Direct measurement of human ankle stiffness during quiet standing: the intrinsic mechanical stiffness is insufficient for stability

Abstract

During quiet standing the human 'inverted pendulum' sways irregularly. In previous work where subjects balanced a real inverted pendulum, we investigated what contribution the intrinsic mechanical ankle stiffness makes to achieve stability. Using the results of a plausible model, we suggested that intrinsic ankle stiffness is inadequate for providing stability. Here, using a piezoelectric translator we applied small, unobtrusive mechanical perturbations to the foot while the subject was standing freely. These short duration perturbations had a similar size and velocity to movements which occur naturally during quiet standing, and they produced no evidence of any stretch reflex response in soleus, or gastrocnemius. Direct measurement confirms our earlier conclusion; intrinsic ankle stiffness is not quite sufficient to stabilise the body or pendulum. On average the directly determined intrinsic stiffness is 91 ± 23 % (mean ± S.D.) of that necessary to provide minimal stabilisation. The stiffness was substantially constant, increasing only slightly with ankle torque. This stiffness cannot be neurally regulated in quiet standing. Thus we attribute this stiffness to the foot, Achilles' tendon and aponeurosis rather than the activated calf muscle fibres. Our measurements suggest that the triceps surae muscles maintain balance via a spring-like element which is itself too compliant to guarantee stability. The implication is that the brain cannot set ankle stiffness and then ignore the control task because additional modulation of torque is required to maintain balance. We suggest that the triceps surae muscles maintain balance by predictively controlling the proximal offset of the spring-like element in a ballistic-like manner.

Influence of wearing an unstable shoe on thigh and leg muscle activity and venous response in upright standing

“…During quiet standing, measurement of low intrinsic ankle stiffness (Loram & Lakie, 2002a), analysis of ballistic character of sways (Loram & Lakie, 2002b) and investigations of balance in an analogous task using a weak spring (Lakie, Caplan et al, 2003) provide increasing evidence that intermittent ballistic-like adjustments in muscle length (Loram & Lakie, 2002b) It is important to note that, although MG activity with the unstable shoe did not differ from barefoot measurements after 8 weeks of training in the experimental group, venous return did not decrease when compared to measurements made before the training period (Figure 3). Moreover, measurements made with the unstable test shoe after training revealed significantly higher venous return than barefoot measurements.…”

Revisão sobre posturografia baseada em plataforma de força para avaliação do equilíbrio

“…Such passive stiffness acts similar to an "elastic" opposed to the torque of gravitational force, which has the tendency to cause a forward fall of the body. Although the estimative of the contribution of the restoring torque due to the passive stiffness varies widely in the literature, it is estimated that this torque ranges about 65% to 90% from the magnitude of the gravitational torque 7,8 . Therefore, more than half of the torque responsible for maintaining our erect posture would be generated by a purely passive component, independent of the direct participation of the nervous system.…”

The impact of obesity on balance control in community-dwelling older women

“…Maintaining upright balance is controlled primarily by the calf muscle that counteracts the destabilizing effect of gravity (Johansson et al 1988). Recent reports indicate that passive stiffness and open loop mechanisms contribute to the generation of the muscle activity required for stance control (Loram and Lakie 2002) which depends on a coordinated effort of the sensory systems (visual, vestibular, and proprioceptive systems). Deficits in these systems result in impaired balance control.…”