There is an inherent risk of injury with any sports participation, but female athletes appear particularly prone to knee injuries, predominantly injury to the anterior cruciate ligament (ACL) (13). A valgus or abducted position of the knee on landing and during cutting has been associated with these ACL injuries (7), with this movement pattern being particularly common in female athletes (4). Women participating in basketball have one of the highest ACL injury rates of all women's sports (8). These injuries have in part been associated with the nature of the sport which involves high-speed changes in direction, jump landing, and pivoting, but more specifically with the particular movement strategies adopted by these female athletes (7). Previous work (5) has shown that female basketball players adopt more valgus positions on landing than female athletes from other sports, which may in part explain their increased risk. All this would indicate the need for prevention programs.
Jump-training programs have been reported within the literature to improve both landing knee valgus (6,13) and functional performance (9). Jump-training programs have also been shown to reduce ACL injury rates in female athletes (6,10,12). The majority of the jump-training programs reported in the literature have been of 6- to 8-week duration, 3 times per week with sessions often lasting from 20 minutes to 1 hour. For most sports coaches, team conditioners, and athletes, this duration and program length is not acceptable. Furthermore, the majority of the jump-training programs reported in the literature incorporate a variety of training elements alongside the jump training (6,9,13), and it remains unclear if it is jump training that makes the difference or a combined strength, flexibility, and jump-training program.
The aim of this study was to assess if an abridged jump-training program could have similar effects to those previously reported, especially if jump training in isolation could match the previously reported successes of the combined programs.
Fifteen women national league division 1 basketball players (mean age 19.1 ± 1 year, range 18-22 years; mean height 1.78-m, range 1.65-1.84 m; and mean weight 64.2 kg, range 59.2-70 kg). All subjects had no history of ACL injury or other knee pathology, significant lower limb pathology (missing greater than 1 month because of injury), lower limb fracture, or surgery and had been injury-free for 3 months before the data collection. All subjects were tested in the early evening and were asked to refrain from any training on the day of testing. All training took place in the same gymnasium at the same time of the day. Written informed consent was obtained from all subjects, and the project was approved by the University research ethics committee.
To simulate the landings encountered during athletic participation, subjects were asked to perform a bilateral drop jump and a more functional jump shot task (9). To orient participants with each task, each subject was asked to perform 3-5 practice trials of both tasks. Once subjects were comfortable with the task, they were asked to perform 3 test trials for each task, with the mean value being recorded.
The drop jump task involved standing on the 30-cm-high bench; the subject was then instructed to drop directly down off the bench onto a mark 30 cm from the bench, landing on both feet and immediately perform a maximum vertical jump, raising both arms to provide countermovement (Figure 1). In the jump-shot task, the subjects were asked to start at the centerline, dribble the ball to the free throw line and then perform a jump shot and land as they would during practice or a game. This was included because it is a much more functional activity and relates more to basketball and during testing the athlete will have a tendency to concentrate on making the shot rather than thinking about and focusing on their knee angle during the drill. As suggested by Kato et al. (9), a functional activity can exaggerate an already malaligned limb.
Two-dimensional frontal-plane-projection angle of knee valgus alignment was measured during the 2 tasks. A digital videocamera was placed at the height of the subject's knee, 2 m anterior to the subject's landing target or the free throw line (depending on the task) and aligned perpendicularly to the frontal plane. The digital images were imported into a digitizing software program (Quintic 4, Quintic Consultancy Ltd., United Kingdom). The angle subtended between the lines formed between the markers at the anterior superior Iliac spine and middle of the tibiofemoral joint and that formed from the markers on the middle of the tibiofemoral joint to the middle of the ankle mortise was recorded as the valgus angle of the knee (Figure 1).
Crossover hop distance was assessed on a 900 cm × 20 cm “course” (2) marked on the gymnasium floor (Figure 2). Subjects stood on the leg to be measured, with the distal aspect of the foot against the start line. The lateral border of the foot was aligned with the contralateral edge of the course; this is shown for the right leg in Figure 2. The subject carried out 3 repetitions for each leg with 1-minute recovery between each trial, and the mean score was then calculated for each leg.
The 15 participants undertook a progressive jump-training program for 4 weeks, 3 times per week (Appendix), each session lasting 15 minutes, under supervision, with all exercises monitored for appropriate landing technique and appropriate instruction given to maintain good technique (11). The program moved from bilateral to unilateral activities while also progressing directional control (11). All subjects attended a minimum of 10 of the 12 sessions.
Means and SDs were calculated for all knee valgus measures. Paired sample t-tests were used to evaluate differences in knee valgus (during the 2 tasks) and crossover hop distance after the training program. The p value was set at p = 0.05.
