The most important findings of the present study were that with an 8-week training program for adolescents (a) combination of balance and plyometric (COMBINED) training induced training responses that were substantially greater for leg stiffness, 10-m sprint, and shuttle run, than plyometric alone (PLYO) and (b) PLYO training improved balance, jump, and squat strength measures to a similar degree as the COMBINED training group. Thus, with the 12- to 15-year-old adolescent boys, an 8-week COMBINED training improved some functional measures more often than PLYO training; however, the COMBINED program did not achieve substantially greater improvements in balance.
However, balance can also be improved with strength training programs. Brooks et al. (14) showed improvements in APAs after an 8-week Pilates exercise program targeting trunk muscles. Seven (39) and 7 (59) weeks of traditional resistance training improved balance scores by 44.8% and 12.4%, respectively. Holviala et al. (35) reported 35.8% and 23.4% improvements in dynamic balance in middle-aged and older women, respectively, after 21 weeks of heavy resistance training. In elderly individuals, electromyostimulation training over 4 (1) and 6 (54) weeks, respectively, improved postural sway/control; however, voluntary strength training was more effective than electromyostimulation in the study by Paillard et al. (54). Strength training would be expected to enhance the ability and speed of postural muscles to return to a more stable position (59) after a balance perturbation.
Similar to the present study, 7 weeks of plyometric training produced similar improvements in dynamic balance measures (decreases in the SD of the center of pressure during hop landings) as a dynamic stabilization and balance training group (52). Plyometrics are a dynamic form of resistance training involving a rapid SSC and can involve both vertical and horizontal displacements of the center of gravity (41). Although specific landing and balance exercises were not included with the PLYO training in the present study, the dynamic nature of the exercises would place a training stress on postural control or equilibrium. Similarly, a number of different types of athletes without specific balance training programs exhibit better balance than the average population because of the dynamic nature of their sports. For example, triathletes have been reported to be more stable and less dependent on vision for posture than controls (53). Gymnasts are reported to be more efficient at integrating and reweighting proprioceptive inputs (61). Because plyometric exercises can provide a spectrum of balance challenges, specific balance or landing exercise may not be necessary for all individuals or activities.
However, the COMBINED training group did show advantages for leg stiffness, 10-m sprint, and shuttle run performances. Sprinting and shuttle runs involve consecutive or repeated unilateral landing and propulsion phases (22). The repetitive unilateral hip flexion and extension movements can place considerable destabilizing torques on the trunk and pelvis (58). While sprinting, the center of gravity pivots over the support foot (sequentially moving outside to inside to outside the limits of the center of gravity) primarily in the sagittal plane (anteroposterior) when transitioning from foot contact to takeoff. In addition, with unilateral foot contacts, the pelvis on the swing leg phase side will drop unless stabilized by the trunk muscles. Behm et al. (5) demonstrated the high intensity of trunk muscle activation induced from running to stabilize the pelvis needed to ensure optimal running performance in both trained (triathletes) and untrained runners. Hence, running provides instability or balance training stresses because of the anteroposterior translation of the body, angular momentum of the limbs, and the vertical or oblique displacements of the pelvis. Shuttle runs with the changes in direction would further displace the center of gravity in all 3 planes of movement. Hence, the rapid perturbations to and oscillations of the center of gravity while sprinting and performing shuttle runs can be considered a far more demanding physiological test of balance than static balance tests.
PLYO training was not clearly different than combined training for CMJ, 1RM leg press, reactive strength, THT, and SLJ. All these measures involved bilateral actions (greater base of support can provide less balance challenges than unilateral actions), and thus the balance benefits from PLYO training were as equal to the task as the COMBINED training. Whereas PLYO and COMBINED training provides similar adaptations for static balance and bilateral tests, the increased emphasis on balance and stable landings in the combined training program can provide an advantage for the more complex, high speed, unilateral, repetitive dynamic tasks that may tax postural control to a greater degree.
Leg stiffness also increased to a greater degree with the COMBINED training group. Carpenter et al. (16) indicated that a stiffening strategy was adopted when individuals were presented with a threat of instability. Increased antagonist activity can contribute to increased joint stiffness (37). Antagonist activity is greater when uncertainty exists in the task (47), which may occur during the flight phase of a drop jump. The role of the antagonist would be to control mechanical impedance (opposition to a disruptive force) (34) and optimally position the limbs and body on surface contact (23). Furthermore, co-contractions are prevalent for joint protection (3).
Balance and perturbation training can significantly improve neuromuscular control by promoting APAs (26). Anticipatory postural adjustments are not unique to the spine and have been shown in peripheral joints as well (20). Repeated exposures to balance and stability challenges result in proactive (45) or feedforward (55) adjustments that would contract (and stiffen) appropriate muscles before landing. Furthermore, the sensitivity of afferent feedback pathways can be improved with balance and motor skill training (12) resulting in quicker onset times of stabilizing muscles (2). Thus, the increased leg stiffness with the COMBINED training group could also have contributed to the greater 10-m sprint and shuttle run performances. Furthermore, the APA or feedforward adjustments could also contribute to injury prevention. For example, balance training has been reported to reduce the incidence of ankle sprains with volleyball players (60).
In conclusion, an 8-week training program for 12- to 15-year-old adolescent boys demonstrated that PLYO training was as effective as COMBINED training for static balance tests and that COMBINED training produced better results for 10-m sprint and shuttle run times, and leg stiffness. The inherent balance challenges associated with plyometric training are sufficient to adequately improve static balance measures and bilateral activities that commence with an adequate base of support (i.e., CMJ, SLJ, squats), whereas the greater stability or balance challenges of consecutive unilateral activities such as high speed sprinting and change of direction running (shuttle) may be enhanced with the addition of balance training to plyometric exercises. The increased leg stiffness scores found with the combined training may also have contributed to the greater improvement in sprint and shuttle run performance.
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