THE TIMING OF MUSCLE STRAIN AND ACTIVATION DURING STEADY SWIMMING IN A SALAMANDER, AMBYSTOMA-MEXICANUM

Muscle function during undulatory swimming is commonly described by th e relative timing of muscle activity (EMG) in the strain cycle and has been studied for several fish species, ranging from anguilliforms to carangiforms. This paper supplies the basic data for steady swimming a t an intermediate speed in a salamander (Ambystoma mexicanum). The str ain cycle of the swimming muscles is estimated using high-speed video recordings (500 frames s(-1)) of an animal swimming in a flow tank. Sy nchronously, EMG signals of six epaxial myomeres were recorded using b ipolar electrodes inserted unilaterally along the body. The neural sti mulation pattern is a head-to-tail travelling wave with a higher speed than the kinematical propulsive wave. The resultant phase delay cause s a different muscle recruitment pattern along the body axis, similar to reports for anguilliform fishes like the lamprey (WILLIAMS et al., 1989) and the eel (GRILLNER & KASHIN, 1976). The anterior trunk myomer es (up to 45% of the total body length L) are activated purely concent rically (i.e., while shortening). Caudal to the 45% L position, the mu scles show an increasing proportion of eccentrical activation. This pa ttern suggests that there is gross positive work delivery along the bo dy axis, with a limited amount of negative work in the tail tip. This is in agreement with the general conclusions for fish of WARDLE & VIDE LER (1994), where this recruitment pattern is associated with a body s hape without a distinct tail blade and with direct and continuous thru st production along the body.