0231–424X/$ 20.00 © 2013 Akadémiai Kiadó, Budapest Acta Physiologica Hungarica, Volume 100 (4), pp. 457–468 (2013) DOI: 10.1556/APhysiol.100.2013.012 First published online September 7, 2013 Short and long latency response due to transition from bilateral to unilateral contraction S Sáfár 1 , B Kopper 2 , V Szakács 2 , J Tihanyi 2 1 Department of Track and Field Faculty of Physical Education and Sport Sciences, Semmelweis University, Budapest, Hungary 2 Department of Biomechanics, Kinesiology and Informatics, Faculty of Physical Education and Sport Sciences, Semmelweis University, Budapest, Hungary Received: October 26, 2012 Accepted after revision: March 25, 2013 The aim of this study was to investigate the effect of fast and slow relaxation of the knee extensor muscle of the dominant leg on torque-time curve of the unilaterally contracting contralateral muscle. Eight adult male subjects were recruited without bilateral deficit. In Task 1 subjects exerted bilateral and unilateral torque and bilateral index was calculated. In Task 2 subjects relaxed the right knee extensors as slow as possible while maximum activation of the contralateral muscle had to be maintained. In Task 3 the relaxation was as fast as possible. During slow relaxation the short latency response (dM1) was 6.6% torque reduction in the left leg. On the contrary fast muscle relaxation resulted in a 7.3% increase. During long latency response the torque increased in both tasks, but did not exceed the torque measured in Task 1. Significant correlation was found between the rate of torque reduction (RTR) and dM1 (r = 0.95, P < 0.001), time to peak (t1) and dM1 (r = 0.812, P < 0.01). The regression analysis indicated that RTR greater or less than –1.0 Nm/ms results in opposite short latency response. We concluded that the different tasks for two knee extensor muscles result in transient interhemispheric effects which are time and rate of torque reduction dependent. Keywords: isometric contraction, knee extensors, torque, bilateral deficit In the ordinary life and especially during sports forces are usually exerted unilaterally, but in some physical activities contra-lateral, homologous muscles should be activated simultaneously. First, Henry and Smith (10) have published that the homologous muscle can produce less force during bilateral muscle contraction compared to unilateral contraction. This difference has been called bilateral force deficit. Since then numerous studies have been carried out on both upper (5, 11, 20, 24, 28, 34) and lower extremity muscles (2, 3, 7–9, 12–19, 23, 24, 27, 28, 32, 34, 35). Studies on comparison of uni- and bilateral force exertion were carried out under different experimental conditions. Namely, the phenomenon was studied under isometric (8, 13–15, 17, 18, 27, 28, 33, 34), concentric (3, 5, 8, 16, 33) and eccentric contractions (5), moreover during multi-joint movement (1, 2, 35, 37). The muscle contractions were elicited voluntarily (34), under reflexively evoked contraction (16), under electrical (34) or transcranial magnetic (4, 6) stimulation in these experiments. In most of the cases authors reported bilateral deficit (BLD) that ranged between 12% and 26% in the lower limbs (3, 15, 17, 28, 34). It was reported that BLD was significantly greater during fast than Corresponding author: Prof. dr. József Tihanyi Department of Biomechanics, Kinesiology and Informatics Faculty of Physical Education and Sport Sciences, Semmelweis University Alkotás u. 44, H-1123 Budapest, Hungary Phone: +36-1-356-7327; Fax: +36-1-356-6337; E-mail: tihanyi@tf.hu