Myer, G.D., K.R, Ford, J.L. Brent, and T.E. Hewett. The effects of plyometric vs. dynamic stabilization and balance training on power, balance, and landing force in ...
Journal of Strength and Conditioning Research, 2006, 20(2), 345-353 © 2006 National Strength & Conditioning Association
T H E E F F E C T S OF PLYOMETRIC VS. DYNAMIC STABILIZATION AND BALANCE TRAINING ON POWER, BALANCE, AND LANDING FORCE IN FEMALE ATHLETES GREGORY D . MYER/ KEVIN R . FORD,' JENSEN L . BRENT,' AND TIMOTHY E . HEWETT^ ^
'Cincinnati Children's Hospital Research Foundation Sports Medicine Biodynamics Center and Human Performance Laboratory, Cincinnati, Ohio 45229; and'^TheUniversity of Cincinnati College of Medicine, Departments of Pediatrics, Orthopaedic Surgery and the College of Allied Health Sciences, Department of Rehabilitation Sciences and the Department of Bioengineering, Cincinnati, Ohio 45229. Myer, G.D., K.R, Ford, J.L. Brent, and T.E. Hewett. The effects of plyometric vs. dynamic stabilization and balance training on power, balance, and landing force in female athletes, ABSTRACT.
J. Strength Cond. We.s. 20(2): 345-;i53. 2006.—Neuroniascular
training protocols that include both plyometrics and dynamic balance exercises can significantly improve biomechanics and neuromuscular performance and reduce anterior cruciate ligament injury risk in female athletes. The purpose of this study was to compare the effects of plyomelrics (PLYO) versus dynamic stabilization and balance training (BAL) on power, balance, strengtb. and landing force in female athletes. Either PI-,YO or BAL were included as a component of a dynamic neuromuscular training regimen that reduced measures related to ACL injury and increased measures of performance. Nineteen bigh school female athletes participated in training 3 times a week for 7 weeks. The PLYO in = 81 group did not receive any dynamic balance exercises and the BAL in - 11) group did not receive any maximum efTort jumps during training. Pretraining vs. posttraining measures of impact force and standard deviation of center of pressure (COP) were recorded during a single leg hop and bold. Subjects were also tested for training effects in strength (isokinetic and isoinertial) and power (vertical jump). Tbe percent cbange from pretest to posttest in vertical ground reaction force was significantly different between tbe BAL and PLYO groups on tbe dominant side ip < 0.05). Botb groups decreased tbeir standard deviation of center of pressure (COP) during hop landings in tbe medial/lateral direction on their dominant side, whicb equalized pretested side to side differences. Both groups increased hamstrings strengtb and vertical jump. The results of this study suggest tbat both PLYO and BAL training are effective at increasingmeasuresof neuromuscular power and control. A combination of PLYO and BAL training may further maximize tbe effectiveness of preseason training for female athletes. core stability, ACL injury prevention, performance training, neuromuscular training, biomechanics, jump training KEY WORDS,
INTRODUCTION
—^_ J I T I ^
nitial interventions aimed at reducing the incidence of anterior cruciate ligament (ACL) injuries in female athletes were developed based on empirical evidence from coaching and training female athletes and from performance enhancement research (11, 15). More current techniques, developed from injury mechanism and objective analysis of training methods, may further reduce traumatic ACL injuries in females (22, 27). The studies that demonstrate
ACL injury reduction are comprehensive, and include multiple training components that may induce the neuromuscular changes required to reduce ACL injury risk. There is evidence that neuromuscular training not only decreases ACL injury risk, hut also that it alters biomechanical risk factors for ACL injury and improves measures of performance in female athletes (14, 15, 22, 26, 27, 29). Effective neuromuscular training protocols have used plyometric power, hiomechanics and technique, strength, balance, and core stability training to induce neuromuscular changes and potential injury prevention effects in females (14, 15, 22, 26, 27, 29). However, it is not known which of these components are most effective and efficient or wbether the effects of the separate components are cumulative. Selective combination of neuromuscular training components may provide additive effects or a single component may provide tbe majority of the biomechanical alterations related to reduction of ACL injury incidence and improved neuromuscular performance in female atbletes 114, 15, 22, 26, 27, 29). Additionally, the time investment or difficulty of teaching these comprehensive protocols may preclude coaches or athletes from undertaking training or may reduce compliance witb training (22, 27, 29). To increase the utilization of ACL injury prevention training, more effective and efficient methods for reducing ACL injury risk and improving performance in female atbletes sbould be developed. When highly effective training methods are developed, dynamic neuromuscular training can be used with high-risk athletes on a widespread basis witb greater compliance. Tbe purpose of this study was to compare the effects of maximum effort plyometric jumping (PLYO) vs. dynamic stabilization and halance training (BALI on power, balance and landing force in female atbletes. Either PLYO or BAL were included as a component of a dynamic neuromuscular training regimen that reduced measures related to ACL injury and increased measures of performance (13, 25, 28). The first hypothesis was that the PLYO study group would demonstrate improvements in power (e.g., vertical jump) while the BAL group would not. Second, the BAL group would improve postural control measures (e.g., center of pressure [COP| sway), while tbe PLYO group would not. Tbe tbird hypothesis was tbat 345
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MYER, FORD, BRENT ET AL.
