Contribution of limb momentum to power transfer in athletic wheelchair pushing

Abstract

Pushing capacity is a key parameter in athletic racing wheelchair performance. This study estimated the potential contribution of upper limb momentum to pushing. The question is relevant since it may affect the training strategy adopted by an athlete. A muscle-free Lagrangian dynamic model of the upper limb segments was developed and theoretical predictions of power transfer to the wheelchair were computed during the push phase. Results show that limb momentum capacity for pushing can be in the order of 40 J per push cycle at 10 m/s, but it varies with the specific pushing range chosen by the athlete. Although use of momentum could certainly help an athlete improve performance, quantifying the actual contribution of limb momentum to pushing is not trivial. A preliminary experimental investigation on an ergometer, along with a simplified model of the upper limb, suggests that momentum is not the sole contributor to power transfer to a wheelchair. Muscles substantially contribute to pushing, even at high speeds. Moreover, an optimal pushing range is challenging to find since it most likely differs if an athlete chooses a limb momentum pushing strategy versus a muscular exertion pushing strategy, or both at the same time. The study emphasizes the importance of controlling pushing range, although one should optimize it while also taking the dynamics of the recovery period into account.

Introduction

One way to improve athletic wheelchair racing speed is to increase energy transfer to the wheelchair. Several variables may affect energy transfer, ranging from equipment design (Slowik and Neptune, 2013, Costa et al., 2009, Jacobson, 1995, MacLeish et al., 1993), athlete anthropometry, handicap and neuromuscular capabilities (Vanlandewijck et al., 2011), to the specific motor control strategy chosen to accomplish the task (Vegter et al., 2015, Rankin et al., 2012, Goosey et al., 1997). In fact, upper limb configuration adopted by an athlete while pushing directly influences a number of limb characteristics likely involved in energy transfer to the wheelchair, including: force production capability, hand stiffness (Rancourt and Hogan, 2009), dexterity and manipulating ability (Yoshikawa, 1990) i.e. its ability to maneuver the hand in space. In addition, athletes are free to choose a specific pushing range, contacting the pushrim at a given attack angle, to further leave it at the release angle, after a given wheel rotation. Specific reasons for selecting one pushing range over another are not yet well understood. For instance, athletes could choose a pushing range to maximize energy transfer to the wheelchair, or to simply reduce muscle fatigue.

Energy transfer to the wheelchair can be achieved in two ways during the push phase, defined as the time period while the hand contacts the pushrim. Firstly, athletes can transfer energy to the wheelchair through muscular exertion during the push phase. Secondly, athletes may take advantage of their upper limb momentum, both angular and linear. Indeed, considerable limb momentum can be accumulated during the recovery phase, defined as the time period while the hand does not contact the pushrim. This momentum can be exploited during the push phase to exert forces on the pushrim. Currently, the contribution of limb momentum to wheelchair pushing is unknown. This question is relevant since it may provide new insights on training strategies that may enhance performance or for prevention of musculo-skeletal injuries (Sosnoff et al., 2015, Boninger et al., 2002).

The purpose of this study was to estimate the potential contribution of upper limb momentum to pushing by computing the power transferred to the wheelchair by a passive, four-bar linkage during a single push. Pushing contribution was compared to the actual power produced by two T54 Paralympic athletes, seated in their own wheelchairs fixed on a high performance ergometer.