M4 Vertical Specialized Locomotion

Topic

Vertical locomotion requires supporting the body's full weight while simultaneously generating the propulsion needed to move upward.

To support body weight without slipping while climbing, the biological friction between the limbs and the substrate must exceed the component of weight acting parallel to the surface; as long as the available friction exceeds this weight component, the animal does not slip. However, if the slope becomes too steep, the weight component parallel to the surface eventually surpasses the available friction, causing the animal to lose its grip. The slope angle at which this transition occurs defines the maximum incline the animal can climb, and the available biological friction varies enormously depending on the surface: ranging from a low value—merely a fraction of the animal's weight—on smooth surfaces, to a value several times the animal's weight when it possesses claws that dig into rough wood.

Alongside friction-based climbing, another form of specialized vertical locomotion is the brachiation seen in gibbons, which swing while suspended from branch to branch. A gibbon hanging from a branch acts as a compound pendulum whose pivot point—the branch itself—also moves; the maximum speed reached at the lowest point of the swing is determined by the conservation of energy, based on the angle from which the animal released the previous branch. In ricochetal brachiation, the gibbon also incorporates aerial phases between branches, reaching speeds of ten to fifteen meters per second during these airborne intervals. Maintaining proper posture during this form of brachiation requires continuous correction of the disturbances introduced by the swinging motion itself: the control system corrects each disturbance by combining a response proportional to the magnitude of the postural deviation—acting as stiffness—with a response proportional to the rate of change of that deviation—acting as damping. This combined correction occurs within twenty to one hundred milliseconds of the disturbance's onset.

In addition to this postural correction, primates possess grasping reflexes that counteract disturbances affecting the hand or foot before the grip is compromised, operating with a latency of twenty to forty milliseconds. The total response time to a disturbance comprises, in sequence, the signal's travel time along the nerve, the synaptic delay between neuron and muscle, and the intrinsic delay of muscle activation—collectively totaling between thirty and eighty milliseconds. It is precisely this total latency that limits the frequency at which the animal can actively compensate for repeated disturbances while supporting its full body weight during this vertical mode of locomotion.

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