the BAL group would improve force dissipation measures and the PLYO group would not. The fourth hypothesis was that the PLYO groups would increase isokinetic strength. The final hypothesis was that hoth PLYO and BAL would improve isoinertial strength measures. METHODS
Experimental Approach to the Prohlem To compare the effects of PLYO versus BAL, a controlled lahoratory cohort experiment was conducted. Female athletes were randomized prior to pretesting to one of two training groups (PLYO or BAL). The PLYO group did not receive any dynamic balance exercises and the BAL group did not receive any maximum effort jumps during training. To evaluate the effects of each protocol the testing included dynamic landing force and COP control, vertical jump, and strength measures.
the final training session). The training sessions were conducted with a minimum 1:3 trainer to athlete ratio, by National Strength and Conditioning Association certified professionals and graduate and undergraduate level student interns to ensure proper institution of protocol guidelines. The subjects were randomized into 2 groups, which allowed for 2 training sessions per day. One group performed a protocol that involved maximum effort plyometric training (PLYO), while the second group performed a protocol that focused on dynamic stabilization and balance training (BAL). This study was approved by the Cincinnati Children's Hospital Medical Center Institutional Review Board.
Procedures Training. The neuromuscular training protocol used in this study was derived from a training protocol, previously shown to reduce biomechanical measures related to ACL injury and increase measures of performance (13, Suhjects 25, 28). The previous protocol was modified to include Nineteen female athletes from a Cincinnati area high only maximum effort plyometric jumps and cut maneuschool participated in this study. The age of the particivers (PLYO) or to include dynamic lower extremity stapants (mean + SD) was L5.9 ± 0.8 years in the PLYO bilization and balance exercises (BAL). The neuromusgroup and 15.6 ± L2 in the BAL group with a range of cular training was conducted on Monday, Tuesday, and 14 to 17 years. All the subjects listed their primary sport Thursday of each consecutive week. Each training session as volleyball, while 12 subjects listed secondary sports inlasted for approximately 90 minutes. Prior to each traincluding 5 in basketball, 2 soccer, 2 softball, and 1 swiming session an active warm-up that included 5 agility ladming. Seventy-nine percent of the subjects had at least 6 der runs was performed. Each of the groups participated years of experience in their reported primary sport, while in 2 types of training per day; either resistance training, none had less than 4 years of experience. Eighty-four perspeed interval training, or PLYO or BAL training decent of the subjects reported previous participation in pending on the group. Tahles 1 and 2 depict the protocols some form of off-season training program. This included for the specific PLYO and BAL sessions respectively tak79'^ in jogging programs, 74*}^ in weight lifting programs, en from the first session of the week in weeks 1, 3, and 42'7f in sprinting programs, and lOVr in a jump training 6. Tables 3 and 4 depict the corresponding strength and program. Height and mass were assessed at the pretrainspeed interval training protocols used by both groups. ing test date and the posttraining test date. The initial The focus of tbe PLYO group included an emphasis on height (mean ± SD) of the participants in the PLYO performing jumping movements with maximum effort group was 169.5± 6.1 cm and mass was 61.4 ± 7.3 kg. In and power, and also performing cutting techniques with the BAL group the initial height and mass (mean ± SD) quick reactions and maximal effort. The athletes received were 168.1 ± 6.9 cm and 66.4 ± 11.8 kg. Follow-up asfrequent feedback regarding the technical performance of sessment of height and mass at the posttest date revealed their jumping and cutting movements. Specifically, the no significant change in mean height while mean weight atbletes were instructed to perform maximum effort increased in the PLYO group to 61.8 ± 7.5 kg and in the jumps without knee valgus, with a focus on improving BAL group to 68.3 ± 11.9 kg (p = 0.01). efficiency and power of the jump. When performing the Parents or guardians signed informed consent prior to unanticipated cutting maneuvers, the athletes were again participation in the study. Initially, 23 subjects were raninstructed to decrease knee valgus motion, maintain domized between the 2 groups. Four athletes did not meet proper knee and foot alignment and position, while atthe preestablished compliance criterion that required tempting to improve the speed and efficiency of the techeach subject participate in at least two-thirds (12 of 18) nique. The exercise intensity progressed by adding comof the training sessions (subject population n = 19). One plexity to the movements, and through the addition of subject made only 11 sessions due to competitive club sea- single limb maneuvers. The exercises performed by tbe son travel, thus excluding her from the study analysis. PLYO group did not include any form of stabilization, Three subjects did not complete necessary training seshold or balance. sions or participate in posttesting, due to injuries during In contrast, the BAL group followed a protocol that outside sport activities (1 acute fibula fracture from an emphasized dynamic stabilization and balance. The atheversion ankle sprain while wearing a rigid ankle ortholetes were instructed on methods to dampen landing force sis, 1 grade 2 and 1 grade 3 inversion ankle sprain). No through sagittal plane fiexion while avoiding positions of acute injuries occurred during the training sessions. The knee valgus. As this protocol progressed, it also increased 4 suhjects excluded from the original 23 due to noncomin difficulty by moving from stable ground surfaces, to pliance were from the PLYO group. The average number relatively unstable surfaces such as Airex pads (Perform of completed sessions for both groups was approximately Better Inc., Cranston, RI), BOSU trainers (TEAM BOSU, 16 (PLYO 16.4 ± 1.5, BAL 15.5 ± 1.4). Canton, OH), and Swiss Balls (Perform Better Inc.). Dynamic balance exercises progressed by altering tbe athAll subjects were pretested 1 week prior to tbe initial letes' center of gravity through perturbations or adding training session. Posttesting was performed approximateexternal weight to the movement or by the addition of ly 8 weeks after the pretest on all subjects (4 days after
