No significant interaction was found for maximum knee flexion angle after the first landing in the drop jump test. However, linear regression analysis showed that in the NMT groups, initial minimum normalized knee distance predicted 59% of the variance in pre-posttest change for knee flexion angle (R = 0.589; SEE = 7.68, p < 0.001), whereas in the RT groups, no significant relation was found (R = 0.189; SEE = 8.13, p = 0.308).
Statistical analysis revealed that for the 1-leg hop test a significant main effect in 1-leg hop distance between sessions was found for both dominant (p = 0.005) and nondominant leg (p < 0.001). The combined NMT groups showed a significantly (p < 0.043) higher effect on 1-leg hop distance for both legs compared to the combined RT groups.
This study investigated the effect of 20 minutes of NMT as added to RT twice a week for 10 weeks on lower extremity kinematics and single leg stability of adolescent team handball players The first result of this study was that a program based on Olsen et al. (35) resulted in improvements in knee valgus angle, contact time, and knee flexion angles (see Tables 2 and 3), thereby confirming the results of earlier studies (28,32,36). These changes are known to be related to a decrease in risk of injury (16,33).
In addition, a novel finding is that the improvement was not sex-specific but occurred in both male and female adolescent team handball players with initial above-average valgus angles (see Tables 1-3). Recently, DiStefano et al. (6) found similar improvements in male and female youth soccer players. However, they used a more general measurement tool (LESS score) and did not compare their intervention to RT.
Importantly, we were able to show that in-season NMT added to regular handball training had a beneficial effect in particular on AAVA athletes (see Figure 2). These improvements are in line with results of DiStefano et al. (6) who found a significantly larger improvement (p < 0.05) in LESS scores for adolescent soccer players of both sexes with poor initial LESS scores as compared to those with moderate to excellent LESS sores. Accordingly, Myer et al. (25) found significant improvements on peak knee abduction torque in “high risk” athletes compared to in “low risk” athletes after an NMT program. In our study, initial minimum normalized knee distance in the NMT groups predicted 52% of the variance in pre-posttest change for minimum normalized knee distance (R = 0.518; SEE = 10.33, p < 0.001), which was even higher than the 40% variance predicted by initial knee abduction moment Myer et al. found in their study. Moreover, Myer et al. only looked at female athletes in a preseason program, whereas DiStefano et al looked at athletes of both sexes in an in-season program as we did in our study. This study is the only one which can attribute the additional effects to NMT, because NMT was compared to RT.
A significant beneficial effect of NMT over RT on single leg hop distance was found for both legs (see Table 4). Although this effect was predominantly caused by a lack of improvement in the RT AAVA group, whereas the NMT AAVA group showed improvements similar to the BAVA groups (see Table 4), no significant interaction of training and valgus group was found.
Female athletes show up to 5 times higher ACL-injury rates compared to male athletes (1,30). Because of these higher rates, most studies on changes in biomechanics after an NMT program have only looked at female athletes (17,27,28,32). However, despite a lower risk, male athletes still account for two-thirds of ACL injuries in sports (19,22) The question arises whether predisposing factors, such as knee valgus alignment, which are more evident in female athletes in general, might also be found in male athletes that are prone to ACL injury. Eleven out of 34 boys (32.4%) in our study showed above-average knee valgus angles. Our cut-off point was based upon the mean value for minimum normalized knee distance of all athletes in this study. Noyes et al. (32) found 75% of boys in their study on the drop jump test to have “distinctly abnormal lower limb valgus alignment” with a cut-off point of 60% for normalized knee distance upon landing. Neither of these cut-off points has been validated for ACL-injury risk. Hewett et al. (16) found a 7.6° (p < 0.01) greater maximum knee abduction angle (measured in a 3d laboratory test) upon landing for female ACL injured athletes compared to noninjured athletes and validated a cut-off point for high risk and low risk female athletes, but this test is not easily applicable in field situations. Although the relation found by Hewett et al. has not been established in male athletes, the combined findings of the present and earlier studies (6,32) indicate that screening for knee valgus angle deficits using simple kinematic analysis should be performed on boys and on girls. Identified AAVA athletes should then be the primary target group for NMT.
