EFFECTS OF INCLINE ON CHAMELEON LOCOMOTION: IN VIVO MUSCLE ACTIVITY AND THE THREE-DIMENSIONAL HINDLIMB KINEMATICS

Arboreal animals, especially lizards, often traverse three-dimensional networks of narrow perches with variable inclines, but the effects of both incline and narrow surfaces on the locomotor movement and function of limbs are poorly understood. Thus, I first quantified the three-dimensional hindlimb kinematics of a specialized arboreal lizard, Chamaeleo calyptratus, moving horizontally, and up and down a 30° incline on a narrow (2.4cm) perch and a flat surface. I compared the flat-surface data of C. calyptratus with those of an anatomically generalized terrestrial lizard, Dipsosaurus dorsalis. Second, I studied the hindlimb muscle activity of C. calyptratus on inclines of -45°, 0°, and 45°. I quantified electromyograms (EMGs) from nine hindlimb muscles, and correlated EMGs with three-dimensional hindlimb kinematics. Inclines had significant main effects for relatively few kinematic variables of C. calyptratus (11%) compared to D. dorsalis (73%). For kinematics of C. calyptratus, the main effects of the flat surface versus round perch were nearly three times more widespread than those of incline. The foot of C. calyptratus was markedly anterior to the hip at footfall, primarily as a result of an unusually extended knee for a lizard. A large amount of knee flexion during early stance may be used by C. calyptratus to actively pull the body forward in a manner not found in most other lizards and vertebrates. Despite the kinematics changing little with incline, the EMGs changed substantially. Most of the changes in EMGs were for amplitude rather than timing, and the hip and thigh muscle EMGs had more conspicuous changes with incline than those of the lower limb muscles. The knee flexion of C. calyptratus during the first half of stance was correlated with two large knee flexors which both increased in amplitude when moving uphill compared to level and downhill. Thus, early stance probably contributes significantly to propulsion in C. calyptratus. During stance, the EMGs of the caudofemoralis muscle in C. calyptratus correlated well with femur retraction, knee flexion, and posterior femur rotation, which provide propulsive forces. Many of the muscles in the hindlimb of C. calyptratus changed activity with incline in a manner similar to the propulsive limb muscles in mammals.