The lumbopelvic-hip complex has been referred to as the core, and it connects the upper and lower extremities (7). Specific core stability training (CST) has been evaluated in several studies related to low back pain and injury prevention (4,20,26,27). Core endurance with low loads, multiple repetition, isometric exercises, balance training, and isolated training of local muscle stabilizations of the lumbopelvic-hip complex has been used as training approach (9,15,24). However, using the same training approach among athletes to improve sport performance has resulted in contradictive results. Schibek (21) and Stanton et al. (23) used a Swiss ball-based CST program that showed significant improvements in core stability. However, they found no improvements in swimming and running performance after the training period. Pedersen et al. (17) and Seiler et al. (22) investigated the effect of CST with the limb suspended in unstable slings among competitive soccer players and experienced golfers. After 8 and 9 weeks of sling exercise training (SET)-based CST program, both groups significantly improved in maximal kicking velocity and maximal clubhead velocity. These different results could be explained by the different training approaches. Some of the core stability programs did not follow the principle of specificity, regarding position timing and functional specificity (23) and the principle of overload (21). Despite, including instability surfaces and exercises, the training program had to follow the principles of specificity and overload to enhance sports performances.
To our knowledge, there are no studies that have attempted progressive sport-specific exercises using CST exercises using unstable slings and compared the effect on maximal throwing velocity among experienced female handball players. Therefore, the aim of this study was to quantify the impact of SET-based core stability program using closed kinetic chain exercises on maximal throwing velocity in team handball players. It was hypothesized that SET would increase maximal throwing velocity.
Experimental Approach to the Problem
A repeated-measures design with 2 groups (SET group and a control group) was used to determine the effectiveness of SET on throwing velocity in female handball players during a 6-week training program.
Twenty-eight female-team handball players (age: 16.6 ± 0.3 years, mass: 63 ± 6 kg, and height: 1.69 ± 0.07 m) from 2 teams were recruited and initially stratified according to team divided into a SET group (SET, n = 16) and a control group (CON, n = 12). Ethics approval was obtained from the local research ethics committee and with current Norwegian law and regulation. Before participating in the study, the subjects, parents, or guardians were fully informed about the protocol, and a written informed consent was obtained before all testing from each subject and their parents or guardians. A questionnaire regarding medical history, age, height, weight, training characteristics, injury history, team handball experience, and performance level was completed before participation. Entry criteria for the study included being free of injuries, training for competitive handball during the study, and at least 5 years of team handball experience. In addition, 4 subjects were excluded (2 in the SET group and 2 the in CON group) because of failed inclusion criteria or illness. Fourteen subjects of the SET group and 10 in the control group participated in the study. To control for potential confounding factors, self-report information regarding total training hours of handball bouts, strength training bouts, and other form of physical training, such as jogging, aerobics, swimming, between the pre and posttest was collected. The subjects trained on average before the intervention period 11.9 (±3.8) h·wk−1. The training included 7.5 (± 3.2) hours for handball bouts, and 1.2 (± 0.3) hours were for general strength training, which was performed at the end of some handball sessions. Both teams had the same amount of this strength training in which the subjects performed only sit-ups, push-ups, dips, squat jumps, etc.
The study was conducted during the middle of the competitive handball season from January to March, and the subjects played weekly matches. All subjects played for 2 leading age-group club teams in the same highest regional division and had an average of 8.0 (±1.4) years of handball experience. The CON group continued their normal handball training and performed their normal club-based training program during the intervention period. None of the CON group used SET before or during the intervention. The subjects in the SET group all attended the same sports high school, located adjacent to the training facility used in the investigation. The SET group replaced 2 technical handball sessions of their normal weekly training with 2 supervised SET sessions. For the 6-week duration of the study, they were allowed to perform the SET program as a group in the daily school schedule.
After a general warm-up of 15 minutes (jogging and throwing drills), throwing performance was tested using a stand-on-the-spot throw (a penalty throw in team handball). A standard handball for women (weight: ∼360 g and circumference: 54 cm) was used. The subjects performed 10 standing throws from a 7-m distance toward a shock absorbing mattress target (Figure 1). Before the pretest, the subjects were familiarized with the test procedure. This activity was undertaken to avoid a learning effect. The subjects were told to throw as fast and as straightforward as possible. Groups of 3 subjects were tested in rotation with a 60-second rest between each of the 10 attempts. The average of the 3 best throws was used in further analyses. The pre and posttests were conducted the week before starting the intervention period and 1 week after ending it.
Throwing velocity was determined by using 2 photocell arrays (Newtest 2000 Sprint Timing System, BY, Finland) placed across the flight path of the ball (Figure 1). The accuracy of the photocells arrays was ±0.001 second. The distance between photocell arrays (3.00 m) and angle between the tripods and the floor (90°) were standardized with a purpose-built frame. The first photocell tripod contained 7 photocells, and the second contained 8 photocells separated by a distance that allowed 2 photocells to be triggered simultaneously by the ball. The distance to the first photocell array was 1.00 m from ball release ([20,26]; Figure 1).
