Abandonment of Linear Synergy During
Negotiation of Obstacles and Stair Climbing

Chris Kirtley

Department of Rehabilitation Sciences,
The Hong Kong Polytechnic University, Hong Kong


A principle of linear synergy between joint moments has been reported in unconstrained upper-limb movements (Gottlieb, 1996), and in the lower-limb during the unconstrained, swing phase of gait (Eng et al, 1997; Kirtley, & Smith, 1997) and kicking movements (Young & Marteniuk, 1995). In the upper-limb, the synergy is abandoned when the hand is made to follow an unusual trajectory, which provides evidence for its neurophysiological basis. In order to determine whether the synergy is also abandoned under less routine lower-limb movements, this study examined the correlation between hip and knee joint moments during the negotiation of obstacles.

The gait of 10 normal adults and 11 normal children was recorded using a Vicon 3D motion analysis system, and the lower-limb joint moments calculated by inverse dynamics. The correlation between knee and hip moments was then determined (by linear regression and principal components analysis) during the swing phase of natural gait, and while clearing obstacles of two different heights (50 and 150 mm) and climbing stairs (of height 200mm)



Linear Regression

Principle Component

Natural Gait

knee = -0.6*hip; r2 = 0.8


Clearing Small Obstacle

knee = -0.5*hip; r2 = 0.5


Clearing Large Obstacle

knee = -0.5*hip; r2 = 0.3


Stair climbing

knee = -0.3*hip; r2 = 0.2



The linear covariance between hip and knee moment can be seen to fall as the difficulty of the task is increased from natural gait, through small, then large, obstacles to stair climbing. This suggests that, for these movements at least, the synergy observed is not simply due to mechanical constraints, but a direct consequence of neural control. The synergy may be employed to simplify control of repetitive motion such as gait, but abandoned when more demanding and less routine tasks are required.


Eng, J. J., Winter, D. A., Patla, A. E. (1997). Intralimb dynamics simplify reactive control strategies during locomotion. Journal of Biomechanics, 30, 581-588.

Gottlieb G. L., Song Q., Almeida G.L., Hong D., and Corcos D. (1997). Directional control of planar human arm movement. Journal of Neurophysiology, 78, 2985-2998.

Kirtley, C. & Smith, R. A. (1997). Proc. 4th Biennial Motor Control & Human Skill Workshop, 53.

Young, R. P., Marteniuk, R. G. (1995). Changes in inter-joint relationships of muscle moments and powers accompanying the acquisition of a mult-articular kicking task. Journal of Biomechanics, 28, 701-713.