Groucho running

Abstract

An important determinant of the mechanics of running is the effective vertical stiffness of the body. This stiffness increases with running speed. At any one speed, the stiffness may be reduced in a controlled fashion by running with the knees bent more than usual. In a series of experiments, subjects ran in both normal and flexed postures on a treadmill. In other experiments, they ran down a runway and over a force platform. Results show that running with the knees bent reduces the effective vertical stiffness and diminishes the transmission of mechanical shock from the foot to the skull but requires an increase of as much as 50% in the rate of O2 consumption. A new dimensionless parameter (u omega 0/g) is introduced to distinguish between hard and soft running modes. Here, omega 0 is the natural frequency of a mass-spring system representing the body, g is gravity, and u is the vertical landing velocity. In normal running, this parameter is near unity, but in deep-flexed running, where the aerial phase of the stride cycle almost disappears, u omega 0/g approaches zero.

Biomechanics and energetics of walking on uneven terrain

“…This suggests that subjects adopted a slightly more crouched posture on uneven terrain, perhaps associated with increased EMG activity in the thigh muscles. Past research has suggested that vertical stiffness decreases with a more crouched posture, for both human running (McMahon et al, 1987) and walking (Bertram et al, 2002). A more crouched limbed posture on uneven terrain might also increase compliance and provide a smoother gait, albeit at higher energetic cost.…”

The three-dimensional locomotor dynamics of African (Loxodonta africana) and Asian (Elephas maximus) elephants reveal a smooth gait transition at moderate speed

“…Why might large mammals change their gait at relatively slower speeds (Fr!0.40)? We speculate that this may be because more compliant limb mechanics can attenuate transient impact forces on the feet (McMahon 1985;McMahon et al 1987;Blickhan 1989), and hence reduce the risk of limb injury incurred by the massive body weight. In addition, bouncing limb mechanics helps to modulate vertical CM oscillations Schmitt et al 2006), which may be particularly critical for the locomotor stability (and efficiency) of large quadrupeds.…”

“…Inverted pendulum-like movement of the body's CM during slower locomotion is characterized by exchange of potential and kinetic energies associated with conservation of mechanical energy via a vaulting mechanism McGeer 1992;Biewener 2006;Biknevicius & Reilly 2006). During running, trotting, hopping or Groucho (compliant) walking/running, kinetic and potential energies fluctuate in phase, but energy is conserved to some extent as the body bounces on somewhat elastic legs McMahon et al 1987;Farley et al 1993). Despite their massive body weights and ponderous motions, elephants are the most economical living land animals, moving very cheaply at their normal walking speed in terms of metabolic cost of transport (J kg K1 m K1 ; African elephant data from Langman et al 1995).…”

Gait selection in the ostrich: mechanical and metabolic characteristics of walking and running with and without an aerial phase

“…Furthermore, grounded running in humans involves considerably larger knee flexion, which requires much greater extensor-muscle force (McMahon 1985). Both of these factors may explain why grounded running in humans is nearly 50% more expensive than normal running at the same speed (McMahon et al 1987), which in turn offers a very reasonable explanation as to why humans avoid this mode of locomotion. It is important to note, however, that there are certain circumstances in which grounded running in humans is, in fact, their preferred gait.…”