Functional activation of the extensor carpi radialis muscles in humans

Objectives: To assess activity of radial wrist extensors caused by isometric radial deviation and extension by using magnetic resonance imaging (MRI) and to assess measures that might be used to normalize T2-weighted data. Design: Two-way analysis of variance (ANOVA) design. Setting: Laboratory and children's hospital. Participants: Three healthy volunteers. Interventions: Ten repetitions of 10-second randomly ordered 30% or 60% of maximum voluntary isometric contractions toward wrist extension or radial deviation. Main Outcome Measures: Average T2 values from T2-weighted MR images of the extensor carpi radialis brevis (ECRB) and the extensor carpi radialis longus (ECRL), flexor digitorum profundus (FDP), and radius marrow were determined across 7 sections and 4 exercise bouts and a preexercise condition. Results: Significant differences across task and across sections were determined. Post hoc analysis revealed differences in activity between proximal and distal ECRB and ECRL during an exercise and differential activation of the same muscle across the 2 exercise tasks. Bone marrow and FDP did not show task-related changes. The range of average T2 values of bone marrow across sections was greater than a muscle (FDP) that was not the target of the exercise protocol. However, FDP did show small but significant differences across sections. Conclusions: T2-weighted MR images can be used to study muscle activation at 30% and 60% of maximum voluntary contractions. The use of inactive muscle and bone marrow for normalizing data requires further investigation.