Variability of concentric angle-specific isokinetic torque and impulse assessments of the knee extensors

Length-tension relationships are widely reported in research, rehabilitation and performance settings; however, several isometric contractions at numerous angles are needed to understand these muscular outputs. Perhaps a more efficient way to determine torque-angle characteristics is via isokinetic dynamometry; however, little is known about the variability of isokinetic measurements besides peak torque and optimal angle. This paper examines the variability of angle-specific isokinetic torque and impulse measures.

Three sessions of concentric (60°·s^-1) knee extensions were performed by both limbs of 32 participants. Assessments were repeated on three occasions, separated by 5-8 d. To quantify variability, the standardized typical error of measurement (TEM) was doubled and thresholds of 0.2-0.6 (small), 0.6-1.2 (moderate), 1.2-2.0 (large), 2.0-4.0 (very large) and  >4.0 (extremely large) were applied. Additionally, variability was deemed large when the intraclass correlation coefficient (ICC) was  <0.67 and coefficient of variation (CV)  >  10%; moderate when ICC  >  0.67 or CV  <  10% (but not both); and small when both ICC  >  0.67 and CV  <  10%.

Isokinetic torque and angular impulse show small to medium variability (ICC  =  0.75-0.96, CV  =  6.4%-15.3%, TEM  =  0.25-0.53) across all but the longest (100°) and shortest (10°) muscle lengths evaluated. However, moderate to large variability was found for the optimal angle (ICC  =  0.58-0.64, CV  =  7.3%-8%, TEM  =  0.76-0.86), and torque and impulse at the beginning and end of the range of motion (ICC  =  0.57-0.85, CV  =  11-42.9%, TEM  =  0.40-0.89). Intersession variability of isokinetic torque and impulse were small to moderate at medium (90-20°) joint angles.

Researchers and practitioners can examine the muscle torque-angle relationship and activity-specific torque outputs within these ranges, without resorting to more strenuous and time-consuming isometric evaluations.