Two adult squirrel monkeys (one of each sex) were trained to locomote on a motor-driven treadmill at seven fixed speeds within 0.89–2.58 m/second. Subsequently, the animals were filmed while locomoting at these speeds, with the films then being used to determine gait parameters. The female monkey exhibited lateral-sequence symmetrical gaits at slow speeds, and transverse and rotatory gallops at fast speeds. The male used lateral-sequence symmetrical gaits at all speeds. The squirrel monkeys exhibited typical relationships between temporal gait parameters and increased speed, with these relationships continuing across the run-gallop transition. An analysis-of footfall intervals (delays) and support patterns indicated that during symmetrical gaits these monkeys emphasized lateral support. During galloping, one delay, the trailing hindlimb to trailing forelimb, was found to be very stable, both across speeds and for both types of galloping. Also during galloping, swing duration was found to be more variable than stance duration.
These findings are interpreted with regard to understanding the neural control of locomotion in tetrapods, and more specifically, in primates. The latter part of this discussion revolves around three basic differences between primate and non primate gaits: (1) utilization of diagonal-sequence gaits by primates (although the squirrel monkeys were atypical in this regard); (2) absence of a running trot in primates; and (3) absence of stepping following spinal cord transection in primates. These three differences reflect greater supraspinal control of locomotion in primates, and possibly intrinsic differences in the coupling of spinal locomotor generators.