Early stabilization of human posture after a sudden disturbance: influence of rate and amplitude of displacement

Abstract

The functional role of short-, medium- and long-latency responses for the maintenance of upright posture was investigated in twenty healthy subjects standing on a platform which could be rotated in pitch around the subject's ankle joints. Tilting the platform toe-up evokes a stretch reflex in the triceps surae muscle (TS, latency 55-65 ms) and at higher speeds and amplitudes of platform displacement a medium-latency response (latency 108-123 ms). Both responses functionally destabilize posture, since they enforce the induced backward displacement of the body. Compensation of body displacement in this situation is achieved by a long-latency EMG response in the anterior tibial muscle (TA 130-145 ms). Platform movement toe-down elicits a rather small medium-latency response in TA (103-118 ms), but no short-latency response. A late compensatory response occurs in the triceps surae muscle (latency 139-170 ms). The mean latency of the late antagonistic EMG response was significantly shorter than that of a voluntary movement triggered by a somatosensory stimulus. Integrals of rectified EMG responses from the two muscles were linearly related to the amplitude and to a smaller degree to the velocity of platform displacement. The slope of this function (gain) varied depending on the direction of ankle displacement and the functional importance of the subsequent EMG responses. Destabilizing short- and medium-latency responses of the stretched muscle had a lower gain relative to amplitude than the late stabilizing response of the antagonist. This functionally adaptive modulation of gain was not seen in relation to the rate of platform displacement.

Differential effects of stimulus characteristics during knee joint perturbation on hamstring and quadriceps reflex responses

“…In accordance to Diener and co-workers, who showed a significant correlation of the size of the MLR but not of the SLR in the ankle muscle triceps surae with the amplitude of perturbation, activation during P2 but not during P1 of the investigated knee joint muscles in the present study was modulated by IMP (Diener, Dichgans, Bootz, & Bacher, 1984). The same conclusions concerning the relationship between stretch amplitude and the size of MLR have been made when applying isolated rotational perturbations of the ankle joint (Gollhofer & Rapp, 1993;Leukel et al, 2009).…”

“…Considering the time constraints, the contribution of cutaneous afferents should have occurred predominantly during P3. In addition to the mechanism described, postural responses and supraspinal contributions have to be considered relevant for quadriceps activation during P3 (Diener et al, 1984;Horak & Nashner, 1986;Mrachacz-Kersting, Grey, & Sinkjaer, 2006;Nashner, 1976) While IMP had significant influence on reflex size, RFD didn't systematically alter neuromuscular activation during P3.…”

“…Depending on the condition of the patient, perturbations with minor velocity and moderate amplitude targeting at MLR might be used for the rehabilitation training of ACL injury. Perturbations with pronounced amplitude and low velocity overcoming the mass inertia of the subject are associated with postural control rather than with joint stabilization, as the major part of reflex responses to this kind of perturbation are due during LLR (Diener et al, 1984;Horak & Nashner, 1986;Nashner, 1976).…”

Changes in the activity of the cerebral cortex relate to postural response modification when warned of a perturbation

“…balance. Postural responses to an external perturbation represent a unique set of behaviors because (1) while they are voluntarily modifiable, they cannot be completely suppressed, and (2) their onset is earlier than a cued voluntary movement but later than that of a segmental spinal reflex (Chan et al, 1979;Diener et al, 1984;Ackermann et al, 1991). Thus, postural responses exhibit characteristics between voluntary and reflexive movement.…”

Influences of H and Medium Latency Reflex Responses of the Soleus Muscle while Receiving with Malleolus Vibration during Upright Standing

“…The two electromyographic (EMG) responses of the soleus (Sol) muscle have been measured in standing subjects during sudden perturbation of a platform [1][2][3] . The first component is the short latency reflex (SLR) that is mediated via fast conducting group Ia fibers from primary muscle spindle endings 4,5) .…”