The SET group performed the CST program twice a week for 6 consecutive weeks. Each session began with a general warm-up of 15 minutes and consisted of 5 exercises in the slings and one on a balance pillow. The total duration of a training session was 75 minutes. There was no less than 48 hours between each session. A dedicated system of adjustable slings (Redcord AS, Arendal, Norway, www.redcord.com) was used instead of a Swiss ball, wobble board, or foam roller. The SET program consisted of 6 specific core and rotational stability exercises performed in a closed kinetic chain (Figures 2 and 3). Four sets of 4-6 repetition maximum (RM) were performed for each exercise, with 1-2 minutes of rest between sets. A rotation was performed in supine abduction exercise (Figure 2A) when the leg was fully abducted to better load trunk rotators involved in a throw.
An experienced expert in SET was present during every training session to make sure the exercises were performed correctly. The training progression incorporated 3 levels of difficulty by increasing the instability using a balance pillow or by increasing the length of the resistance arm (Figure 4).
Exercise selection and intensity gradation were determined from a pilot study and experience with similar subjects. The first level of each exercise was performed for the first 4 training sessions. The second level was performed from the fifth to eighth session. After 8 sessions, subjects who completed the exercises correctly were progressed to a third difficulty level. A fourth intensity level was introduced to the dynamic crunch and push-up exercises (Figures 2C and 3C) by applying manual resistance during the concentric phase. All exercises were performed in a slow and controlled motion except for the concentric phase of the push-up exercise and the one-leg squat. This was done to stress the neuromuscular system to stabilize the core by adjusting to the external loads (26).
To compare the effects of training on throwing velocity, a 2 (training group) × 2 (test time point) factor analysis of variance with repeated measures for the factor time point was used. The throwing velocity had an intraclass correlation coefficient of 0.994 and a coefficient of variation of 2.5%. Statistical power equations to determine a minimum study population at the p ≤ 0.05 level with a power of 0.8 revealed a sample of minimum of 6 subjects in each group. The throwing velocity was the dependent variable and by estimating ES = 4.97 and α = 0.05 for single-sided results in pairwise analyses, our sample size (n = 24) yielded a calculated power of 0.867. Alpha was set at p = 0.05 to determine statistical significance.
There was no difference in maximal throwing velocity between the SET and CON groups at pretest (p = 0.364). After training, the SET group demonstrated a significant increase in throwing velocity (4.9%; p = 0.01), although the throwing velocity of CON was statistically unchanged (p = 0.418; Figure 5).
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.
Four of the 6 exercises were aimed at strengthening the lumbopelvic area (Figures 2 and 3). The push-ups and one-leg squat exercises trained the strength and stability of the shoulder girdle and 3D control of the femur, respectively (Figure 3). These exercises extended the core stabilization load from the knee via the hip, to the shoulder. The nature of the equipment led to improved torso and shoulder stabilization during throwing enhanced by the activation of the musculature. The slings are inherently unstable, meaning that the SET group-enhanced activation of the musculature involved the stabilization of the shoulder and torso. By training the stability and strength of the upper and lower extremities, an increased stability between each joint might reduce the loss of energy between the segments (1,4-6,15,18). Training in unstable conditions, as with slings and balance pillows, may therefore have reduced this loss of force output (1,4,5). A reduction of force output might influence the throwing velocity in the posttest (13,16,18,25).
A challenging aspect of sports training is to maximize training transfer to performance (8,23,29). Several core-training programs aimed at reducing low back pain and preventing injury have suggested the use of low load and high repetitions or holding time (2). However, the goal of the exercise selection and muscular load range in the present study was to improve performance in a movement requiring very high levels of muscle recruitment. Therefore, a low-volume, high-intensity core-training regime was employed. It is unclear whether a more traditional low-load, high-repetition training core-training program would elicit the same transfer to performance. Another important aspect of the specificity of a training program is the plane of movement and axis of rotation. Several recommended core-training exercises are only performed in 1 or 2 planes of motion and without rotational instability. In sport performance such as throwing, kicking, serving, etc., the rotation of the core along the vertical axis is a critical part of the motion (13,15,25). An advantage of the present training program may be the 3D instability created by the sling environment.
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.
Whether the changes in throwing performance observed here were caused by greater strength of the core and subsequent greater contribution of the hip and trunk to segmental velocity summation or a more stable core resulting in a reduced loss of force transfer between the segments cannot be determined from the current study.
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 effectiveness of exercises performed on unstable equipment has demonstrated the likelihood of reducing injuries. Exposing the joints for destabilization force during training may encourage an effective neuromuscular pattern and increase force production. This study showed that a core-training program of progressively unstable closed kinetic chain can improve a highly specific performance task. The training exercises have to resemble the sport-specific demands and the core exercises should also focus on the rotation of the core muscles. Thus, the kinetics of throwing are similar to other sports involving segmental summation. It is therefore likely that the improvements observed in the study may translate to other sports and improve specific performance tasks.
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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