Agonist and antagonist muscle activation during maximal and submaximal isokinetic fatigue tests of the knee extensors A. Hassani, D. Patikas, E. Bassa, K. Hatzikotoulas, E. Kellis, C. Kotzamanidis * Laboratory of coaching and Performance, Department of Physical Education and Sport Science, Aristotle University, Thessaloniki, Greece Department of Physical Education and Sport Science at Serres, Aristotle University, Serres, Greece Received 15 March 2005; received in revised form 9 November 2005; accepted 23 November 2005 Abstract The purpose of this study was to examine the differences in electromyographic activity of agonist and antagonist knee musculature between a maximal and a submaximal isokinetic fatigue protocol. Fourteen healthy males (age: 24.3 ± 2.5 years) performed 25 maximal (MIFP) and 60 submaximal (SIFP) isokinetic concentric efforts of the knee extensors at 60° s À1 , across a 90° range of motion. The two protocols were performed a week apart. The EMG activity of vastus medialis (VM), vastus lateralis (VL) and biceps femoris (BF) were recorded using surface electrodes. The peak torque (PT) and average EMG (aEMG) were expressed as percentages of pre-fatigue max- imal value. One-way analysis of variance indicated a significant (p < 0.05) decline of PT during the maximal (45.7%) and submaximal (46.8%) protocols. During the maximal test, the VM and VL aEMG initially increased and then decreased. In contrast, VM and VL aEMG continuously increased during submaximal testing (p < 0.05). The antagonist (BF) aEMG remained constant during maximal test but it increased significantly and then declined during the submaximal testing. The above results indicate that agonist and antagonist activity depends on the intensity of the selected isokinetic fatigue test. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Electromyography; Fatigue; Knee muscles; Maximal isokinetic; Submaximal isokinetic; Coactivation 1. Introduction Numerous studies in the past have analyzed agonist and antagonist EMG activity of the knee muscles during max- imal isometric [16,23,25] isokinetic [11–14,21,28,29,31,32] and submaximal isometric [7,10,22,24] fatigue protocols. It appears that both agonist and antagonist activity are muscle action and intensity dependent. During maximal isometric fatigue tests of the knee extensors the agonist activity decreases in parallel with tor- que decline while antagonist activity is constant [23,25]. In contrast, research findings during isokinetic fatigue tasks are conflicting, as agonist activity has been found to increase [13,14], decrease [29] or increase and then decrease [31,32] during the test. Antagonist activation during maxi- mal isokinetic tests also appears not to be influenced by agonist muscles fatigue [12,13]. Studies on isometric tests [7,10] have suggested that there are differences in the mechanisms that regulate max- imal and submaximal fatigue tasks. This takes the form of a different pattern of agonist and antagonist activity between maximal and submaximal isometric tests and has been attributed to differences in motor unit activation strat- egy [7,22–25]. Particularly, when subjects perform isomet- ric efforts at a given submaximal force level until exhaustion, torque output is maintained constant for a per- iod of time until fatigue occurs [22]. This phase of constant torque is accompanied by an increase of agonist and antag- onist activity [22,24]. Isokinetic tests demonstrate different activation patterns compared with isometric tasks [1,2,20] and therefore 1050-6411/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.jelekin.2005.11.006 * Corresponding author. Tel./fax: +30 2310 992221. E-mail address: kotzaman@phed.auth.gr (C. Kotzamanidis). Journal of Electromyography and Kinesiology 16 (2006) 661–668 www.elsevier.com/locate/jelekin