Human balancing of an inverted pendulum: position control by small, ballistic‐like, throw and catch movements

Abstract

In standing, there are small sways of the body. Our interest is to use an artificial task to illuminate the mechanisms underlying the sways and to account for changes in their size. Using the ankle musculature, subjects balanced a large inverted pendulum. The equilibrium of the pendulum is unstable and quasi‐regular sway was observed like that in quiet standing. By giving full attention to minimising sway subjects could systematically reduce pendulum movement. The pendulum position, the torque generated at each ankle and the soleus and tibialis anterior EMGs were recorded. Explanations about how the human inverted pendulum is balanced usually ignore the fact that balance is maintained over a range of angles and not just at one angle. Any resting equilibrium position of the pendulum is unstable and in practice temporary; movement to a different resting equilibrium position can only be accomplished by a biphasic ‘throw and catch’ pattern of torque and not by an elastic mechanism. Results showed that balance was achieved by the constant repetition of a neurally generated ballistic‐like biphasic pattern of torque which can control both position and sway size. A decomposition technique revealed that there was a substantial contribution to changes in torque from intrinsic mechanical ankle stiffness; however, by itself this was insufficient to maintain balance or to control position. Minimisation of sway size was caused by improvement in the accuracy of the anticipatory torque impulses. We hypothesise that examination of centre of mass and centre of pressure data for quiet standing will duplicate these results.

Balancing with positive feedback: the case for discontinuous control

“…Thus, for a subject standing in neutral alignment, the feedback at the ankle is positive and not negative. A second surprising observation is that the muscle controlling movements for balance control are not continuous but are intermittent and pulsatile (Loram & Lakie 2002;Loram et al 2006). Similar conclusions concerning positive feedback and intermittent control have been obtained from studies of a closely related experimental paradigm, namely stick balancing at the fingertip Hosaka et al 2006).…”

The influence of artificially increased hip and trunk stiffness on balance control in man

“…The inverted pendulum model may be valid for a restricted number of movements in a single plane, including those induced by small and slow horizontal support surface translations. Recently, however, this concept has been challenged for motion occurring during quiet stance (Aramaki et al 2001;Loram and Lakie 2002). Indeed, as body motion increases in amplitude and direction from that of quiet standing (Fitzpatrick et al 1992(Fitzpatrick et al , 1994Winter et al 1996Winter et al , 1998Gatev et al 1999;Accorneo et al 1997), to that induced by a support surface perturbation in the pitch plane alone (Cordo and Nashner 1982;Allum et al 1993;Horak et al 1997), and finally to that induced by combined roll and pitch plane perturbations (Moore et al 1988;Maki et al 1994aMaki et al , 1994bHenry et al 1998b;Carpenter et al 1999;Allum et al 2002), the multi-link nature of human postural corrections becomes increasingly prominent.…”

“…The inverted pendulum or multi-link concepts of human postural control can be tested directly by splinting various joints, but such experiments are rare and were restricted to the pitch plane (Loram and Lakie et al 2002;Peterka 2002). Here we tested the hypothesis that normal balance control is highly dependent on a multi-link mode of movement by stiffening the hips and trunk (but not the ankles and knees) with two different rigid corsets.…”

Tendon structural and mechanical properties do not differ between genders in a healthy community‐dwelling elderly population

“…12 A proposed explanation for this finding is related to the tendon's primary function of force transmission. Stiffer tendon structures enable more rapid force transfers than compliant systems and thus increase the speed at which the muscle tendon complex corrects the ''catch and throw'' actions involved in maintaining balance 13 and consequently improves balance performance. 14 Tendon properties also have the capacity to affect muscle force output and function, as muscle follows a hyperbolic force-velocity relationship.…”