The Catchlike Property in Skeletal Muscle: Influence of Contraction and Muscle‐Type on Augmented Contractile Function

In slow and fast motor units of skeletal muscle, discharge rates are modulated to match and preserve target force levels. Although relatively constant patterns in firing rates are most common, two or more closely‐spaced pulses at the onset of activation may appear in both human and mammalian skeletal muscles. The outcome of this aberration in motor unit firing pattern has been shown to increase both the rate and extent of force development; an effect referred to as the catchlike property (CLP). The purpose of this study was to explore the contraction (i.e. isometric vs. concentric) and muscle‐type (fast vs. slow twitch) dependence of the CLP in extensor digitorum longus (EDL) and soleus muscles of the mouse (n=10 for all data). The addition of a single pulse 10ms at the onset of an otherwise constant frequency train (CFT) was utilized to augment force and work output of the contractile event, referred to as the catchlike‐inducing train (CLT). When compared to the CFT, the peak force during the CLT of soleus muscles was increased to 1.15±0.03 and 1.39±0.03 during isometric (ISO) and concentric (CON) contractions, respectively. In EDL muscles, however, forces were increased to 1.47±0.05 and 1.20±0.04 for ISO and CON contractions; demonstrating a muscle and contraction‐type dependence (p <0.05). Similarly, the work done and/or force‐time integral increased to 1.34±0.03 and 1.21±0.03 (soleus), and 1.43±0.05 and 1.18±0.02 (EDL), for ISO and CON contractions, respectively. However, when normalized for the number of pulses within each contraction, no net increase in work was evident. Therefore, the catch‐like property may serve primarily as a mechanism for augmenting initial force levels rather than as a modulator of contractile efficiency within skeletal muscle.