Biomechanical examination of a commonly used measure of spasticity

Background. An increase in the prevalence of neurological disability puts pressure on service providers to restrict costs associated with rehabilitation. Spasticity is an important neurological impairment for which many novel and expensive treatment options now exist. The antispastic effects of these techniques remain unexplored due to a paucity of valid outcome measures. Aim. To develop a biomechanical measure of resistance to passive movement, which could be used in routine clinical practice, and to examine the validity of the modified Ashworth scale. Study design. Repeated measure cross-section study on 16 subjects who had a unilateral stroke one-week previously and had no elbow contractures. Outcome measures. Simultaneous measurement of resistance to passive movement using a custom built measuring device and the modified Ashworth scale. Passive range of movement and velocity were also measured. The “catch”, a phenomenon associated with the modified Ashworth scale, was identified by the assessor using a horizontal visual analogue scale and biomechanically quantified using the residual calculated from a linear regression technique. Results. Half the study population had a modified Ashworth score greater than zero. The association between the two measures was poor (κ=0.366). The speed and range of passive movement were greater in subjects with modified Ashworth score “0” (P<0.05). Resistance to passive movement was higher in the impaired arm (P<0.05) and tended to decrease with repeated measures and increasing speeds. Conclusions. A device to measure resistance to passive movement at the elbow was developed. The modified Ashworth scale may not provide a valid measure of spasticity but a measure of resistance to passive movement in an acute stroke population.