Influence of joint position on electromyographic and torque generation during maximal voluntary isometric contractions of the hamstrings and gluteus maximus muscles

Study Design: Repeated measures analysis of joint angle effects on hip and knee muscle electromyographic (EMG) activity. Objectives: To simultaneously determine angle-dependent changes in maximal voluntary isometric contraction (MVIC) torque and EMG activity during hip extension and knee flexion. Background: Procedures for normalizing EMG data and for determining torque-angle relationships for various joint motions both entail asking subjects to exert an MVIC. The implicit assumption in these paradigms is that magnitude of the EMG response is at a constant, maximum level so that observed angle-dependent variations in torque are due to mechanical factors, such as muscle length and muscle moment arm. Methods and Measures: Fifty subjects (25 men and 25 women) participated in this study (age, 23.5 ± 4.6 y; range, 18-38 y). Subjects performed maximal isometric knee flexion at 4 knee angles and maximal isometric hip extension at 4 hip angles. The dependent variables were normalized root-mean-square EMG and torque. The process for normalizing EMG and torque data consisted of determining the largest mean value for each subject across testing positions for the muscle of interest. That value was designated as corresponding to 100% MVIC, and all other data for that muscle were expressed as a percentage of the MVIC value. Repeated measures was used to determine angle-dependent changes in normalized MVIC-torque and MVIC-EMG values for each muscle group. Results: Mean torque-angle relationships were generally consistent with previous reports, though considerable intersubject variability was observed. There were significant angle-dependent differences in maximal EMG for both the hamstring and gluteus maximus muscles. Mean percentages of hamstring MVIC-EMG at knee angles of 30° (81 ± 19) and 60° (82 ± 22) were greater than at 0° (68 ± 20) or 90° (74 ± 20). The mean percentage of gluteus maximus MVIC-EMG at a hip angle of 0° (94 ± 10) was greater than at 30° (84 ± 13), 60° (80 ± 14), or 90° (64 ± 20), and gluteus maximus maximal voluntary isometric EMG at 90° was less than at all other angles. These differences could not be explained solely by muscle length-dependent effects on EMG amplitude, suggesting that despite instructions for maximal effort, motor unit activation was not maintained at a constant, maximal level throughout the range of motion. The form of the EMG/angle relationships differed markedly from the torque-angle relationships. Conclusions: These findings have implications for the use of MVIC-EMG for reference values in EMG normalization procedures and for the interpretation of mechanisms underlying the torque-angle relationships observed in vivo.