Vastus lateralis single motor unit EMG at the same absolute torque production at different knee angles

• Published in: Journal of applied physiology (1985) 2009-07, Vol.107 (1), p.80-89
• Main Authors: Altenburg, T. M, de Haan, A, Verdijk, P. W. L, van Mechelen, W, de Ruiter, C. J
• Format: Article
• Language: English
• Subjects: Adult; Biological and medical sciences; Electromyography; Female; Fundamental and applied biological sciences. Psychology; Humans; Isometric Contraction - physiology; Knee; Knee - physiology; Male; Motor ability; Motor Neurons - physiology; Muscle Stretching Exercises - methods; Musculoskeletal system; Quadriceps Muscle - physiology; Recruitment, Neurophysiological - physiology; Studies; Torque
• ISSN: 8750-7587; 1522-1601
• DOI: 10.1152/japplphysiol.90322.2008

1 Institute for Biomedical Research into Human Movement and Health, Manchester Metropolitan University, Cheshire, United Kingdom; 2 Research Institute MOVE, Faculty of Human Movement Sciences, VU University Amsterdam, Amsterdam, The Netherlands; and 3 EMGO Institute and Department of Public and Occupational Health, VU University Medical Centre, Amsterdam, The Netherlands Submitted 26 February 2008 ; accepted in final form 16 May 2009 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 %maximum voluntary contraction (%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 ± 22.2 N·m) and not different ( P > 0.05) at knee angles 10° more flexed (35.2 ± 17.9 N·m) compared with recruitment threshold at AngleTmax (41.8 ± 21.4 N·m). 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 ± 2.1 Hz. 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 VL 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. recruitment threshold; discharge rate; muscle length; quadriceps muscle Address for reprint requests and other correspondence: C. J. de Ruiter, Research Institute MOVE, Faculty of Human Movement Sciences, VU Univ. Amsterdam, Van der Boechorststraat 9, 1081 BT, Amsterdam, The Net