Submitted on February 26, 2008
Revised on May 13, 2009
Accepted on May 16, 2009
Vastus lateralis single motor unit EMG at the same absolute torque production at different knee angles
Teatske Maria Altenburg1, Arnold de Haan2, Peter WL Verdijk1, Willem van Mechelen3, and Cornelis Johannes de Ruiter4*
1 Research Institute MOVE
2 Research Institute MOVE, VU University Amsterdam
3 VU University Medical Centre
4 Institute for Fundamental and Clinical Human Movement Sciences
* To whom correspondence should be addressed. E-mail: c_j_de_ruiter{at}fbw.vu.nl.
Single motor unit electromyographic (EMG) activity of the knee extensors was investigated at different knee angles with subjects (n=10) exerting the same absolute submaximal isometric torque at each angle. Measurements were made over a 20° range around the optimum angle for torque production (AngleTmax) and, where feasible, over a wider range (50°). Forty six vastus lateralis (VL) motor units were recorded at 20.7±17.9%MVC together with the rectified surface EMG (rsEMG) of the superficial VL muscle. Due to the lower maximal torque capacity at positions more flexed and extended than AngleTmax, single motor unit recruitment thresholds were expected to decrease and discharge rates were expected to increase at angles above and below AngleTmax. Unexpectedly, the recruitment threshold was higher (P<0.05) at knee angles 10° more extended (43.7±22.2Nm) and not different (P>0.05) at knee angles 10° more flexed (35.2±17.9Nm), compared to recruitment threshold at AngleTmax (41.8±21.4Nm). Also, unexpectedly the discharge rates were similar (P>0.05) at the three angles: 11.6±2.2, 11.6±2.1 and 12.3±2.1Hz. Similar angle independent discharge rates were also found for 12 units (n=5; 7.4±5.4%MVC) studied over the wider (50°) range, while recruitment threshold only decreased at more flexed angles. In conclusion, the similar recruitment threshold and discharge behavior of vastus lateralis motor units during submaximal isometric torque production suggests that net motor unit activation did not change very much along the ascending limb of the knee-angle torque relationship. Several factors such as length-dependent twitch potentiation, which may contribute to this unexpected aspect of motor control, are discussed.
