Effect of Instability in Legpress Testing on Strength & Muscle Activity in Functional Ankle Instability

Abstract

Compensating instable situations is an important functional capability to maintain joint stability, to compensate perturbations and to prevent (re-)injury. Therefore, a reduced maximum strength and altered neuromuscular activity is expected by inducing instability to high loading test situations. Possible effects are not clear for induced instability during maximum legpress tests in healthy and furthermore in subjects with functional ankle instability (FAI). PURPOSE: First, to compare maximum strength and lower leg muscle activity between stable (S) and unstable (UN) maximum legpress tests. Second, to evaluate the association between FAI and effect of instability during testing. METHODS: 18 male subjects (12 healthy/6 subjective FAI, age: 28±4yrs, height: 180±8cm, weight: 80±9kg, physical activity: 6±5h/wk) were included and their ankle function was quantified by the Foot and Ankle measure (FAAM) questionnaire. Five maximum strength test with leg press isokinetic device in concentric (CON) and eccentric (ECC) mode were measured. Muscle activity were recorded by EMG of m. tibialis anterior (TA), m. peroneus longus (PL) and m. soleus (SOL). Peak force (FPeak, Nm) for maximum strength and root mean square (RMS, Hz) for EMG amplitude of TA, PL and SOL were calculated. Comparisons of conditions (S vs UN) were analyzed descriptively and with paired T-tests. For association, Pearson correlation was applied using FAAM score and RMS differences (condition UN – S). RESULTS: UN lead to a significant peak force reduction of 10.1% (CON) and 13.7% (ECC) significantly (p<0.001). RMS of PL in CON and TA in CON and ECC mode were 26.2%, 59.7% and 35.8% respectively significant higher in UN footplate in comparison with S (p<0.01). In addition, no correlations between FAI and loss of strength or changes in muscle activity have been found. CONCLUSION: Reduction in peak force and increased muscle activity confirmed the expected increased effort to compensate instability. The missing association between FAI and amount of altered strength or muscle activity might be attributed to a low level of FAI in the included subjects.