PLYOMF.TRIC VS. DYNAMIC BALANCK TRAINING
TABLE 1. Plyometric training at weeks 1, 3, and 6. Plyometric training
Reps
Week 1
Athletic position Wall jumps Squat jumps Tuck jump (with thighs parallel) Line jumps (side to side) Line jump lateral max vertical Lunge jump 180 Jumps (height) Broad jump vertical + stept Bounding in place
15 10 10 10 10 15 20
Forward jumps over barriers + stept Forward barrier jumps with middle box + stept Box drop max vertical + stept Box drop + stept
6 10 10
Week 3
Wall jumps Squat jumps Tuck jump (with abdominal crunch) Tuck jump (with butt kick) Barrier jumps (front to back) Barrier jumps (side to side) Hop, bop, hop-athletic position + stept 180" Jumps (height) Broad jump, jump, jump vertical + stept Bounding for distance Lateral barrier hops with staggered box reaction Back drop-box toucb-max vertical + stept Lateral box drop max vertical Power steps
15 15 15 15 15 15 15 6 6
6 10 5 8
Week 6
Squat-tuck jumps Barrier hops flat (front to back) Barrier hops flat (side to side) Crossover hop, hop, hop-athletic position + stept 180'Jump.s (height) Broad jump, jump, jump vertical + stept
3 Barrier hop-reaction (3-way) Forward-backward hops over barriers + step*'" Box drop-180°-box drop-max vertical + stept Box drop-ISO" + stept Lateral box drop-broad jump +
12 12 12 10 15 6 3
6 8
Box drop max vertical-broad jump + stept
Approach max vertical Crossover step-ski stop max vertical '''' Time in seconds. t Exercise ends with a quick reaction step. single limb maneuvers. In addition, this group performed exercises which focused on maintaining dynamic stabilization, with movements designed to strengthen the core musculature of the body. These exercises included torso flexion, extension, and rotational strength maneuvers. Testing. A single-leg hop and balance test was performed a total of 6 times (3 randomized trials on each side) using an AccuPower portahle force platform (Ad-
347
vanced Mechanical Technology, Inc., Watertown, MA). Subjects initiated the movement while balancing on 1 foot, and were instructed to hop forward 50 cm and balance for 10 seconds after the landing on the same foot. Standard deviation of center of pressure (COP) in the medial-lateral (COPMI.) and anterior-posterior (COP^p) directions, and maximum vertical ground reaction force were calculated for each trial with Matlab software version 7.0 (Mathworks, Natick, MA) Previously, measurements on the portable platform have been shown to provide reliable measures (3). Knee extensor and knee fiexor strength was assessed isokinetically using a dynamometer (Biodex Medical Systems, NY). Isokinetic testing bas previously been shown to be a reliable measure of quadriceps (r ^ 0.968) and hamstrings ir = 0.848) strengtb in children and adolescent populations (12). The patient was secured in a seated position on the dynamometer with her trunk perpendicular to the floor, hip flexed to 90°, and knee also flexed to 90". Stabilization straps were secured at the waist, distal femur, and distal sbank, just proximal to the medial malleolus. The test session consisted of 10 knee fiexionextension repetitions for each leg at 300°- second '. Peak flexion and extension torques were recorded. A warm-up set consisting of 5 knee flexion-extension repetitions for each leg at 300°- second ' was performed prior to data collection. The vertical jump was measured on an MXl vertical jump trainer (MXP Sports, Reading, PA). Prior to the test, each subject's overhead reach was determined with the subject reaching directly overhead with both hands up toward the ball, the midline of the basketball was aligned with the distal interphalangeal joint of the right and left middle fingers. The subject was told to use a natural over-head reach (no exaggerated superior rotation of the shoulder girdle). The digital readout of the system was zeroed to subtract reach from jump height and provide actual vertical displacement during the vertical jump testing (8, 26). Prior authors have demonstrated that counter movement vertical jump testing has a test-retest reliability of 0.99 (31). Prior to isoinertial strength testing the athlete was instructed on the proper form for squat, hang clean, and bench press exercises. Tbe athlete was instructed to perform practice repetitions with the standard barbell. After the exercise orientation, the trainer chose a weight they estimated the athlete could lift 5 or fewer times. The test was accepted provided the repetitions completed were 8 or less. If the athlete performed more than 8 repetitions, more weight was added and they were retested. The 1 repetition maximum (IRM) was predicted with the equation introduced by Wathen et al. I IRM = 100 * rep wt/ [48.8 + 53.8 ^•^exp(-.075^'^reps)Ii (21, 34). The squattesting required the athlete's thigh to be parallel to the fioor for each repetition. The hang clean testing required tbe athlete to clean the bar onto her front deltoids and stand with full body extension for each repetition. The bench press testing required the athlete to touch tbe barbell to ber cbest and return to full arm extension for eacb repetition. Kravitz and colleagues demonstrated tbat predicted IRM testing provided acceptable levels of accuracy (20). Statistical Analyses Statistical means and standard deviations for each variable were calculated for each group. All dependent van-
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MYER, FORD, BRENT ET AL.