Our study has several shortcomings: (a) The group assignment in our study was based on a statistical method using the mean value and SD for normalized knee distances of all athletes after the first test. Like the classifications of Noyes et al. (32) or DiStefano et al. (6), the way athletes were classified cannot be generalized to other athletes nor has our method been validated by comparison with other measures or prospective studies. Future research is needed to validate a cut-off point for knee injury risk in some simple field test determining whether or not an athlete should be subjected to NMT. (b) A considerable difference in training intensity for the BAVA athletes in the NMT group compared to the RT group could have influenced the results of our study. Our results however have shown the largest differences in improvements in AAVA athletes in the NMT group compared to the RT group and for these groups no significant difference in mean training exposure was found. (c) Groups in our study were not randomly assigned. Possible bias could have been that differences in training methods in different clubs or teams could have accounted for differences in outcome. However, both groups trained for the same sport and did not change training methods compared to the previous season. In addition,
AAVA and BAVA athletes in either NMT or RT groups did participate in training sessions together. Still, further research in randomized controlled studies is needed to confirm the results of our study.
Based on the results of this study, it should be considered to add in-season NMT is to RT of adolescent team handball players of both sexes. Twenty minutes of NMT twice a week during a period of 10 weeks at the start of the season may significantly reduce knee valgus angles, and improve single leg stability, specifically in athletes with initial above-average knee valgus angles This NMT should consist of a warm-up including agility exercises, balance, and coordination exercises both on a wobble board and a mat and strength- and plyometric exercises at the end of the training.
This study was sponsored by the RGF Zuid-Holland, regional department of the Dutch Physical Therapy Association. De Haagse Hogeschool, Dep. Bewegingstechnologie, The Hague, The Netherlands provided software for contact time measurement. There was no conflict of interest. Specials thanks to Eugene Rameckers, for assistance in manuscript preparation and Koos Herrewijnen for making software adjustments.
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Appendix 1: Testing Procedures
Subjects first received a standardized specific warm-up including jogging, squat exercises, jump-strides, and jumping up and down a bench. After that, subjects started the testing procedure. For each individual test, the jumping procedure was explained and demonstrated. After the instruction, each subject was allowed 2 warm-up jumps before the actual tests were done. No instructions were given with respect to the actual technique of the jumps. However, for the Drop Jump Test, subjects were instructed not to pull up their legs during the jump.
Drop Jump Test
The procedure of this test has been described by Noyes et al. (32). Subjects were asked to jump from a box (30 cm high), land on both feet, and immediately jump vertically as high as possible and land on the same spot again. Markers (for measurement of knee kinematics in the frontal plane with a video camera) were placed at the greater trochanter, on the center of the patella and at the lateral malleolus of both legs. Some adjustments were made compared to the procedure as described by Noyes et al. (32). First, subjects were asked to wear stretch shorts and low cut shoes and socks that left the ankle free to minimize marker movement. Second, an additional marker was placed on the lateral epicondyl of the left knee to measure the amount of flexion with a laterally placed camera (operating at 30 frames per second, placed on a stand 102 cm in height, positioned 366 cm on the left side of the box). Third, instead of landing on the ground, subjects were asked to land on a contact mat (Conrad, Netherlands, type 750188; 71 × 40 cm), placed at a 20-cm distance from the box. They then had to jump immediately as high as possible and land on the same mat again. The contact mat was connected to a laptop computer. A computer program (Jumptest2-XP, Netherlands) which registers contact time and flight time in milliseconds and computes jump height (JH) in millimeters (JH = ½g (½tflight)2), was used to measure the contact time after jumping from the box, This measurement was used to estimate the jump height of the maximal vertical jump. The use of a contact mat has been shown to be a reliable way to measure jump height (24). The highest jump (of 2 trials), measured by means of the contact mat, was used as reference for all analyses. This procedure had significant (p < 0.01) high test-retest reliability for absolute (cm) and normalized (% of hip distance) knee-separation distances (ICC: prelanding, 0.74 [SEM: 0.6 cm] and 0.68 [SEM: 2.4%]; landing, 0.93 [SEM: 0.5 cm] and 0.91 [SEM: 1.9%]; take-off, 0.95 [SEM: 0.5 cm] and 0.94 [SEM: 1.6%], respectively) and minimum knee distance (ICC: absolute 0.96 [SEM: 0.4 cm], normalized 0.95 [SEM: 1.5%]).
