Magnetic resonance imaging and electromyography to measure lumbar back muscle activity.

Study Design: Mixed model analysis of muscle functional magnetic resonance imaging (MRI) and electromyography (EMG) changes in lumbar muscles during trunk extension exercise at varying intensities.
Objective: To gain insight within the relationship between muscle functional MRI and activity of the lumbar back muscles, which is related to exercise intensity.
Summary of Background Data: It is known that muscle activity during exercise induces a force-sensitive T2 increase; however, it is not known how sensitive this T2 change is. In addition, the association between MRI and EMG measurement was investigated.
Methods: Multifidus and erector spinae muscle activity was investigated during a trunk extension exercise at 5 increasing loads (from 40% to 80% of 1 repetition maximum), with both MRI and EMG. Data were analyzed using mixed model analysis.
Results: Our results indicate a linear relationship between MRI and exercise intensity; for both muscles an increase of 10% exercise intensity corresponds with an increase of the T2 value with 1.18 (0.89, 1.47) ms. Also for EMG there is a linear relationship with exercise intensity; an increase of 10% exercise intensity corresponds with an increase of 6.98 (5.33, 8.62) microV. Furthermore, a linear association between MRI and EMG is acceptable. For the multifidus, an increase of 1 muV (EMG) corresponds with an increase of 0.168 (0.117, 0.219) ms (MRI). For the erector spinae, an increase of 1 microV corresponds with an increase of 0.078 (0.042, 0.114) ms.
Conclusion: Both muscle functional MRI and EMG have specific (dis-) advantages and therefore have to be seen as complementary techniques. Nevertheless, our results support the validity of each method and indicate that MRI and EMG can be used independently to quantify lumbar muscle activity.