Effects of an intermittent exercise fatigue protocol on biomechanics of soccer kick performance E. Kellis, A. Katis, I. S. Vrabas Department of Physical Education and Sports Sciences at Serres, Aristotle University of Thessaloniki, Thessaloniki, Greece Corresponding author: Eleftherios Kellis, PhD, TEFAA Serres, Serres 62100, Greece. Tel: 130 2310 991044, Fax: 130 2310 991046, E-mail: ekellis@phed-sr.auth.gr Accepted for publication 16 August 2005 The purpose of this study was to examine the effects of fatigue on biomechanical indices of soccer kick perfor- mance. Ten male amateur soccer players performed max- imal instep kicks prior to, in the middle and after the implementation of a 90 min intermittent exercise protocol. Three-dimensional data, ground reaction forces (GRFs) and segmental moments were measured during the kick while blood lactate and ammonia concentrations were monitored throughout the protocol. Analysis of variance designs with repeated measures indicated a significant increase in ammo- nia (Po0.01) and lactate levels (Po0.01) following fatigue. The GRFs and joint displacement curves during the kick remained unaltered after fatigue (P40.01). However, post- fatigue maximum angular velocity of the shank, the net moments acting on the shank and the resultant joint moments were significantly lower compared with the corresponding pre-exercise values (Po0.01). The velocity of the ball was 24.69 m/s prior to the protocol and significantly decreased to 21.78 m/s after (Po0.01). Simi- larly, the ball/foot speed ratio significantly (Po0.01) declined from 1.40  0.12 (pre-fatigue) to 1.33  0.18 (post-fatigue). The present results suggest that an exercise protocol that simulates soccer game conditions results in significant impairment of soccer kick performance. This could be attributed to alterations of the function of the neuromuscular system and force generation capacity, which may have altered the mechanics of soccer kick performance. Fatigue is indicated by a reduction of maximal force or power that is associated with sustained exercise and is reflected in a decline in performance (Rah- nama et al., 2003). Numerous studies have examined the effects of various fatigue protocols on compo- nents of physical performance, such as muscle strength and power (for reviews, see Byrne et al., 2004; Millet & Lepers, 2004), but fewer have exam- ined fatigue effects on performance of multisegment movements frequently used in sports (Nicol et al., 1991; Bruggemann, 1996; Forestier & Nougier, 1998; Rodacki et al., 2001; Madigan & Pidcoe, 2003). These studies vary, depending on the type of protocol used and the activity examined. Some studies found significant effects of local muscle fatigue protocols on the performance of multi- segment tasks such as handball throwing (Forestier & Nougier, 1998) and vertical jumping (Rodacki et al., 2001). Other studies applied sport-specific exercise protocols such as long-distance running (Nicol et al., 1991; Bruggemann, 1996), repetitive landings (Madi- gan & Pidcoe, 2003) or lifting (Sparto et al., 1997) and reported a significant decline in leg power, maximum isometric force and activity of the quadriceps (Nicol et al., 1991) as well as alterations in ground reaction force (GRF) and joint kinematics of running (Wil- liams et al., 1991; Bruggemann, 1996; Mizrahi et al., 2000), landing (Madigan & Pidcoe, 2003) and lifting (Sparto et al., 1997). Although the above studies demonstrated significant effects of fatigue on biome- chanics of multisegment tasks, they are specific to the protocol or the task examined. Striking movements are common in many sports (soccer, tennis, veolleyball, etc.) and in everyday life. Kicking represents an example of such motion. Soccer kick is the result of coordinated segmental actions aiming to produce the highest possible ball velocity to a certain target (goal). Numerous studies have examined the biomechanics of kicking, mostly under non-fatigued conditions (for a review, see Lees & Nolan, 1998). To our knowledge, only one study (Lees & Davies, 1988) examined the effects of fatigue on soccer kick perfor- mance and reported a lower maximum velocity of the foot and the ball following a 6 min step exercise proto- col. The authors suggested that fatigue caused a lack of coordination between the upper and lower leg (Lees & Davies, 1988). However, the exercise protocol used differs compared with the intermittent exercise proto- cols frequently applied in soccer (Reilly & Thomas, 1976; Rahnama et al., 2003) and full kinematic data Scand J Med Sci Sports 2006: 16: 334–344 COPYRIGHT & BLACKWELL MUNKSGAARD 2005 Printed in Singapore . All rights reserved DOI: 10.1111/j.1600-0838.2005.00496.x 334