Digital video clips of 2 cameras (1 positioned in front and 1 lateral) were captured and saved to the hard drive of a computer for storage and analysis. Images from the frontal camera were analyzed using Software for Analysis of Jumping Mechanics (Sportsmetrics, Cincinnati, OH, USA) for measurements in the coronal plane. As described by Noyes et al. (32), still images of predetermined phases of the drop jump were selected, and in these images, markers were identified in a predetermined sequence. The software calculates the distances between the markers on the hips, knees, and ankles as absolute hip-, knee- and ankle- distances. In addition, the software provides normalized knee and ankle distances (as a percentage of the hip distances in the same jump). These normalized knee and ankle distances are indicative of knee valgus alignment.
The images from the lateral camera were analyzed with a software program developed for goniometric measurements (Fotonaarhoek, The Hague, Netherlands). The frame in which the athlete showed maximum knee flexion was captured for analysis. Knee flexion angle was measured as the angle between the lines from the markers on the left hip and ankle to the marker on the lateral epicondyle of the knee.
One-Leg Hop Test
The 1-leg hop test was performed as described by Noyes et al. (31) and has been shown to have high test-retest reliability (ICC: 0.92-0.96; SEM 4.56-4.62 cm) in healthy subjects by several authors (3,38). Subjects were asked to stand on 1 leg, jump as far as possible, land on the same leg and keep their balance. Upper extremity movement was not restricted. Subjects alternately hopped twice for each leg starting with their left leg. Hop distance was measured from toe to toe. The mean hop distance of the 2 trials per leg for each test was used for calculation. Furthermore, athletes were asked which leg they would use to kick a ball as far as possible. This leg was labeled as dominant leg. In the total population, 86% (n = 69) were right dominant.
Appendix 2: Exercise Program
While landing from a jump, players were instructed to flex their knees and keep them in a straight line with hip and ankle without kneeing-in. The latter instruction was also given for balance exercises on the wobble board. In addition, athletes were instructed to make a soft landing and cutting maneuvers, by flexing their knees during all plyometric and agility exercises. Athletes trained in couples and were instructed to correct each other for incorrect posture or techniques.
Warm-Up Exercises (6 minutes)
- Without ball: Jogging, running forward with knee lifts, running forward with heel strikes, running backward with side steps, side shuffle, carioca, running forward with upper body rotations, running with increasing speed, shuttle run.
- With ball: Running with sidesteps and bouncing, stop jumps with knees bend 90°, small 1-leg jumps with landing on alternative legs, side to side jumps.
- Alternate exercises: Running with side steps, 1-leg hop twice left, twice right and stop jump, running and cutting, jump shot pass and landing with bend knees, cutting and jump shot.
Mat Exercises (4 minutes)
- Couples standing on 1 leg and throwing a ball (both legs).
- Jump shot from a step box and landing on 2 legs.
- Step down from a box on 1 leg while catching a ball (both legs).
- Balance fight on 2 legs and 1 leg.
- Stop jump on 2 legs while catching a ball and immediately jump 180° (both sides).
Balance Board Exercises (4 minutes)
- Standing on 2 legs with knees bent and throwing a ball.
- Squat exercise (on 2 legs and 1 leg) while throwing a ball.
- Standing on 1 leg with bent knee while throwing a ball.
- Standing on 1 leg with bent knee and bouncing (later with eyes shut).
- Balance fight on 2 legs.
(Two exercises 15-20 seconds wit 30 seconds rest per session) on a mat
- Wall jumps.
- Skate jumps.
- Forward lunges.
- Side to side jumps on 2 legs (later on 1 leg).
- Broad jumps.
- Tuck jumps.
- Scissor jumps.
- 180° jumps.
- One-leg hop and stick (3 seconds, both legs).
- Squat jumps.
- Up down and 180° vertical.
- Two legged jump forward and backward (later on 1 leg).
- Two legged cross jumps (later on 1 leg).
- Triple broad jump and vertical.
Strength Exercises in Couples
- Squat 2-3 series, 8-10 reps, weight up to max 10% bodyweight.
- Nordic hamstring 2-3 series 3-10 reps.