Effects of fatigue on the catchlike property in a turtle hindlimb muscle.

The purpose of this report was to test for the possibility that a catchlike, force-enhancing property, attributable to a particular stimulation pattern, could be evoked in non-mammalian turtle muscle, just as it has been shown in mammalian muscle. We tested for the presence of this property in dynamic lengthening and shortening contractions, as well as in the more commonly studied isometric contractions. A second aim was to note the effects of fatigue on the catchlike property, if the latter was present. The force response of the external gastrocnemius muscle in the adult turtle, Pseudemys ( Trachemys) scripta elegans, was compared for a control, constant-frequency 10 Hz, 1-s duration stimulation pattern using 0.1-ms pulses vs. the same pattern, but with two additional pulses within the first 100-ms interspike interval of the control stimulus train. This latter train produced a pronounced and prolonged enhancement of muscle force, which was attributed to a catchlike effect. It was greatly increased when the muscle was in a fatigued state. The extent of this force enhancement was significantly different for the three contraction types, and generally in the order: isometric>lengthening>shortening contraction. These differences were greater in fatigued vs. fresh muscle. Comparative aspects and potential mechanisms underlying the catchlike effect are discussed.