The between session (test-retest) reliability of the method for assessing knee valgus was tested on a separate group of players. The knee valgus angles for a group of 5 players for both drop jump and jump shot were assessed; these were then reassessed 30 minutes later. For drop jump, comparison of first and second measurements using intraclass correlation coefficient (model 2,1) revealed a strong correlation of r = 0.92 (p = 0.001) between the 2 measurements and no statistically significant differences (p = 0.67) on testing with repeated measures t-test. Mean difference between the 2 measurements was 0.7° (±0.5°) with a 95% confidence interval of 0-1.5°. These data then give a minimal detectable difference score of 1.2° calculated using the formula presented by Eliasziw et al. (3); this indicates that any differences in knee valgus angle on drop jump between pre and posttraining measures larger than 1.2° are real differences and not related to measurement error. For the jump-shot task, intra-class correlation (ICC)2,1 revealed a strong correlation r = 0.89 (p = 0.005), with no significant differences (p = 0.7); mean difference between the 2 measures was 0.9° (±0.7°), with a 95% confidence interval of 0-1.1°, and a minimal detectable difference of 2.2°. The between-session reliability of the crossover hop test was as follows: ICC2,1 revealed a strong correlation r = 0.94 (p = 0.003), with no significant differences (p = 0.12); mean difference between the 2 measures was 28.7 cm (±31.3), with a 95% confidence interval of 0-56 cm, and a minimal detectable difference of 79 cm.
Figure 3 shows the average knee valgus angles for both the drop jump and jump shoot pre and posttraining. From Figure 3 it can be seen that the drop jump knee valgus angle on the left leg was on average reduced 9.8° (p = 0.002); on the right leg, the reduced value was 12.3° (p = 0.0001); during the jump shot, the knee valgus angle in the left leg showed a mean reduction of 4.5° (p = 0.035), and the right leg value was reduced by 4.3° (p = 0.01). After training, crossover hop distance showed an average percentage improvement on distance jumped of 73.6% (p = 0.001), with the left leg improving by an average of 111 cm (p = 0.001) and the right leg 110 cm (p = 0.001), shown in Figure 4.
The study undertaken demonstrated that a 4-week (12 sessions) jump-training program brought about significant improvements in crossover hop distance and knee valgus angles on drop-jump landing and a functional jump-shot test. These results reflect those of previous studies that have investigated the effects of a jump-training program on landing strategies (6,9,13). It is difficult to provide direct comparison of this study's result and the other training studies to be found in the literature because of the variety of measurement methods used, for instance Noyes et al. (13) used knee separation distance. The only study where direct comparison could be made is that of Kato et al. (9), whofound training to bring about a 41% decrease in knee valgus during jump-shot landing, which reflects the 36% decrease found in the present study.
With the exception of the study of Kato et al. (9), the other studies reported above have used training programs of 6- to 8-week duration, 3 times per week with sessions often lasting from 20 minutes to 1 hour. All of the above studies also used what could be described as combined training programs incorporating not only jump training but also strength and flexibility training (6,9,13). The study undertaken was the first to demonstrate comparable improvements in landing and function (crossover hop) using jump training only, the program undertaken was also of a much shorter duration.
Injury-prevention programs that require substantial time are likely to suffer noncompliance, and the motivation to participate in training without perceived performance-enhancement effects might be limited (11). Both of these factors should be taken into consideration when attempting to incorporate injury-prevention training programs into an athlete's training schedule. There is limited research on the ideal duration of a prevention program. What is clear from the literature is that the outcomes from the shorter duration programs such as the one undertaken here and that of Kato et al. (9) and Mandelbaum et al. (10) match those with longer training periods (6). Similarly, the functional performance outcomes are also as equally positive.
The available literature provides little clarity as to the actual components required within an ACL injury prevention program. The majority of the studies highlighted above that have incorporated jump training have not done so in isolation but in combination with strength, flexibility, and balance training. Training protocols that focused on resistance training alone have not been shown to reduce ACL injuries (11), and balance board training has been shown to have both positive (1) and negative results (14) on ACL injury rates. Intuitively, it would appear that the common feature which is “adding value” to all these programs is the jump-training element; the findings of this study would support that conjecture, but considerable research would still need to be conducted to confirm this. Even from the findings of this study, it needs to be investigated if it was the jump training that proved the important element or the instruction on how to land and the learning of that skill.
Previous research has shown jump-training programs combined with flexibility and strength training to be successful in reducing knee valgus angle on landing, which has then been linked to reduced knee injury rates. The study undertaken showed similar improvements in knee valgus angles and hop distances. The study undertaken achieved these results with an abridged program concentrating only on jump-landing training over a considerably shortened training session duration and training period.
A progressive jump-training program of both short session and program duration can have similar positive effects on landing strategies and functional performance as longer duration mixed-content training programs. Positive benefits in terms of injury prevention have been shown when athletes undertake jump training; previously reported programs may have had limited utility to coaches because of their duration, so using an abridged program could still produce positive benefits and may prove more user friendly.
The author wishes to thank the players and coaches from the club involved in the data collection and Claire Hall BSc (Hons) GSR and Kaye Humberstone BSc (Hons) GSR for their help with data collection.
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Appendix: Jump-Training Program
Session 2: as in session 1
Keywords:© 2010 National Strength and Conditioning Association
knee abduction; crossover hop; jump training