Biomechanics Research Querterly for Exercise and Sport ©2006 by the American Alliance for Health, Physical Education, Recreation and Dance Vol. 77, No.3, pp. 296-303 Effects of Movement Velocity and Maximal Concentric and Eccentric Actions on the Bilateral Deficit D. Clark Dickin and Danny Too This study was performed to examine the effects of movement velocity and maximal concentric and eccentric actions on the bilateral deficit. Eighteen female participants performed maximal unilateral and bilateral knee extensions concentricaUy and eccentrically across six movement velocities (30,60, 90, 120, 150, and 180·/s). &!peated measures analyses of variance revealed significant differences (p < .025) between bilateral and summed unilateral contractions both concentricaUy and eccentricaUy at each velocity tested. Post hoc analyses revealed that the degree of bilateral deficit increased as movement velocity increased for concentric actions (e.g., 17-33 % deficit, for 30 and 180 0 /s, respectively), and an increasing trend was seen for eccentric actions (e.g., 18-25 % deficit,for 30 and 180 0 /s, respectively). These findingr suggrm that with increased velocity, a decreased or incomplete activation of fast twitch muscle fibers may have occurred in bilateral actions when compared to unilateral actions. Key words: parallel elastic, serial elastic, stretch reflex, stretch shortening cycle S imultaneous voluntary contractions of homolo- gous muscles have been shown to produce less force than the same muscles contracting unilaterally. This difference has been termed the bilateral deficit and has been recorded for a number of different muscles of the upper and lower extremities during both isometric and concentric actions (Oda & Moritani, 1994; Ohtsuki, 1983; Vandervoort, Sale, & Moroz, 1984; Vandervoort, Sale, & Moroz, 1987). Deficits in force production during concentric bi- lateral actions have been shown to increase as a func- tion of increased movement velocity (Vandervoort et al., 1987). Although research has not determined the Submitted: March 23, 2004 Accepted: September 25, 2005 D. Clark Dickin is with the Department of Health, Physical Education, Recreation, and Dance at the University of Idaho. Danny Too is with the Department of Physical Education and Sports at the State University of New York, College at Brockport. cause of the bilateral deficit, a number of explanations have attempted to address this phenomenon. One ex- planation may be that activated antagonistic muscula- ture during maximal contractions results in a reduction in overall force production (Cresswell & Ovendal, 2002; Jakobi & Cafarelli, 1998). A second possible explana- tion could be an incomplete activation of primarily fast twitch muscle fibers (Koh, Grabiner, & Clough, 1993; Ohtsuki, 1983; Vandervoort et al., 1984; Vandervoort et al., 1987), evidenced by an increasing deficit as a function of increasing velocity. Third, this increasing deficit as a function of increased movement velocity may be attributed to decreased time available to de- velop cross-bridges in the actin-myosin complex dur- ing contraction (Huijing, 1992). Fourth, limitations of the central (Oda & Moritani, 1994; Van Dieen, Ogita, & De Haan, 2003) and peripheral (Archon tides & Fazey, 1993; Ohtsuki, 1983) nervous system have also been proposed to explain the bilateral deficit. However, to date, the cause of the bilateral deficit in force pro- duction has not been adequately determined. There is a paucity of literature on the topic ofbilat- eral deficits in force production with eccentric actions. This can be attributed to the lack of machines with at- tachments that allow for the testing of maximal bilateral RIlES: September 2006 296