Effects of String Stiffness on Muscle Fatigue after a Simulated Tennis Match

• Fabre Jean-Bernard
• Martin Vincent
• Borelli Gil
• Theurel Jean
• Grélot Laurent

We tested the influence o f string stiffness on the occurrence o f forearm m uscle fatigue during a tennis m atch. Sixteen tennis play­ ers perform ed tw o prolonged sim ulated tennis m atches w ith low-stiffness or high-stiffness string. Before and im m ediately after exercise, m uscle fatigability was evaluated on the forearm muscles during a m axim al interm ittent gripping task. G roundstroke ball speeds and the profile o f acceleration o f the racquet fram e at collision w ere recorded during each match. The peak-to-peak am plitude o f acceleration and the resonant frequency o f the fram e w ere significantly greater with high-(5060 ± 1892 m /s2 and 204 ± 29 Hz, respectively) than w ith low-stiffness string (4704 ± 1671 m /s2 and 191 ± 16 Hz, respectively). The m axim al and the averaged gripping forces developed during the gripping task were significantly reduced after the tennis m atch with high-(-1 5 ± 14%, and-2 2 ± 14%, respectively), but not with low-stiffness string. The decrease o f ball speed during the sim ulated m atches tended to be greater with high-than with low-stiffness string (P-.06). H ence, playing tennis with high-stiffness string prom otes forearm m uscle fatigue developm ent, w hich could partly contribute to the groundstroke ball speed decrem ent during the game. Keywords: sport engineering, electrom yography, gripping force, im pact, racquet C om posite m aterials have allowed racquet engineers to design tennis racquet fram es by considering the physics o f the sw ing, w ithout being lim ited by the structural lim itations o f wood. C on­ sequently, m odern racquets are generally stiffer, lighter, and have a low er sw ing w eight (ie, m om ent o f inertia) than old wood racquets.1 These structural modifications have therefore prom oted a change in playing style from one based on the correct execution o f the tech­ nique to one characterized by power and spin.2 Indeed, although sw ing speed does not significantly vary as a function o f racquet m ass, it is significantly increased when sw ing w eight decreases.1 Thus, modern racquets allow the players to swing the racquet faster.3 This contributes to the faster pace o f m odern tennis, together with the im proved fitness o f tennis players, and larger racquet heads that allow players to hit the ball faster, w hile still m aintaining a good ball co n tro l.1 M oreover, despite being lighter, com posite fram es are stiffer than traditional frames. A s a result, stiff fram es allow a reduction o f the ball energy consum ed in bending the racquet, thereby im proving hitting power.4 T he com bination o f the increased fram e stiffness and sw ing speed has led to increased im pact forces,2 w hich have to be resisted for by the action o f the upper lim b m uscles. In that context, em pha­ sis has been put on the contribution o f the string properties to the reduction o f im pact forces.2 Indeed, the m ain purpose o f the string is to absorb the kinetic energy from the relative ball-racquet velocity, Jean Bernard Fabre is with CNRS, ISM UMR 7287, Aix-Marseille Univer­ sity, Marseille, France, and with the Research Department, ESP-Consulting, Aix-en-Provence, France. Vincent Martin is with the Laboratory of Meta­ bolic Adaptations to Exercise in Physiological and Pathological conditions (AME2P, EA 3533), Blaise Pascal University Clermont-Ferrand, France. Gil Borelli and Jean Theurel are each with the Research Department, ESP-Consulting, Aix-en-Provence, France. Laurent Grdlot is with CNRS, ISM UMR 7287, Aix-Marseille University, Marseille, France. Address author correspondence to Jean Bernard Fabre atjb.fabre@esp-consulting.fr. turn it into potential energy, and then give that energy back to the ball. For a sam e m om entum variation (ie, mass x speed variation o f the ball-racquet system), if the friction betw een the ball and the stringbed during the collision is neglected, the im pulse o f the ball in the stringbed (ie, im pact force x contact time) does not change, w hatever the string properties. However, it is generally accepted that a string with a low stiffness bends m ore than one w ith high stiffness during the collision, thereby increasing the contact tim e and thus lim iting the im pact forces transm itted from the racquet to the player.5 The impact forces destabilize the racquet, w hich m ainly tend to recoil, thereby generating a high torque at the wrist joint. This phenom enon is also influenced by the location o f ball im pacts:3 the racquet rotation within the hand is increased for off-center im pacts (below or above the longitudinal axis o f the racquet, for exam ple).6 Therefore, a high gripping force is required to prevent excessive rotation o f the racquet at im pact.6-9 However, it is widely accepted that the repetition ot high force gripping contractions generates m uscle fatigue, w hich reduces maximal gripping force.10 The use o f high-stiffness string, w hich translates into increased im pact forces, could thus result in the increase o f the required gripping strength, thereby prom oting fatigue developm ent. T he tension o f the string may also affect the intensity o f im pacts. Low ering tension would have the sam e effect that low-stiffness string; that is, the slightly longer contact time would translate into a higher outgoing ball speed, as long as the deform ation o f the string is perpendicular to the string plane, and a low er im pact force. However, when the strings begin to m ove w ithin the string plane, rubbing against each other, som e elastic energy is lost.3 M oreover, playing with a low string tension also translates into reduced control o f the ball.3 Thus it does not seem recom m endable to play with a low string tension. In that context, players may be prone to choose high-stiffness string, w hich allow s a better control o f the ball trajectory. However, it can be reasonably suggested that the m echanical characteristics o f the string could influence the occurrence o f forearm m uscle fatigue during tennis play, by contributing to the intensity o f im pact 401