Experimentally Induced Anterior Knee Pain Affects Voluntary Activation Of The Quadriceps

Abstract

PURPOSE Pain affects muscle function, but the exact mechanisms underlying the interactions are largely unknown. Joint pathology may influence the muscles that surround the joint, reducing the ability to voluntarily activate all of the motor units and/or reducing the maximum voluntary contraction of a muscle. Knee pathology is associated with altered quadriceps function and experimental studies have identified that induced muscle pain affects muscle function. This study aimed to investigate whether experimentally induced pain in the knee would have an immediate effect on quadriceps muscle function (maximum voluntary contraction and voluntary activation). METHODS 10 healthy individuals (5 male, 5 female) with no current or recent past history of knee pain/injury were recruited. All individuals were tested in two conditions; (i) when they had no knee pain; and (ii) while they had experimental knee pain (immediately following an injection of 2.5 ml (5%) hypertonic saline into their fat pad). The order of testing (pain and no pain) was randomized. Quadriceps function was measured as (i) maximal isometric torque (5 second contraction) on a KinCom dynamometer 14 60 degrees knee flexion; and (ii) voluntary activation, measured using the interpolated twitch technique during maximal isometric contraction. Surface electrodes were placed over the quadriceps muscle and used to deliver twitches (Digitimer DS7AH) at supramaximal intensities (constant current, 300–350mA). Voluntary activation was calculated as 100 × (1 - a/b), where a is the amplitude of the interpolated twitch and b is the amplitude of a twitch obtained from the resting muscle. RESULTS The mean intensity of pain induced in the knee at the time of testing was 6 on a 10 cm visual analogue scale. Experimentally induced anterior knee pain resulted in significantly reduced quadricep muscle torque (mean difference 0.29 Nm.kg-1; 95%CI: 0.09; 0.49) and voluntary activation (mean difference 7%; 95%CI: 4; 11) in the experimental leg. CONCLUSIONS Induced anterior knee pain resulted in significant reductions in the voluntary activation of the quadriceps muscle. Thus rehabilitation of knee conditions must focus on pain reduction and on restoring quadriceps muscle activation. KMC supported by a National Health and Medical Research Council (Australia) Health Professional Training Fellowship (Regkey no 209168).