Comparison of MRI with EMG to study muscle activity associated with dynamic plantar flexion

This study compared magnetic resonance imaging (MRI) and surface electromyography (EMG) to evaluate the effect of knee angle upon plantar flexion activity in the triceps surae muscles [medial & lateral gastrocnemius (MG, LG) and the soleus (SOL)]. Two weight & height matched groups performed identical protocols, twelve (6M, 6F) in the MRI group, twelve (8M, 4F) in the EMG group. Subjects plantar flexed dynamically for 2 min at 25% of 1-repetition maximum voluntary contraction (1-RM). Exercise was performed with the knee extended (0° flexion), flexed (90°), and partially flexed (45°). In the MRI group spin-echo images were acquired before and immediately following each exercise session. T2 times, calculated at rest and after exercise by fitting the echoes to a monoexponential decay pattern with a least-squares algorithm, were compared with EMG data. In the EMG group a bipolar electrode was used to collect samples were from the MG, LG, SOL, and anterior tibialis (TA) during exercise at each knee angle, MRI also examined the peroneus (PER). At 0° flexion MRI demonstrated a significant post-exercise T2 increase in the MG (p ≤ 0.001), LG (p ≤ 0.001), and PER (p ≤ 0.01), with no T2 change in the SOL or TA. At 90° flexion there was a significant T2 increase in the SOL (p ≤ 0.001) with no significant T2 change in the MG, LG, PER, or TA. At 45° T2 increased significantly in the SOL (p ≤ 0.001) and LG (p ≤ 0.05), but not the MG, PER, or TA. EMG produced similar results with the exception that there was significant activity in the TA during the relaxation cycle of the 90° protocol. We conclude that: 1) Soleus activity is measurable by MRI; and 2) MRI and EMG produce similar results from different physiological sources, and are therefore complementary tools for evaluating muscle activity.