a second stimulus are markedly reduced; little or no change in the characteristics is seen with cooling of the slow twitch muscle. These results are examined in the light of some observations made on the pattern of summation in frog sartorius muscle at different temperatures (2 to 28°C). Cooling significantly increases the degree of activation occurring during the twitch in the fast twitch muscle fibers, whereas it has little or no effect on the degree of mechanical activation in the slow twitch muscle fibers. Furthermore, the results suggest that the tension, the rate of rise of tension, and, perhaps the time to peak tension of the isometric twitch in both the fast twitch and the slow twitch muscle fibers at 37 to 38°C are limited by the degree of mechanical activation and that a second stimulus applied shortly after the first can greatly overcome the effect of that limitation.

Changes produced by a preceding tetanus in the isometric twitch myogram were examined in cat soleus (slow twitch) muscle in situ, at 37°C, and with supramaximal nerve stimulation. A tetanus produced a depression in the twitch tension (P) accompanied by a shortening of time to peak (TP) and time to half-relaxation (HR), and a slight depression in the maximum rate of tension rise (MR). Recovery of P, TP, and HR, after a standard tetanus of 100 Hz for 300 ms, showed an early-fast-phase (time constant of 0.8 to 0.9 s) and a late-slow-phase (time constant of 70 to 80 s), whereas that of MR showed only the late phase of recovery. Analysis of the data indicated the occurrence of two post-tetanic influences which produce different maxima of tension depression at zero test interval and have different rates of recovery. The twitch tension depression, when examined at the same test interval (0.7 s), increased with increase in the number of stimuli in the preceding tetanus. The relationship between the post-tetanic twitch tension and the number of stimuli in the tetanus was approximately logarithmic and was similar for tetanic frequencies of 50, 100, and 200 Hz. Unlike in the fast-twitch muscles, there was no clear evidence of a post-tetanic influence which may potentiate the twitch tension: the most prominent post-tetanic effect, in this muscle, was an acceleration of the tension relaxation in the twitch.

The influence of temperature (range 35 to 20°C) on the isometric contractile properties of fast-twitch extensor digitorum longus (EDL) and slow-twitch soleus (SOL) muscles of the male C57 mice was studied in vitro, with direct stimulation. Cooling from 35 to 25°C resulted in an average twitch tension potentiation of 42% in EDL and 13% in SOL muscles. Further cooling to 20°C did not produce further potentiation in EDL muscles. The time-to-peak tension and the time to half-relaxation increased 2.4 to 3.2 times for a 10°C cooling in both muscles. The maximum tetanic tension was little changed in cooling from 35 to 30°C, but was depressed 16 to 19% in cooling to 20°C in both muscles. These results from mouse fast and slow muscles were compared with previously published data from muscles of the rat and the cat. The behaviors of the slow SOL muscles in the three species were found to be significantly different, but they could be explained on the basis of their muscle fiber-type composition.