Movement control strategies during jumping in a lizard (Anolis valencienni)

Abstract

Whereas maximal performance is subjected to specific control criteria, sub-maximal movements theoretically allow for an infinite number of control strategies. Yet sub-maximal movements are predominant in the locomotor repertoire of most organisms and often little understood. Previous data on sub-maximal vertical jumping in humans has suggested that a movement effectiveness criterion might best explain the observed control strategy employed. Here we test the generality of this criterion in jumping by inducing lizards to jump both at a range of distances as well as a range of take-off angles. Our results show that while movement effectiveness appears to best explain jumping for different take-off angles, a ‘push harder’ strategy (i.e. mostly increasing the force output of the system), is used in the control of distance jumping. Thus, our data support the generality of the movement effectiveness criterion for vertical jumping, but not for distance jumping. Sub-maximal distance jumping in the lizard Anolis valencienni appears to be governed by a relatively simple control strategy that allows a rapid response. This accords well to the ecological circumstances in which long jumps are typically used (escape from predators).

Introduction

Jumping is a complex behavior involving the coordination of multiple segments to obtain the rapid displacement of center of mass away from its initial position (Bobbert and van Ingen Schenau, 1988; Aerts, 1998; Ashby and Heegaard, 2002; Gregersen and Carrier, 2004). Maximal jumping has been intensively studied and involves an optimization of the neuromuscular control (Bobbert and van Ingen Schenau, 1988; Selbie and Caldwell, 1996; Seyfarth et al., 1999; Spagele et al., 1999; Zajac, 2002; Hof, 2003). However, whether these same control strategies also apply to sub-maximal movements is unknown.

Yet, the majority of movements made by organisms are not maximal effort movements (Irschick and Losos, 1998), making it of paramount importance to understand how sub-maximal movements are controlled (Van Zandwijk et al., 2000; Vanrenterghem et al., 2004). As the control of sub-maximal movements can theoretically be achieved in an infinite number of ways underlying general principles are often invoked to simplify the control of complex movements (Van den Berg, 2000; Aerts et al., 2001). Two major control strategies for sub-maximal locomotor movements have recently been proposed in literature: (1) an increase or decrease of the force output of the system without changing the basic descending control and kinematics (Boyd and McLellan, 2002); (2) an optimization of the energy expended during the actual movement (Vanrenterghem et al., 2004). The biggest difference between the two strategies is that few changes in kinematics are expected in the first case. However, changes in kinematics involving the recruitment of light distal segments (foot, lower limb) before the heavier proximal segments such as upper limb or body (thus conserving energy when possible) are expected for the latter control strategy.

A recent study investigating the control of sub-maximal jumping in humans demonstrated that during vertical sub-maximal jumping, movement effectiveness is the most likely control criterion (Vanrenterghem et al., 2004). Movement effectiveness was associated with changes in the proximal segments and the invariance of changes in distal segment angles. Thus, energy expenditure was optimized during the jump by minimizing rotations of heavy proximal segments when possible (Vanrenterghem et al., 2004).

Here, the generality of the movement effectiveness criterion for jumping is tested by investigating the control of jumping in the lizard Anolis valencienni. Anolis lizards in general, are ideal subjects to study jumping as they often jump in their natural habitat to move around, or to escape from predators (Losos, 1999; Toro et al., 2003, Toro et al., 2004). As they live in complex three-dimensional arboreal habitats they not only jump different distances, but also jump to supports (branches) positioned at a wide variety of angles, distances and heights from the original perch. Movement effectiveness as a control criterion was tested for two different goal-directed jumping tasks in A. valencienni: jumping far and jumping at different angles, where jumping at different angles is used here as a proxy for vertical sub-maximal jumping. As an alternative control criterion the ‘push harder’ hypothesis is evaluated (i.e. increasing the force output of the system with few changes in kinematics; Boyd and McLellan, 2002). Specifically it is predicted that if movement effectiveness is the control strategy used, changes in distal segment angles (ankle, knee) during jumping should be constant and should thus have little or no predictive value for explaining changes in jump distance or take-off angle. Alternatively, changes in peak force should be strongly correlated with jump distance or take-off angle for the ‘push harder’ hypothesis to be valid.