TABLE
2. Core stability training at weeks 1, 3, and 6. Core stability training
Time*
Week 1 Deep hold position Box butt touch Line jump (forward)-deep hold Line jump (lateral)-deep hold Box drop-deep hold Single leg squat-deep hold BOSU(F)t deep hold BOSU(F)T drop squats BOSU(R)t jump stick landing-deep hold BOSU(R)t hoth knees deep hold B0SU(R)1; crunches BOSU(R)$ swivel crunch (feet planted) BOSU(R)$ single leg pelvic bridges BOSU{R)$ supermans
5 20
Reps 5 8 8 4 10 6 8 8 10 35 40 12 12
Week 3
BOSU(F)t drop stick-deep hold BOSU(F)T deep hold partner perturbations Box drop (lateral)-deep hold Single leg line hop (front-back)-deep hold Single leg line hop (side-side)-deep hold Single leg squat-heel touches Swiss ball both knees deep hold BOSU(R):i: single leg step-stick deep hold Double crunch Table double crunch Table double swivel crunch Table reverse hyperextensions B0SU(R)1: lateral crunch B0SU(R)1: swimmers
10 20
4 8 8 10 20
6 25 15 8 12 10 10
Week 6
Double BOSUlFit deep hold (partner perturbations) BOSU(F)t drop single leg airex stick deep hold BOSU(R)$ single leg deep hold partner ball toss Swiss ball both knees deep hold (partner perturbations) BOSU(R)t single leg (4 + way] hop stick deep hold BOSU(F)t single leg ball pick up Airex walking lunges BOSU(F)t single leg squats BOSU(F)t single leg deep hold (partner perturbations) Straight leg lifts with toe punch Straight leg lateral double crunch BOSU(R)$ double crunch BOSU(R)$ opposite swivel crunch (feet up) Swiss ball reverse hack hyperextensions
10
5 5
25 20
30
15 12 15 12 12
* Time in seconds. t BOSU flat side up. t BOSU round side up.
ables (vertical ground reaction force, SD of COP, isokinetic and isoinertial strength and vertical jump) were calculated for percent change and analyzed with a 1-way analysis of variance (ANOVA) to determine differences between groups. A mixed-design repeated measures ANOVA {2 X 2) was also used to test for the effect of training and training group on vertical jump height and IRM strength variahles. A mixed-design repeated measures ANOVA ( 2 X 2 X 2 ) was used to test for the effect of training, training group and side on vertical ground reaction force, COPMT., COP^J, and isokinetic strength. Statistical analyses were conducted in SPSS for Windows, Release 12.0 (SPSS Inc., Chicago, Illinois). Comparison of prior biomeehanical data during dynamic tasks found that the ratio of within group to between group differ-
ences was 0.73 (23). Based on these data, a power analysis revealed that to achieve 90% statistical power in the current study, with an exploratory alpha level of 0.05, a minimum of 8 subjects per group (PLYO and BAL) were required. Therefore, we recruited a team with at least 16 athletes that could he randomized into 2 groups. RESULTS
The BAL and PLYO groups demonstrated differing effects of training on force dissipation during single leg landings. Figure 1 is a representative graph of landing ground reaction force profiles in a BAL trained athlete pretraining vs. posttraining. The percent change from pretest to posttest in vertical ground reaction force was significantly different between the BAL and PLYO groups on the domi-
PLYOMETRIC VS. DYNAMIC BALANCE TRAINING
TABLE 3. Strength training at weeks 1, 3, and 6. Strength training
TABLE 4. Speed training at weeks 1, 3, and 6. Reps Speed training
2 1 2
2 2 1 1 1 T-H
Week 1 Dumbbell hang snatch Bench butt touch Barbell squat Bench press Lying leg curl Lat pulldowns Ball squat dumbbell floor touches Dumbbell shoulder press Russian hamstring curl Seated cable row Hip abduction-adduction at 60° and 120° sec Double crunch
Sets
1 1
2
12 10 12 12 10 15 10 15 10 15 10 20
Week 3
Hang clean Leg press Dumbbell incline press PVont lunges + press Inverted lying pull-ups Stretch dumhbell dead lift 3-Way dumbbell shoulder circuit Bench reverse hyperextensiona Knee flexion-extension at 120° and 300''-sec Band good mornings Back extensions Ankle circuit
10 8 10 10 10 12 10 12 10 20 12
Week 6
Dumbbell hang snatch Barbell squat Single leg band assisted squat Bench press Lying leg curl Lat pulldowns Dumbbell shoulder press Band shoulder press Russian hamstring curl Standing cable row Hip ahduction-adductiun at 60° and 120°-sec Double crunch
349
5 5 8 5 8 8 8 8 15
Week Run Run Run Run Run Run Run Run
(6 mph) (8-10 mph) (6-9 mph) (6-8 mph) (6-8 mph) (8-10 mph) (10 mph) (10-12 mph)
Grade {%)
Time*
0 0 10 15 20 15
10 20
10
10
0
8
10
12
15 20 25 30 35 0 5
8 12 10 8 6 20 15
10 15 20 20 10 0 5 0
10 10 10 6
#of Sets
20 20 20 10
Week 3
Run (10-14 mph) Hold (10-12 mph) Run (10 mph) Run (9 mph) Run (8 mph) Run (8 mph) Run (10-12 mph) Retrograde (3-5 mph) Week 6
Run (12-14 mph) Run (12-14 mph) Run (10-13 mph) Run (12-14 mph) Run (12-16 mph) Run (14-20 mph I Retrograde (5~9 mph) Retrograde (6-12 mph)
6 6 10 6
Time in seconds.
