The fiber type profiles and contractile properties of selected elbow flexors and extensors of cat were investigated. FG, FOG, and SO fiber types were differentiated by staining cross sections of the muscles for ATPase and NADH diaphorase activities. Isometric contractile characteristics of whole muscles were measured with quasi-isolated muscle preparations electrically stimulated through their nerves. Elbow extensors ranged in mean fiber type composition from fast-twitch superficially located muscles such as the LTT with 85% fast-twitch (FG and FOG) fibers to deep slow-twitch muscle like ANC which consisted solely of SO fibers. The major flexors, BI and BR, both consisted primarily of fast-twitch fibers (75-80%). Mean contraction times (CT) and fatigue indexes (FI) for the ANC and MT were 78 ms and 0.87 and 60 ms and 0.76, respectively. For the LTT, LGT, BI, and BR, mean CT were all within 30 ± 2 ms, and the mean FI ranged from 0.33 to 0.44 (excluding LGT). Fast and slow muscle CT and FI could be predicted from SO and oxidative (SO + FOG) compositions, respectively. When stimulated at progressively increasing frequencies, the increase in percent of maximal tetanic tension for slow muscles (ANC and MT) was greatest from 5 to 15 pulses/s and for fast muscles (BI, BR, and LTT) between 35 and 50 pulses/s. The steepest portion of the tension-frequency curves for fast and slow elbow muscles coincided with the minimal and maximal primary range firing frequencies computed from previously published data for fast and slow alpha motoneurons. Extensors are divided into fast and slow groups: slow, uniarticular extensors are available at the elbow as they are at the knee and ankle for postural support, while their faster and larger synergists are capable of rapid and more forceful movements. Flexors at these three hinge joints, which are generally inactive during standing posture, are composed primarily of fast-twitch fibers.
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