Recent evidence suggests that changes in pre-programmed motor control may have a role to play in the development of chronic Functional Instability of the Ankle Joint (FAI), a disabling condition characterised by a tendency for the foot to repeatedly sprain or give way. A common mechanism of injury for subjects with FAI is the foot giving way at heel strike during the running cycle. Reber et al (1993) demonstrated activity of ankle musculature in the late swing phase of the running cycle in preparation for impending forces at heel strike. This pre-programmed muscle activity could be altered in FAI subjects. The purpose of this study was to compare activity of ankle and knee muscules pre and post heel strike in a group of FAI subjects and a control group. The study subjects included 7 subjects with unilateral FAI and 6 age and sex matched control subjects. Subjects ran on a treadmill at a velocity of 8kmph whilst electromyographic data was sampled at a frequency of 2000Hz from their quadriceps (VM), soleus (SOL), peroneus longus (PER) and tibialis anterior (TA) muscles using surface electrodes. Data from 5 consecutive running cycles for each subject was extracted for analysis, filtered (20-500Hz), full wave rectified, normalised with respect to peak amplitude, and averaged for each subject. Integral EMG activity during the 150ms linear envelopes relating to the periods immediately prior to and post landing were calculated for each subject and group means were obtained. T-tests were used to test for significant differences between groups. FAI subjects demonstrated significantly higher VM and SOL activity in the period prior to heel strike (P<0.05) compared to the control group. There were no differences in pre heel strike activity of PER or TA or in post heel strike activity of any of the 4 muscles analysed.Table 1. IEMG activity (%max activity.150ms) pre and post heel strike. Values are Means±SD. *=significant difference from column to left (P<0.05)These results demonstrate that pre-programmed motor control of muscle activity is altered during the running cycle in subjects with FAI. Changes were observed in activity of muscles at both the ankle and knee joints indicating that the change in motor control associated with FAI affects both local and adjacent joint function. The causes and consequences of observed changes are not apparent from these results and need further investigation.