DROP JUMP TRAINING STIMULUS INDUCES DIFFERENT QUALITATIVE ADAPTATIONS ON THE ELECTROMYOGRAPHIC (EMG) PATTERN OF THE LEG EXTENSOR MUSCLES
AbstractINTRODUCTION Jumping technique can dramatically affect drop jump performance (Warren et al., 1995), which suggests that the quality of the training stimulus may also influence the neuromuscular control of drop jump exercises. The present study was designed to investigate the changes in performance characteristics and in neuronal activation patterns, induced by different drop jump training stimulus. Thirteen healthy males were involved in an 8 weeks training program, followed by a detraining period o 4 weeks and a new training period of 4 additional weeks. The subjects exercised 3 to 4 times per week. They performed reactive.DJ from their best drop height. All the training sessions were supervised. During the sessions of the first 4 weeks the subjects were informed on the flight time of each jump and a reactive jump with maximal effort with a short contact time were continuously demanded by the supervisor. On the training sessions from week 4 to week 8 the supervisor only informed the subjects about the flight time of the jump. Finally, on the last 4 weeks the subjects were informed both on the flight and contact time of each jump. The testing procedures took place before and after each 4 weeks. The subjects performed reactive drop jump (DJ) exercises from the heights of 25, 40, 55 and 70 cm. The vertical ground reaction forces and the surface electromyograms (EMG) of the triceps surae muscles (GAS and SOL) were recorded. The EMGs were full-wave rectified and integrated (iEMG) over different functional phases (Schmidtbleicher et al., 1988): Preactivation phase (PRE) (100ms before ground contact); Reflex Induced Activation phase (i.e. the activation phase from 40 ms to 120 ms after impact); Late EMG Response phase (LER) (activity from 120 ms until the end of contact). RESULTS Table 1 summarises the results observed on the jump heightlcontact time ratio (JHCT) during the training process. Table 1- Mean and standard deviation of the jump height contacV contact time ratio (JHICT), for drop jump exercises from 25 cm (DJ25), 40 cm (DJ40), 55 cm (DJ55) and 70 cm (DJ70), during the training The increase on the jump height/contact time ratio (JHCT) corresponds to the training periods where the subjects were continuously instructed to jump reactively and received feedback information on the jump height and contact time of each individual drop jump. A decrease on the jump height/contact time ratio (JHCT) occurred when the feedback was only the jump height of the drop jumps. The EMG results showed a qualitative shifting in the EMG-patterns toward an accented RIA-phase activation, when the JHCT ratio increased. Additionally the changes in contact time were correlated negatively (r=-0.70 ; -0.85, pc0.001) with the iEMG of the RIA-phase for GAS and SOL muscles. CONCLUSION These results revealed that feedback on the jumping performance, produced clear differences on the biomechanics of the jump. Qualitative adaptations on the EMG pattern induced by a strength training program with drop jump exercises, are only observed if the jumping technique allows for a good jump height/contact time ratio. These parameter should be used to monitor the quality of reactive strength training stimulus.
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