Modem training concepts highlight the importance of a complex exercise set-up during the same training unit (Baker, 2003; Baker & Newton, 2005; Knauer & Mallow, 2006). A short-term improvement of the movement speed by contrast or complex-training methods using the effect of a `postactivation potentiation" and an increased "muscle stiffness" have been discussed (Robbins, 2005). From both a scientific and practical point of view, different designs for these kinds of interventions are presented. There are two conflicting theories about the optimum load for the improvement of power and movement speed (Surakka, Alanen, Aunola, Karppi & Pekkarinen, 2006; McBride, Triplett-McBride, Davie & Newton, 2002). One exercise method is to use heavy loads to induce recruitment of high-threshold fast Type II motor units by the size principle (Sale, 1987), and another method is to use light loads to maintain the specificity of the exercise velocity and to maximise the mechanical power output (Wilson, Newton, Murphy & Humphries, 1993). Studies with load controlled velocity training show that the speed at which an individual trains, results in a velocity specific change in muscle activation (McBride et al., 2002). When the intent of the muscle action is explosive, training with a specific load (and thus velocity) results in velocity specific increases in muscle activation. Thus it appears that the velocity of the movement, as controlled by the load, plays a key role in improving high-velocity performance capabilities and possible neural mechanism adaptations. Several studies focused on the kinetics of tennis service, on the importance of the service speed for the performance outcome in tennis and on different training concepts to improve service quality (Kleindder & Mester, 1991; Treiber, Lott, Duncan, Slavens & Davis, 1998; Kraemer, Ratamess, Andrew, Fry, Triplett-McBride, Kozins, Bauer, Lynch & Fleck, 2000; Ferrauti & Weber, 2000). Besides the importance of velocity-specificity, as discussed above, biomechanical similarity between the alternated training exercises in the complex training design is needed for optimal transfer and thereby improvement in functional performance. In tennis, several experts transfer the idea of complex training on the improvement of the tennis service, combining technical training with training of throwing power (Bom, 2002; Goebel, 2004). The major contributors to the mean linear velocity of the centre of the racket head at impact are internal rotation of the upper arm (54.2%), flexion of the hand (31%), horizontal flexion and abduction of the upper arm (12.9%), and racket shoulder linear velocity (9,7%) (Elliot, 1995). In overhead throwing actions similar mechanics are observed. This similarity explains the findings of Goebel (2004) that a strong consistency between the service and throwing velocity with tennis players exists. Although muscle strength plays a role in overhead sports performance, there is argue that light load, velocity-specific power and plyometric training also contributes to this. There are no controlled studies about the short or long term effects of a load controlled velocity complex training intervention on the functional performance (serve speed and precision) of the tennis serve. In this study the complex training method for service improvement include upper-body exercises during one training unit that closely simulates the velocity and acceleration profiles and therefore contraction dynamics associated with the service performance. In so doing, the emphasis was put on the high-velocity kinetic chain of the dominant arm in order to maintain the speed and form specificity for the major contributors to serve speed.
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