Improvements over time of average jumping heights for three experimental groups, free-jumping, 10%-assisted-jumping, and 20%-assisted-groups, during 10 week training periods are introduced in Fig 1. Averages of the highest improvement of the jumping height of each individual subject during 10 week training period for three training groups are also introduced in Fig 2. All three groups significantly improved in jumping heights during 10 week training period (p0.001 by One-Factor ANOVA). Fisher's PLSD Test (ANOVA) also proved that the assisted-jumping, both 10%-assisted and 20%-assisted, was superior to free-jumping training in developing vertical jumping ability (p0.005). However, the same test also proved that there was no significant difference in the effect between 10%-assisted and 20%-assisted training methods.
Discussion:
It was found that free-jumping training is more effective than weight training such as squatting, and the combined training of free jumping and squatting is the most effective to develop vertical jumping ability. On the other hand, it is known through the studies of force-power relationship that peak power is attained at the force of 30% of maximum voluntary muscle contraction (%MAX), and the most effective way to train power output is to train with the load of 30 %Max. The force-power studies suggest that the effectiveness of the free jumping training on vertical jumping ability is because the load applied to the leg muscles by free jumping may be suited to 30 %MAX for the jumpers. However, other investigators have suggested that only in the single-joint activities the peak power output is attained at 30 %MAX but in the multi-joint activities such as squatting the peak power is achieved at around 50 %MAX.
In this study, it was clear from the results that suspension training was more effective than free jumping training for developing vertical jumping ability. Apart from the force-power relationship, the authors of this study are more concerned about the neural adaptation (improvement in the muscle-nerve systems) in the subject's leg muscles with regard to the improvement in the jumping height by the suspension training. It is well known that muscle activities in a wide sense result from activation of prime movers, appropriate co-contraction of synergists, and inhibition of antagonists. In a local sense they result from motor unit activation (motor unit recruitment and firing frequency), reflex potentiation, and motor unit synchronization. In the suspension training, the upward assistance by a rubber tubing seemingly reduces the value of the body weight by 10 or 20% and helps the subject to increase the takeoff velocity which should be faster than in free jumping. By repeating this fast motion in certain intervals for certain period of time, ten repetitions, three times a week, and for ten weeks in this experiment, the neuro-muscular systems in subjects' leg muscles may be enhanced and eventually they may learn the fast takeoff action even without being assisted. This idea of neural adaptation is supported by studies which report that in short training periods or during the early stage of training strength gain (and power gain) in muscles is attained without hypertrophy in the muscles.
There was no significant difference between the 10%-assisted and the 20%-assisted groups in the improvement in the vertical jumping ability even though the average increment in jumping height of the 20%-assisted group was more than that of the 10%-assisted group. The reason for this is unclear. There may be a suitable value or a limitation in the amount of assistance in the suspension training. However, further study on a larger group of studies may be necessary to understand this problem. Also, we could not clarify exactly which factor or factors in neuro-muscular systems increase in motor unit synchronization, more appropriate co-contraction of synergists, increased inhibition of antagonist, more appropriate synchronization of contralateral upper or lower limbs, etc., affected the improvement in vertical jumping ability after the suspension training. Further investigations utilizing biomechanical and also electromyographic methods may clarify some aspects of this problem.
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