Pitch then power: limitations to acceleration in quadrupeds

Abstract

Rapid acceleration and deceleration are vital for survival in many predator and prey animals and are important attributes of animal and human athletes. Adaptations for acceleration and deceleration are therefore likely to experience strong selective pressures--both natural and artificial. Here, we explore the mechanical and physiological constraints to acceleration. We examined two elite athletes bred and trained for acceleration performance (polo ponies and racing greyhounds), when performing maximal acceleration (and deceleration for ponies) in a competitive setting. We show that maximum acceleration and deceleration ability may be accounted for by two simple limits, one mechanical and one physiological. At low speed, acceleration and deceleration may be limited by the geometric constraints of avoiding net nose-up or tail-up pitching, respectively. At higher speeds, muscle power appears to limit acceleration.

Figure 1.

Free-body diagram of the stride-average forces acting on a generic quadruped of unvarying body geometry, assuming acceleration/deceleration are powered predominantly by limb torques. ⊕ denotes CoM, vertical ground reaction force (opposes weight), and horizontal force that accelerates/ decelerates (−) the animal. We consider only during braking and during propulsion. L_leg is the length of leg, L_cran and L_caud are the distances from CoM to hip/shoulder joint, respectively.

Figure 2.

(a–c). The body geometry used to create limiting stride-averaged accelerations for maximally accelerating greyhounds and accelerating and decelerating horses. The maximum net nose-up (a,b) or tail-up (c) pitching acceleration is determined by constraining the total ground force vector (dotted line) to be at the mean (mid-stance) hind (a,b) or fore (c) foot position and assuming that geometry is largely constant. The resulting maximum acceleration (d,e) and deceleration (f) predictions are indicated by horizontal grey bars. Bar thickness denotes 1 s.d. due to the range of body geometry measured (n = 5, horses and dogs). (d) Acceleration data for greyhounds (70 strides, 10 dogs). (e) Accelerating polo ponies (160 strides, five ponies). (f) Decelerating polo ponies (160 strides, five ponies). Each data point indicates mean speed and mean acceleration for a single stride; bar ends denote initial and final velocity of the stride. At low speeds, maximal accelerations are consistent with pitch avoidance. At higher speeds, lower maximum accelerations are achieved, consistent with a power constraint. Body-mass specific power requirements for the curves are 60, 30 and −23 W kg⁻¹ for figure parts d–f, respectively.