PRE -POST
30
nant side (p < 0.05). Figure 2 shows the BAL group reduced their impact force by 7.0% while the PLYO group had an increase of 7.67/. The non-dominant side showed a similar trend, although the results were not statistically significant (BAL -5.49'^, PLYO -hO.3%, p = 0.33). The percent increase from pretest to posttest was not different between groups for any other measured variables {p > 0.05). Figure 3 shows the effects of the two different training regimens on dynamic force stabilization during single-leg landing. Both the PLYO and BAL groups decreased COPML on their dominant side (side X training interaction; p < 0.05) during landing onto a force plate from a single-leg hop, which equalized pretested side to side (dominant to nondominant) differences. Neither BAL nor PLYO training affected COP^p during single-leg landing on the portable force platform {p > 0.05). Figure 4 demonstrates the relative effect of the 2 training regimens on strength, as measured on an isokinetic dynamometer. Both groups increased isokinetic hamstrings peak torque {p < 0.01), and hamstrings to quadriceps ratio (p < 0.01). Both training protocols also significantly improved vertical jump (p < 0.001, Figure
Time (ms]
1. Mean pretest (dotted linel and posttest (solid line) for the ground reaction forces measured during the single leg landing from a representative subject in the dynamic stabilization and balance training group. FIGURE
5) and predicted IRM measures of bench press (p < 0.001), hang clean {p < 0.001) and parallel squat (p < 0.001; Table 5). DISCUSSION
Comprehensive neuromuscular training can lead to simultaneous improvements in athletic performance and movement hiomechanics in female athletes (13, 25, 28). The training protocol in the present study used similar
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MYP:U, FORD, BRENT ET AL.
HBAL • PLYO
BAL Dominant
FIGURE 2. Vertical impact force change from pre- to posttest was calculated as a percent change (negative represented a reduction in force) and analyzed. '''Percent reduction of peak impact three was significantly different between groups p < 0.05. BAL = dynamic stabilization and balance training; PLYO = maximum effort plyometric jumping.
SAL Oomnarl
BAL Nondomlnant
PLYO Oomiranl
PLYO Nondominant
FIGURE 3. SD of center of pressure in the medial-lateral direction p < 0.05. Pre = pretest; Post = posttest; BAL = dynamic stabilization and balance training; PLYO = maximum effort plyometric jumping. strength and speed training components as a previously studied comprehensive protocol (26). Hov^'ever, the protocol was modified by excluding either dynamic balance or maximum effort plyometric exercises. The 2 different protocols demonstrated similar effects on neuromuscular power, body sway and dynamic strength. However, the decreased impact force demonstrated hy the BAL group may suggest that balance and dynamic stabilization training are important to improve force attenuation strategies when landing from a single-leg hop. TABLE 5.
BAL Nondominant
PLYO Dommant
PLYO Nondi
4. •''^Peak hamstrings torque normalized to body weight p < 0.01. PRE = pretest; POST = posttest; BAL = dynamic stabilization and balance training; PLYO - maximum effort plyometric jumping. FIGURE
5. '^'''"•'Vertical jump in the dynamic stabilization and balance training I BAL) and maximum effort plyometric jumping (PLYOl groups p < 0.001. PRE = pretest; POST = posttest. FIGURE
The findings supported the hypothesis that there would be a difference between the 2 groups in decreasing landing force when landing from a single-leg hop. As expected, the BAL group significantly reduced impact landing forces while the PLYO group (without balance/hold exercises) did not. This result may be related to the inclusion in the BAL group protocol of force dissipation techniques, which used both double and single-leg deep knee flexion exercises that emphasized coronal plane knee control. It was demonstrated previously that plyometric training that incorporates dynamic stabilization exercises can reduce impact landing forces (151. The re-
Predicted 1 repetition •maximum calculations." BAL
Bench press (kg) Hang clean (kg) Squat(kg)
PLYO
Pre
Post
Pre
Post
p value
33.0 ± 4.2 30.2 ± 4.2 43.9 ± 10.9
40. 5 ± 3. 7 42..4 ± 2. 4 82..1 ± 8.4
30.9 ± 5.8 27.5 ± 3.5 44.0 ± 5.8
36. 3 ± 6 .7 40. 3 ± 5 .5 81. 2 ± 9 .0
P < 0.001 P < 0.001 P < 0.001
Pre = pretest; Post = posttest; BAL = dynamic stabilization and balance training; PLYO - maximum effort plyometric jumping.
