Christopher L. Rawdon, Christopher Ingalls, FACSM, Feng Yang. Georgia State University, Atlanta, GA.

Background: Although much is known about the causes of isolated skeletal muscle strength loss after contraction-induced muscle injury, no studies have examined the extent to which individual muscles in a muscle group (e.g., quadriceps) experience differential injury and strength loss following this type of injury. To address this gap, we first sought to examine the validity of two methods (i.e., Biodex dynamometer & kinematic analysis) to measure the strength of individual quadriceps muscle torque produced by electrical stimulation. Methods: 7 young adults (30.3±3.9 y/o) were enrolled. On Day 1, their peak isometric torque during a maximal voluntary contraction (MVC) was assessed followed by peak isometric tetanic torque produced by electrical stimulation (20 & 80 Hz) of the vastus medialis, rectus femoris and vastus lateralis muscles using the Biodex. After 15 min, peak concentric torque created by electrical stimulation of individual quadricep muscles was estimated via kinematic analysis (i.e., Vicon) of a weighted leg (5% of body weight). Peak concentric torque (T) was calculated as T=Iα+Twt, where I is the leg-foot-weight segment’s moment of inertia about the knee, α is angular acceleration of the same segment, and Twt is torque induced by segment’s gravity. I and α were obtained based on the anthropometric model and the collected kinematic data, respectively. This procedure was repeated on Day 2 (<7 days from Day 1). Results: The sum of the 20 & 80 Hz torque for three quadricep muscles was 18.2%±5.2% and 38.8±7.1% of the MVC, respectively. For all conditions, the mean isometric torques collected on the Biodex on Day 1 was comparable to Day 2. Moreover, the mean Biodex concentric torque for Day 1 was not significantly different from Day 2. The mean Biodex isometric torque was strongly correlated with the mean kinematic-based torque (r=0.90, p<0.001). Both Biodex and kinematic analysis showed a strong test-retest reliability (Biodex ICC=0.95 [95% CI: 0.90-0.97]; Vicon ICC=0.98 [95% CI: 0.96-0.98]). The reliability of both methods to measure individual muscle torque was further supported by the Bland-Altman results with only 1 out of 42 difference points apiece falling outside the lines of agreement. Conclusion: Both the Biodex dynamometer and Vicon motion capture analysis were reliable and may be useful methodologies for measuring the individual quadricep muscle torque produced by electrical stimulation.

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