Pitch then power: limitations to acceleration in quadrupeds.

Sarah B Williams,Alan M Wilson,Huiling Tan,James R Usherwood

doi:10.1098/rsbl.2009.0360

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.

Highlights

Acceleration requires power from muscles to increase the kinetic energy of the centre of mass (CoM)
The model is similar to that proposed by Gray (1944) for animals standing on inclined surfaces and is simplified by applying the following assumptions and constraints: (i) a net pitching acceleration over a stride is avoided; (ii) the body geometry is constant: this assumption is deliberately simplistic, ignoring motions of the head with respect to the CoM (contrasting with a similar formulation expressed for lizards (Aerts et al 2003)); and (iii) accelerations are presumed to be powered by torque of the limb, rather than limb extension (Biewener 1989; Williams et al 2009)— the feet are drawn directly beneath the hip/shoulder in figures 1 and 2
The muscle-specific powers associated with near-maximal accelerations for a range of bipeds are high: accelerating turkeys (Roberts & Scales 2002) can reach a mean of 55– 60 W kg21 over a complete gait cycle

Summary

Introduction

Acceleration requires power from muscles to increase the kinetic energy of the centre of mass (CoM). Acceleration and deceleration may be limited by the geometric constraints of avoiding net nose-up or tail-up pitching, respectively.

Results

Conclusion