Physiological alterations of maximal voluntary quadriceps activation by changes of knee joint angle

The purpose of this study was to investigate the influence of different angles of the knee joint on voluntary activation of the quadriceps muscle, estimating the ability of a subject to activate a muscle maximally by means of voluntary contraction. Isometric torque measurement was performed on 6 healthy subjects in 5° intervals between 30° and 90° of knee joint flexion. Superimposed twitches at maximal voluntary contraction (MVC) and at a level of 60% and 40% of the MVC were applied and the voluntary activation estimated. At between 30° and 75° of knee flexion, the maximal extension torque increased at an average rate of 2.67 ± 0.6 Nm/degree, followed by a decline with further flexion. However, throughout the joint‐angle range tested, voluntary activation increased on average by 0.37%/degree with a maximum at 90° of flexion. Due to the influence of joint position it is not possible to generalize results obtained at the knee joint angle of 90° of flexion, which is usually used for the quadriceps twitch‐interpolation technique. Consequently, it is useful to investigate voluntary activation deficits in knee joint disorders at a range of knee joint angles that includes, in particular, the more extended joint angles used frequently during daily activity. © 2001 John Wiley & Sons, Inc. Muscle Nerve 24: 667–672, 2001