Thirteen of the 14 subjects performed all 6 exercises at level 3 at the end of the training period. The subjects that were not able to train at level 3, trained at level 2 in the Superman exercise and the Push-up exercise (Figures 3A, C) after ending the training period. The rest of the exercises were performed at level 3. At the end of the training period, 7 subjects performed the Dynamic Crunch exercise at level 4 and 5 subjects the Push-up exercise at this level. The subjects performing the exercises at level 4 were not significantly different in change in throwing velocity after the training period compared with others from the SET group (Push-ups, p = 0.38 and Dynamic Crunch, p = 0.09).
The purpose of this study was to determine the effect of a SET-based core stability program on maximal throwing velocity among handball players. The main result of this study supports our hypotheses. Closed kinetic chain exercises in unstable slings improved maximal throwing velocity among 16-year-old female handball players. The SET group showed a ∼5% improvement in maximal throwing velocity, whereas the control group did not change. The performance enhancement observed in the present study is consistent with other studies involving more specific training of the shoulder girdle and traditional high-intensity strength training. Barata (3), Ettema et al. (10), and Gorostiaga (12) used general strength training programs (3 × 6RM, 8-12RM and pyramid training) on young handball players. After a 6- to 9-week training period, the increase in throwing velocity was between 1.4 and 6.9% (3,10,12). Prokopy et al. (18) recently also employed closed-kinetic chain exercise-based training methods in slings and reported a 3.4% of throwing velocity among NCAA division I softball players. Considering the short intervention period, the magnitude of the improvement observed suggests performance benefits comparable to or better than other investigated training modalities.
The increase in throwing velocity of the SET group might be explained by an increase lumbopelvic rotational stability and strength. Thirteen of the 14 subjects in the SET group progressed from the first level to the third level of all of the exercises during the training period. The third level required greater stability and strength to be performed correctly than the first level. It is therefore likely that the SET group improved their core strength and/or improved the neuromuscular coordination of the core. The core receives, adds, and transfers energy from the proximal segments to the distal segments (14,15). Many factors such as proximal segment force production, proximal-to-distal force production (11,19), segmental decelerative capacity (11), segmental function, and postural stability affect throwing velocity. Exercises causing both strength and stability of the core might affect an athlete's ability to activate the muscles in a more coordinated way or generate more force (28). Changes in coordination, increased force generation, or both might improve rotational force generation and transfer. This may explain the significant enhancement in maximal throwing velocity after the training period. This hypothesis is supported by Kibler's findings (14). All subjects were active, experienced, and well-trained handball players in the middle of their competitive handball season. The subjects were tested in the most common and easiest technical throw in handball.
Difficulty in progression of core exercises might be another reason why others have not demonstrated any effect on sport performance (21,23). By increasing the instability and resistance arm, the difficulties of the exercises were progressed and provided a sufficient overload to the core musculature during the training period. By using slings and the balance pillows, the exercises became unstable. By doing so, we tried to activate the local stabilizing muscle of the core in an independent co-contraction of the global muscles (7,28). Trying to maximize the generated and transferred energy from the core, the muscles had to adjust the force based on feedback provided by the neural system (15,28). The neural subsystem continuously had to ensure sufficient stability and desired joint movements (15,28). Therefore, it was important that the exercises were performed correctly and in a controlled motion. The qualities of the execution among the exercises were an important part of the intervention and one of the reasons the SET expert was present during every training session.
Future research should therefore focus upon what changes during core training and how it affects sport performance such as throwing, kicking, serving, etc., among elite players. Currently, there is no validated test battery to evaluate core stability in athletes and potentially identify athletes that can benefit from targeted training. Therefore, future research should seek to establish a core stability test battery that involves dynamic muscle actions.
In conclusion, a unique functional, 3D, core stability program consisting of progressively unstable closed kinetic chain exercises for hips and torso significantly improves throwing velocity among handball players. High levels of core strength and stability may be an important precondition for generating high rotational velocities in multisegmental movements such as throwing.
The authors would like to thank the subjects at Gimle High School for their enthusiastic participation in this study. This work was supported by the Faculty for teachers training and sports, Sogn og Fjordane University College, Norway, and Institute of Public Health, Sport and Nutrition, University of Agder, Norway. The results of the present study do not constitute endorsement of the product by the authors or the NSCA.
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