Clinical Measures Predicting Knee Extensor Muscle Activation During a Maximal Voluntary Isometric Contraction

Abstract

Open Access Research Articlefiber pennation angle) but also by its neurophysiological properties (e.g., motor unit recruitment, rate coding, motor unit synchronization). Several studies have shown that the deficits in knee extensor strength found after knee injuries are attributable more to decreased knee extensor muscle activation than to muscle atrophy due to disuse [1][2][3]. The inability to maximally activate the knee extensors in the absence of frank tissue damage can alter athletic performance, which often relies on optimal contraction of this lower-extremity muscle group [1].The interpolated twitch and central activation ratio methods are considered to reliably assess the percent of a muscle group's mass that is activated during a voluntary contraction [4,5]. Interestingly, the knee extensors have, on average, been observed to have a relatively low activation during a maximal voluntary isometric contraction (MVIC) compared to other muscles or muscle groups, including the biceps, brachialis, adductor pollicis, tibialis anterior, and ankle plantarflexors [5]. Activation of the knee extensor during a MVIC in otherwise healthy individuals has been reported on average to be in the 85-95% range, while the MVIC activation for other muscle groups exceeds 95%. We have reported a similar mean value for the knee extensors (i.e., 86%) in apparently healthy young adults but individual values in the study ranged greatly from 64 to 99% [6]. The lower knee extensor activation levels observed in healthy individuals may not affect typical activities of daily living, but they could limit athletic performance and/or affect injury prevention.Knowing knee extensor activation during a MVIC could be useful clinical data for optimizing athletic performance, determining an athlete's ability to return to sport, and design of strengthening and rehabilitation programs. While it is possible to assess activation by performing the interpolated twitch and central activation ratio methods in the research laboratory, the techniques are not practical to perform in a clinical setting or on the training field because they require expertise and training not possessed by most clinicians, and utilize equipment not commonly found in the clinic. Also, the techniques are often AbstractStrength of a muscle group is determined by its physiological cross-sectional area and one's ability to activate the muscle group. Injury and disuse are associated with a decreased ability to maximally activate a muscle group. Clinical orthopedic measures such as manual muscle testing may provide an indication of muscle strength but no direct insight into the extent a muscle group is being activated. The objective of our studies was to determine the standard clinical orthopedic measures and/or medical history that can best predict muscle activation during a maximal voluntary isometric contraction (MVIC) of the knee extensors. Forty young, healthy subjects completed a pilot study (Study A), which included a medical history assessment and a comprehensive battery of low...