Comparison of MRI with EMG to study muscle activity associated with dynamic plantar flexion.
Level 3 - non-randomized controlled study
Non-randomized matched comparative physiological study
PubMed 14599535 · doi:10.1016/s0730-725x(03)00183-8
What was done
Two height- and weight-matched cohorts (MRI group: n = 12, 6 males, 6 females; EMG group: n = 12, 8 males, 4 females) performed identical 2-minute dynamic plantar flexion protocols at 25% of 1-repetition maximum across three knee angles: 0° (extended), 45° (partially flexed), and 90° (flexed). In the MRI group, spin-echo images were acquired before and immediately after exercise to calculate T2 relaxation times in the medial gastrocnemius (MG), lateral gastrocnemius (LG), soleus (SOL), tibialis anterior (TA), and peroneus (PER). In the EMG group, bipolar surface electrodes recorded activity from the MG, LG, SOL, and TA during exercise.
What was found
At 0° knee flexion, MRI demonstrated significant post-exercise T2 increases in MG (p <= 0.001), LG (p <= 0.001), and PER (p <= 0.01), with no T2 change in SOL or TA. At 90° flexion, T2 significantly increased only in SOL (p <= 0.001), with no change in MG, LG, PER, or TA. At 45° flexion, T2 increased significantly in SOL (p <= 0.001) and LG (p <= 0.05), but not in MG, PER, or TA. EMG showed concordant activation patterns across angles, except for detecting significant TA activity during the relaxation cycle at 90°. Absolute T2 values and EMG amplitudes were not reported in the abstract.
Why it matters
This study demonstrates that knee angle alters the recruitment profile of the triceps surae—isolating the soleus at 90° flexion while engaging the gastrocnemii at full extension—and confirms that MRI T2 mapping is a viable alternative or complement to surface EMG for assessing deep and superficial muscle recruitment.
Limits
The sample size was small (n = 24 total, split into two 12-person groups rather than a within-subject repeated-measures design across both modalities). The abstract omits absolute baseline and post-exercise values for T2 times and EMG signals. The protocol evaluated only a single low load (25% 1-RM) during dynamic contractions in healthy volunteers.
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- supports Performing seated calf raises with bent knees largely disengages the gastrocnemius because it crosses the knee joint, thereby isolating the soleus muscle.