PLYOMETRIC VS. DYNAMIC BALANCE TRAINING
suits of the current study may indicate that the improved force dissipation strategies at landing are more related to the dynamic stabilization component of the training, rather than the plyometrics component. The inability of the PLYO group, without held landings and balance drills, to decrease impact force may indicate that the incorporation of exercises that focus on both force dissipation and dynamic balance may be necessary to obtain post training force reduction during single limb landing. Both groups significantly improved their COP^i on their dominant side following training, which equalized pretested side to side (dominant to nondominant) differences. Protocols that use balance training only, or when used in combination with plyometric training, can decrease side to side differences in strength measures (10, 15). Females often demonstrate leg dominance, which is an imbalance between muscular strength and joint kinematics between contralateral lower extremity measures during dynamic tasks (7, 13). Female athletes may generate lower hamstrings torques in the non-dominant leg than in the dominant leg (15). Ford et al. showed that adolescent female athletes had significant side-to-side differences in maximum knee valgus angle compared to males during landing (7). Side-to-side imbalances in muscular strength, flexihility and coordination have been shown to he important predictors of increased injury risk (2, 13, 16). Knapnik et al. demonstrated that side-to-side balance in strength and flexibility is important for the prevention of injuries and when imbalances are present, the athlete has an increased risk of injury. Baumhauer et al. also found that individuals with muscle strength imbalances exhibited a higher incidence of injury (2). Hewett et al developed a model to predict ACL injury risk with high sensitivity and specificity. Nearly half of the parameters in the predictive model were side-to-side differences in lower extremity kinematics and kinetics (13). Side-to-side imbalances may increase risk for both limbs. Over-refiance on the dominant limb can put greater stress and torques on the dominant knee, while the weaker knee may be at risk due to an inability of the musculature on that side to effectively absorb the high forces associated with sporting activities. Therefore, training programs should target side-to-side imbalances in females. However, the results of the study may suggest that the mode of exercise used may not be as critical as the focus on single limb exercises (24). Both protocols in the current study progressed to single leg activities to increase exercise difficulty and intensity, which may be related to the improved symmetry in COP^,, after training for both groups. Both the PLYO and BAL significantly improved the COP measures along the medial/lateral axis as opposed to no change in the COP^,,. The improvements in force dissipation and postural control were expected in the BAL group; however, it was hypothesized that the PLYO group would not improve postural sway with training. The medial/lateral COP improvements by both groups contrast the findings of Paterno et al. who used a combined dynamic balance and plyometric protocol (28) and found that improvements in body sway measures occurred in the anterior/posterior plane. The difference in results may be related to the difference in measurements of dynamic balance. The current study measured COP control of the athlete in relation to a stable surface, while Paterno et al. measured body sway on an unstable platform
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(28). The study results may be indicative of a more sportspecific and injury relevant training adaptation, since the athletes in the current study performed a more sports related task that required a dynamic ground based landing with immediate control of the ground reaction forces to maintain halance and COP control. The role of COP control and improved balance in reducing lower extremity injury risk (4, 32, 33) further illustrates the positive nature of this improved measure. Both the BAL and PLYO groups showed large increases in open chain isokinetic hamstring measures (Figure 5). Complimentary to the increased hamstring strength, both groups significantly improved their hamstring strength relative to their quadriceps strength (H/Q ratio). These results were expected as previous authors have demonstrated that isolated plyometric and balance training can improve H/Q ratio (10, 36). Additionally, both groups received some form of isolated hamstrings exercise per training bout. Fry and Powell hypothesized that generalized strength training may not improve H/Q ratio unless hamstring isolated exercises are incorporated into the protocol (9). Electromyographic analysis demonstrates that during the leg press hamstring co-activation is significantly reduced compared to the squat exercise (35). Therefore, the observed improvements in the H/Q ratio may be a combinatory effect between training variables (PLYO and BAL) and the isolated hamstring strengthening and squat exercises incorporated into both groups' protocols. The increased H/Q ratio from both PLYO and BAL groups should he considered a positive adaptation to injury prevention training as low H/Q ratios have been related to increased risk of traumatic lower extremity injury in female athletes (30). Both the PLYO and BAL group improved their strength measures (squat and hang clean) and power (vertical jump) measures. Fatouros and colleagues also found combinatory effects of plyometrics and resistance training to not only increase jump performance but leg strength (5). The PLYO increases were also similar to the results demonstrated by others who found the effects of plyometrics appear to be combinatory with resistance training. Adams et al. reported that a combined plyometric and squat training program significantly increased vertical jump versus training with squats or plyometrics alone (1). We hypothesized that the PLYO group would demonstrate increased strength and power similar to previous reports (15, 26). However, improvements in vertical jump with a protocol that focused on dynamic stabilization and balance was unexpected. The measured improvements in vertical jump may be more related to the resistance training included in the protocol, which may be an important component in all injury prevention protocols (18, 19). Training protocols that focus on resistance training alone may not reduce ACL injury rates. However, there is inferential evidence that resistance training may reduce injury risk due to the beneficial adaptations that occur in hones, ligaments and tendons following training (6, 17). Protocols that supplement plyometric and technique training with strength training may significantly reduce ACL injuries in female athletes (15). Thus, it appears that resistance training may be effective at reducing knee injuries when combined with other training components. However, the efficacy of a single-faceted resistance training protocol on ACL injury prevention has yet to be demonstrated in the literature.
352 MvER, FORD, BiiRNT KT AL. PRACTICAL APPLICATIONS
The goals of the study to evaluate the effects of including maximum effort plyometric jumps or dynamic lower extremity stabilization exercises in a dynamic neuromuscular training program shown to reduce measures related to ACL injury and increase measures of performance were met. The results of this study indicate that both PLYO and BAL training are effective at increasing measures of lower extremity neuromuscular power and control as well as decreasing leg dominance. However, the BAL component may be more important to improve single limb force attenuation strategies. A combination of PLYO and BAL training may further maximize the effectiveness of preseason training for female athletes.
15.
Plyometric training in female athletes. Decreased impact forces and increased hamstring torques. Am. J. Sports Med. 24: 765-773. 1996. 16.
17.
