Original researchEffects of a plyometric program on vertical landing force and jumping performance in college women
Introduction
Female athletes participating in sports that require jumping and/or rapid changes in direction (e.g., soccer, basketball, and volleyball) are up to six times more likely to suffer a non-contact anterior cruciate ligament (ACL) injury compared to their male counterparts participating in the same sport (Arendt & Dick, 1995; Arendt, Agel, & Dick, 1999; Harmon & Ireland, 2000; Henry & Kaeding, 2001). More concerning is that even though a great deal of attention has been given to this particular injury, the rate of ACL injuries has remained constant for female athletes over the past 13–15 years (Agel, Arendt, & Bershadsky, 2005; Mihata, Beutler, & Boden, 2006). The multi-factorial nature of non-contact ACL tears has been described elsewhere (AAOS, 2001); however, women display biomechanical and neuromuscular characteristics that differ compared to men when performing common athletic tasks such as jumping, landing, and changing directions (Chappell, Yu, Kirkendall, & Garrett, 2002; Hanson, Padua, Troy Blackburn, Prentice, & Hirth, 2008; Henry & Kaeding, 2001; Landry, McKean, Hubley-Kozey, Stanish, & Deluzio (2007a), Landry, McKean, Hubley-Kozey, Stanish, & Deluzio (2007b); Lephart, Ferris, Riemann, Myers, & Fu, 2002a). The differences in basic motor skills and neuromuscular activation patterns between men and women may be a contributing factor in the elevated risk of ACL injuries that has been observed in female athletes. Improving these potential risk factors typically includes strategies aimed at altering faulty kinematic (motion) patterns, which subsequently helps to improve kinetic (force) and neuromuscular characteristics upon landing from a jump or when changing directions (Caraffa, Cerulli, Projetti, Aisa, & Rizzo, 1996; Chimera, Swanik, Swanik, & Straub, 2004; Griffin, 2001; Hewett, Lindenfeld, Riccobene, & Noyes, 1999; Silvers & Mandelbaum, 2001).
Injury prevention programs such as Sportsmetrics™, which aim to improve faulty movement mechanics and neuromuscular characteristics, incorporate plyometric drills and use specific instructional cues focused on how to land properly (Hewett, Stroupe, Nance, & Noyes, 1996; Silvers & Mandelbaum, 2001). In addition to differences in muscular firing patterns during common athletic maneuvers (Chappell, Creighton, Giuliani, Yu, & Garrett, 2007; Landry, McKean, Hubley-Kozey, Stanish, & Deluzio (2007a), Landry, McKean, Hubley-Kozey, Stanish, & Deluzio (2007b)), female athletes have less knee and hip flexion when landing from a jump (Lephart, Ferris, & Fu, 2002b), commonly referred to as a ‘stiff’ landing. Women also have greater internal tibial rotation during a single-legged landing (Nagano, Ida, Akai, & