Muscle activation during maximal voluntary eccentric and concentric knee extension

Abstract

The aim of this investigation was to study the relationships among movement velocity, torque output and electromyographic (EMG) activity of the knee extensor muscles under eccentric and concentric loading. Fourteen male subjects performed maximal voluntary eccentric and concentric constant-velocity knee extensions at 45, 90, 180 and 360 degrees.s-1. Myoelectric signals were recorded, using surface electrodes, from the vastus medialis, vastus lateralis and rectus femoris muscles. For comparison, torque and full-wave rectified EMG signals were amplitude-averaged through the central half (30 degrees-70 degrees) of the range of motion. For each test velocity, eccentric torque was greater than concentric torque (range of mean differences: 20%-146%, P less than 0.05). In contrast, EMG activity for all muscles was lower under eccentric loading than velocity-matched concentric loading (7%-31%, P less than 0.05). Neither torque output nor EMG activity for the three muscles changed across eccentric test velocities (P greater than 0.05). While concentric torque increased with decreasing velocity, EMG activity for all muscles decreased with decreasing velocity (P less than 0.05). These data suggest that under certain high-tension loading conditions (especially during eccentric muscle actions), the neural drive to the agonist muscles was reduced, despite maximal voluntary effort. This may protect the musculoskeletal system from an injury that could result if the muscle was to become fully activated under these conditions.

Modelling the maximum voluntary joint torque/angular velocity relationship in human movement

“…The a min values for the activation level at high eccentric velocities were similar (66% and 72%) and the inflection points of the differential activation profile both occurred near an angular velocity of zero (Table 1, Figures 3c, 4c). These values for a min are consistent with the results of Westing et al (1991) who observed reduced EMG activity values in the range 69% -85% for eccentric velocities of 360 o s -1 compared to 360 o s -1 concentric velocities. They are also consistent with the results of Westing et al (1990) who found that maximum voluntary eccentric torque was 81% -83% of that obtained when tolerable electrical stimulation was superimposed.…”

“…Westing et al (1990) investigated the effect of supplementing voluntary activation with electrical muscle stimulation during isovelocity knee extension and found that eccentric torque increased by more than 20% while concentric torque appeared to be unchanged. Subsequently Westing et al (1991) measured mean full-wave rectified EMG on three knee extensors during maximum voluntary isovelocity eccentric and concentric knee extensions. They found that EMG activity was lower under eccentric loading than concentric loading and did not appear to change across eccentric velocities, while EMG increased with increasing concentric velocities.…”

Fluctuations in acceleration during voluntary contractions lead to greater impairment of movement accuracy in old adults

“…activity at any given movement speed. In general, the lesser EMG during an eccentric contraction when lowering a given load (6,42) is attributed to a lower discharge rate and the recruitment of fewer motor units (6,23,34). The reduction in discharge rate likely includes an increased variability in discharge (12,23,29,30), which can decrease twitch fusion and increase the fluctuations in the forces exerted by individual motor units (12,14).…”

The torque–velocity relationship in large human muscles: Maximum voluntary versus electrically stimulated behaviour

“…Consequently, maximum voluntary eccentric strength is much lower than one might expect based on maximum isometric measurements and in vitro tetanic force-velocity behaviour. EMG studies have pointed to a 10−30% reduction in the neural drive of the agonist muscle under the high loading conditions of eccentric and low concentric maximum voluntary knee extensions (Stauber, 1989;Westing et al, 1991;Kellis and Baltzopoulos, 1998;Babault et al, 2001). This is regarded to be an involuntary mechanism to protect the human body against excessive strain and injury (Westing et al, 1991).…”