KRAEMER, W.J., N.D. DUNCAN, AND J.S. VOI.EK. Resistance
18.
training and elite athletes: Adaptations and program considerations. J. Orthop Sports Phys. Ther 28:110-119. 1998. KHAEMER, W.J., K. HAKKINEN, N,T. TRIPi.E'rr-McBRIDE, A.C. FRY, L . P . KOZIRIS, N.A. RATAMESS, J.E. BAUER, J.S. VOLEK, T. McCoNNELL, R.U. NEWTON, S.E. GORDON, D . CUMMINGS, J. HAUTH, F . PuLi.o, J.M. LYNCH, S.A. MAZZETTI, AND H.G.
KNUTTGEN. Physiological changes with periodized resistance training in women tennis players. Med. Sci. Sports Exerc. 35: 157-168. 2003. 19.
ADAMS, K,, J.P. O'SHEA, K.L. O'SHEA, AND M . CLIMSTEIN. The
effect of six weeks of squat, piyoinetric and squat-plyometric training on power production. J. Strength Cond. Res. 6:36-41. 1992. 2.
BAUMHAUER. J . , D. ALOSA, A. RENSTWJM, S. TKKVINO, AND B .
BKENT. J.L., K.R. FoKD, G.D. MYER. A.D. HAKRISON, AND T.E.
20.
21.
5.
7.
8.
FORD, K.R., G.D. Mvi-Ui, R.L. SMITH, R.N. BYRNES, S.E. Doi'i-
9.
RAK, AND T.E. HEWET'I'. Effects ol' an overhead goal on drop vertical jump maneuvers. J. Strength Cimd. Res. 19. 2005. FRY, A.C, AND D.R. POWELL. Hamstring/quadricep parity with three different weight training methods. J. Sports Med. Phys. Fitness 27:362-367. 1987.
11.
24.
25.
NO, AND p. Succop. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes. Am. J. Sports Med. 33: 492-501. 2005. 14.
26.
MYER, G.D., K.R. FORD, J.P. PALUMBO, AND T.E. HEWETT. Neu-
romuscular training improves performance and lower-extremity biomechanics in female athletes. J. Strength Cond. Res. 19: 51-60. 2005. 27.
MYKLEBUST, G., L. ENCEBRETSEN, L H . BRAEKKEN, A. SK-JOL-
BERG, O.E. OLSEN, AND R. BAHR. Prevention of anterior cruciate ligament injuries in female team handhalt players: A prospective intervention study over three seasons. CUn. J. Sport Med. 13:71-78. 2003. 28.
PATERNO, M.V., G.D. MYER. K.R. FORD, AND T.E. HEWETI',
Neuromuscular training improves single-limb stability in young female athletes. J. Orthop. Sports Phys. Ther. 34:305317. 2004. 29.
PETERSEN, W., C. BRAUN, W. BOCK, K. SCHMIDT, A. WEIMANN, W. DRESCHER, E . EiLiNG, R. STANGE, T. FUCHS, J . HEDDERICH,
AND T. ZANTOP. A controlled prospective case control study of a prevention training program in female team handball players: The German experience. Arch. Orthop Trauma Surg. 2005. 30.
SoDERMAN, K., H. ALEREDSON, T. PIETILA, AND S. WERNER.
Risk factors for leg injuries in female soccer players: A prospective investigation during one out-door season. Knee Surg. Sports Traumatol Arthrosc. 9:313-321. 2001.
HEWKT-1', T.E., M.V. PATKRNO, AN[> K,R. NOYES, Differences in
single leg balance on an unstable platform between female and male normal, ACL-deficient and ACL-reconstructed knees. In: Proprioception and Neuromuscular Control in Joint Stability. S. Lephardt and F.H. Fu, eds. Champaign, IL: Human Kinetics, 1999. pp. 77-88.
MYER. G.D., K.R. FORD, AND T.E. HEWETT. The effects of gen-
der on quadriceps muscle activation strategies during a maneuver that mimics a high ACL injury risk position. J. Electromyogi: Kinesiol. 15:181-189. 2005.
HEWE'IT, T.E., G.D. MYER. AND K.R. FORD. Decrease in neu-
HEWETT, T.E., G.D. MYER, K.R. FORD, R.S. HEIDT, JR., A.J. CoLOsiMO, S.G. MCLEAN, A.J. VAN DEN BOGERT, M.V. PATER-
MYER, G.D., K.R. FORD, AND T.E. HEWEIT. Rationale and clin-
ical techniques for anterior cruciate ligament injury prevention in female athletes. J. Athl. Train. 39:352-364. 2004.
romuscular control about the knee with maturation in female athletes. J. Bone Joint Surg. Am. 86-A:1601-1608. 2004. 13.
MCLEAN, S.G., R.J. NEAL, P . T . MYERS, AND MR. WALTERS.
Knee joint kinematics during the sidestep cutting maneuver: Potential for injury in women. Med. Sci. Sports Exerc. 31:959968. 1999.
HEWETT, T.E., T.N. LINDENFEI.D, J.V. RICCX>RENE, AND F.R.
NOYES. The efl'ect of neuromuscular training on the incidence of knee injury in female athletes. A prospective study. Am. J. Sports Med. 27:699-706. 1999. 12.
23.
HEITKAMF, H.C., T. HDRSTMANN, F , MAYER, J. WF.I.I.ER, AND
H.H. DiCKHUTH. Gain in strength and muscular balance after balance training. Int. J. Sports Med. 22:285-90. 2001.
