PURPOSE: To determine which exercise most selectively and effectively activates tibialis posterior. METHODS: Five healthy adults (two men, three women; mean age 31 yr) with an Arch Index (AI) within 1 SD from norm performed three exercises, separated by 1-wk intervals. The exercises were: 1) closed chain resisted foot adduction (foot adduction), 2) unilateral heel raise (heel raise), and 3) open chain resisted foot supination (foot supination). Magnetic resonance transaxial images were obtained immediately before and after exercise using a 1.5-T MRI system. Changes in pre-to postexercise signal intensity were compared across five muscles: tibialis posterior, tibialis anterior, medial gastrocnemius, soleus, and peroneus longus. Postexercise signal intensity was normalized to baseline preexercise signal intensity. RESULTS: Tibialis posterior signal intensity increased after each exercise. The greatest TP increase (50 +/- 6%) occurred after foot adduction, whereas the mean increase in the other muscles was less than 5%. After the heel raise exercise, the signal intensity increase in TP was 27% (+/- 11%), soleus 39% (+/- 8%), peroneus longus 57% (+/- 14%), and medial gastrocnemius 99% (+/- 12%). The signal intensity of tibialis anterior decreased 4% (+/- 2%). After foot supination, the TP signal intensity increased 26% (+/- 7%), whereas the mean change in the other muscles was less than 10%. Multivariate analyses of variance revealed a significant difference in muscle activation between exercises. Posthoc analysis showed greater activation of TP during foot adduction than foot supination (P = 0.021). CONCLUSION: In individuals with a normal AI, TP was activated selectively and most effectively during foot adduction. Knowledge of selective activation of a muscle is necessary to provide an optimal environment for muscle strengthening and/or tendon rehabilitation.
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