Figure 1. Bipedal human walking. Footprints show step and stride lengths. Lower panel is a gait diagram with horizontal lines representing the time when each foot is on the floor. LF, left foot; RF, right foot, a, double-support phase; b, single-support phase; c, aerial plane. Note that the x axis is relative time; the actual time taken to complete a running stride is less than that for a walking stride.

Figure 2. Graph of relative stride length (, stride length; h, hip height) plotted against ‘relative speed’ (u, speed; g, gravitational acceleration) for walking and running humans and horses, and half-bounding bandicoots (*).

Figure 3. Gait diagrams for commonly used quadrupedal and bipedal gaits. LH, left hind foot; LF, left fore foot; RF, right fore foot; RH, right hind foot; R, right foot; L, left foot.

Figure 4. Stiff-legged walking model showing the arc travelled by the centre of mass (COM). For the COM to travel at speed u in the arc of a circle of radius h, there must be an acceleration u²/h towards the centre of the circle. Therefore, maximum walking speed is approximated by the equation: u_max = Ögh, where g is gravitational acceleration. Potential and kinetic energies (PE and KE) associated with the COM oscillate out of phase of each other during walking.

Figure 5. (a) Diagram of a hindlimb of a kangaroo indicating the ground reaction force (GRF) and the tension (T) in the calf muscles, required to balance this force. R and r are the moment arms of these two forces about the ankle joint and r/R is the effective mechanical advantage. (b) Graph indicating how the effective mechanical advantage (EMA) increases with body size as animals adopt more upright limb postures. Hopping kangaroos are an exception to this rule.

Figure 6. Lines connecting the three supporting feet produce a triangle of support. If the centre of mass (•) falls within this area then the animal is in balance and is stable. Arrows indicate which foot is being lifted and moved forwards in this walking gait.

Figure 7. Graph showing how maximum running speed varies with body mass. The solid line represents the mean performance for placental mammals over the whole size range, but note that there is a large degree of scatter of data (not shown) about this line. Open triangle, porcupine; filled square, cheetah; filled triangles, kangaroos and wallabies.

Figure 8. Graphs of how stride frequency varies (a) as an individual animal increases speed (W, walk; T, trot; G, gallop; B, bound) and (b) as a function of body mass at physiologically equivalent speeds (i.e. the trot–gallop transition).

Figure 9. Tracings from a video sequence of a kangaroo using a bipedal hop. Kinetic and potential energies of the body (1, 2) are temporarily converted to strain energy in tendons (3, 4) and are returned to the body during elastic recoil (5–7), helping accelerate the body forwards and upwards.

Figure 10. Mass-specific oxygen (= energy) consumption as a function of speed. Data are derived from treadmill studies. Dotted lines indicate the predicted curves for placental mammals of the masses indicated. Solid lines show empirical data for macropodid marsupials, with the 5-kg and 18-kg hopping animals uncoupling the cost of locomotion from speed.