Reflex ankle stiffness is inversely correlated with natural body sway.

We aimed to determine whether effective ankle stiffness (EAS), measured during slow unperceived perturbations of stance, is related to natural anterior-posterior body sway. Because the perturbations are not perceived, any neural component of the response to perturbation is assumed to be "reflex", in the broad sense of an involuntary response to a stimulus. Subjects stood on a force platform for three 10-min trials. EAS was obtained from the average slope (Δτ/Δα) of the relation between ankle torque (τ) and ankle angle (α), recorded during repeated perturbations delivered at the waist by a weak spring. EAS was normalised using the subject's "load stiffness" (LS), calculated from mass (m) and height (h) above the ankle joint (m·g·h). Sway was obtained from fluctuations in ankle angle prior to perturbation. Variation in EAS and sway between subjects provided spread of data for correlation. There were no significant changes in EAS or sway across trials. All subjects had higher EAS than LS and mean EAS (1124 N m/rad) was significantly greater (p<0.01) than mean LS (531 N m/rad). There was a strong significant inverse correlation between mean sway and mean normalised EAS (r=-0.68, p=0.03). We conclude that the body, in response to slow unperceived perturbations, simulates an inverted pendulum with a stiffness of about twice LS and that EAS is largely generated by neural modulation of postural muscles. The inverse correlation between EAS and body sway suggests that the reflex mechanisms responding to perturbation also influence the extent of natural sway.

The effect of foot wedging on electromyographic activity in the erector spinae and gluteus medius muscles during walking.

The use of foot orthoses for treatment of low back pain (LBP) has received some attention in the literature, mainly from a clinical or theoretical perspective. It has been proposed that this treatment alleviates pain by altering muscle activity in the area of the low back but there is no direct evidence of such an effect. The objective of this study was to determine the effects of different types of foot wedging on the bilateral surface electromyographic activity of erector spinae (ErSp) (L3 level) and gluteus medius (GlMed) of 13 participants without LBP. Activity in ErSp had a significantly earlier onset during the gait cycle with bilateral heel lifts and bilateral lateral forefoot wedging. GlMed activity had a significantly later onset with bilateral heel lifts, and with an unilateral heel lift on the ipsilateral side (P<0.0125). No significant amplitude changes were demonstrated in either muscle for any of the forms of wedging tested. These results show that foot wedging can produce measurable changes in timing of muscle activity within the low back and pelvis during the gait cycle. Further investigation is required to determine whether this effect contributes to the alleviation of LBP.