AHN, A.N.: Using in vivo stimulation and strain conditions to explain how two muscles function differently under identical conditions

Even when experiencing identical strain and stimulation conditions, two leg muscles of the cockroach, Blaberus discoidalis, function differently. Both muscles are innervated by the same, single excitatory motor neuron, extend a single degree-of-freedom joint, and have similar twitch kinetics and similar force-length and force-velocity properties when maximally stimulated. When both muscles were strained cyclically and stimulated phasically under identical conditions (8 Hz; 15% strain; in vivo stimulation pattern determined during running), muscle 178 generated mechanical energy over a cycle (10.1 W kg^-1; n = 5), while muscle 179 absorbed net mechanical energy (-14.7 W kg^-1; n = 6). Forces generated during shortening determined the main difference in mechanical function between the two muscles. Although the force-length relationships of muscles 178 and 179 were similar when maximally stimulated, the force-length relationships when submaximally stimulated showed that muscle 178 generated higher forces at shorter lengths than muscle 179. Differences between the two muscles in their submaximal force-length relationships and in force enhancement after a passive stretch could account for approximately 85% of the difference in peak force generated between the two muscles during oscillatory contractions. Using the in vivo conditions and examining the history-dependent properties of these muscles provide clues to how the muscles function differently under identical cyclical conditions. Understanding the functional consequences of submaximal stimulation and contraction history can reveal differences in force generation that are not apparent during maximally stimulated muscle in isometric and isotonic measurements.