MANDELBAUM, B.R., H.J. SILVERS, D . WATANABE, J. KNARR, S. THOMAS, L. GRIFFIN, D.T. KIRKENDALL, AND W.J. GARRETT. Ef-
fectiveness of a neuromuscular and proprioceptive training program in preventing the incidence of A(3L injuries in female atbletes: Two-year follow up. Am. J. Sports Med. 2005.
FORD, K.R., G.D. MYER, AND T.E. HEWETI-. Valgus knee motion
during landing in high school female and male basketball players. Med. Sci. Sports Exerc. 35:1745-1750. 2003.
10.
22.
FATUUROS, I.G., A.Z. JAMUKTAB, D . LEONTSINI, T. KYRIAKOS, N. AGGELOUSI.S, N . KOH'I'OPOULOS, AND P. BUCKENMEYEK. Eval-
uation of plyometric exercise training, weight training, and their combination on vertical jumping performance and leg strength. J. Strength Cond. Res. 14:470-476. 2000. 6. FLKCK, S.J., AND J.E. FAI.KEL. Value of resistance training for the reduction of sports injuries. Sports Med. 3:61-68. 1986.
LESUER, D.A., J.H. MCCORMICK, J.L. MAYHEW, R.L. WAS.SER-
STEiN, AND M.D. ARNOLD. The accuracy of prediction equations for estimating 1-RM performance in the bench press, squat, and deadlift. J Strength Cond. Res. 11:211-213. 1997.
CARAFFA, A., G. CERULLI, M. PROJETTI, G. AISA, AND A. Ry/.zo.
Prevention of anterior cruciate ligament injuries in soccer, A prospective controlled study of proprioceptive training. Knee Surg. Sports Traumatol. Arthro.sc. 4:19-21. 1996.
KRAVITZ, L., C. AKAI^N, K. NOWICKI, AND S.J. KINZEY. Predic-
tion of 1 repetition maximum in higb-school power lifters. J. Strength Cond. Res. 17:167-172. 2003.
HKWE'IT. Reliability of single leg landings on a portable force platform. Med. Set. Spurts Exerc. 37(Suppll:S400. 4.
KRAEMER, W.J., S.A. MAZZETTI, B.C. NINDI., L.A. GOTSIIAI.K, J.S. VOLEK, J.A. BUSH, J.O. MAJtx, K. DOHI, A.L. GOMEZ, M. MILES, S.J. FLECK, R.U. NEWTON, AND K, HAKKINEN. Effect of
resistance training on women's strengtb/power and occupational performances. Med. ScL Sports Exere. 33:1011-1025. 2001.
BEYNNON. A prospective study of ankle injury risk factors. Am. J. Sport Med. 23:564-570. 1995. 3.
KNAPIK, J.J., C.L. BAUMAN, B . H . JONES. J.M. HARRIS, AND L.
VAUCHAN. Preseason strength and flexibility imhalances associated witb athletic injuries in female collegiate athletes. Am. J Sports Med. 19:76-81. 1991.
REFERENCES 1.
HEWETT, T.E., A.L. STROUPE, T.A. NANCE, AND F.R. NOYES.
31.
STOCKBRUCJGER, B.A., AND R.G. HAENNEL. Validity and reli-
ability of a medicine ball explosive power test. J. Strength Cond. Res. 15:431-438. 2001.
PLYOMETRIC VS. DYNAMIC BALANCE TRAINING 32.
33. 34. 35.
TROPP, H., J . EKSTRAND, AND J. GILLQUIST. Stabilometry in
functional instability of the ankle and its value in predicting injury. Med. Sci. Sports Exerc. 16:64-66. 1984. TROFF, H., AND P . ODENRICK. Postural control in single-limb stance. J. Orthop. Res. 6:833-839. 1988. WATHEN, D. Load assignment. In: Essentials of strength training and conditioning. T.R. Baechle, ed. Champaign, IL: Human Kinetics, 1994. pp. 435^39. WiLK, K.E., R.F. EscAMiLLA, G.S. FLEISIG, S.W. BARRENTINE, J.R. ANDREWS, AND M.L. BOYD. A comparison of tibiofemoral
joint forces and electromyographic activity during open and closed kinetic chain exercises. Am. J. Sports Med. 24:518-527. 1996. 36.
WiLKERSON, G.B., M.A. COLSTON, N . L SHORT, K.L. NEAL, P.E.
HoEWiscHER, AND J.J. PiXLEY. Neuromuscular changes in female collegiate athletes resulting from a plyometric jumptraining program. J. Athl. Train. 39:17-23. 2004.
353
Acknowledgments The authors would like to acknowledge funding support from National Institutes of Health Grant R01-AR049735-01A1. Tbe authors would also like to thank Coach Kerry Buttkovich and tbe Division 1 2004 Ohio State Semifinalist and 2005 State Champion Seton High School Volleyball team for their participation in this study. Tbe authors would also like to acknowledge Hanni Cowley, Elizabetb Brougber, Dr. Jon Divine, Adrick Harrison, Rachel Heyl, Rachel Mees, Monica Naltner, Carmen Quatman, Nick Palumbo, Mark Paterno, Annie Schmolt, and Merry Jo Ford for their assistance with training and testing of athletes and for their input to the manuscript edits. Address correspondence to Gregory D. Myer, greg. myer@cchm c. org
