Muscle Recruitment During Terrestrial Locomotion: How Speed and Temperature Affect Fibre Type Use in a Lizard

Synchronized electromyography and cinematography were used to study the activity of the red and white regions of the iliofibularis muscle in savannah monitor lizards, Varanus exanthematicus (Bosc), during locomotion. Analysis of variance on results from four individuals moving at speeds of up to 1.5 km h⁻¹ at two body temperatures (25 and 35°C) revealed that all kinematic variables were significantly affected by speed but none was affected by temperature. Hence, patterns of limb movement at any speed were similar at both temperatures. However, this similarity resulted from differences in muscle activity. Analysis of variance on electromyographic variables for activity in the red and white regions showed widespread significant effects of both temperature and speed. The red region was active at all speeds, and it displayed regular bursts of activity which usually occurred when the foot was above the ground, the femur was being abducted and the knee flexed. Variables measuring the intensity of red region activity generally increased with speed until a maximum was attained and no further change occurred with additional increases in speed. The speed at which maximum red activity was attained at 25°C was less than that at 35°C. For equal locomotor speeds, amplitudes of electromyograms (EMGs) from the red region at 25°C were greater than those at 35°C. In contrast to the red region, the white region was active only above some threshold speed, and activity was often rather irregular compared to that of the red region. At 25°C the threshold speed for recruitment of the white region (0.9 km h⁻¹) was less than that at 35°C (1.3 km h⁻¹). The relationship between locomotor speed and activity per minute for the red region was very similar to the relationship between speed and rate of oxygen consumption described in previous studies of lizards, and the threshold speed for recruitment of the white region was also similar to the maximal aerobic speed previously reported for this species. Hence, lizards increase speed and compensate for lower temperature by increasing intensity of activity within the red region and recruiting fibres in the white region. We suggest that compensation for muscle function at decreased body temperature may involve recruitment of greater numbers of motor units.