PATTERNS OF HIND-LIMB MOTOR OUTPUT DURING WALKING IN THE SALAMANDER DICAMPTODON TENEBROSUS, WITH COMPARISONS TO OTHER TETRAPODS

Based on similarity of motor patterns of lizards, crocodiles, birds an d mammals, various authors have concluded that a number of homologous muscles across these taxa demonstrate neuromuscular conservatism. This hypothesis remains untested for more basal taxa. Therefore, a quantit ative electromyographic study of the hind limb during treadmill walkin g (mean speed of 0.75 SVL/s) in the salamander Dicamptodon tenebrosus was undertaken. Muscles located ventrally on the hind limb become acti ve just before foot placement on the substrate, and maintain activity through the first half of the stance phase. Dorsally located muscles b egin activity at or just before the start of the swing phase, and fire through the first half of swing. Several muscles showed a secondary E MG burst during the stride. The second burst in most ventral muscles o ccurred in late stance. In all dorsal muscles with double bursts, the second burst occurred in the middle of stance. Comparison of electromy ographic onset and offset values for Dicamptodon to those for presumed homologues in other tetrapods reveals similarity in activity patterns for all ventral and two dorsal muscles despite anatomical rearrangeme nts, supporting the hypothesis of neuromuscular conservatism for some muscles but not others.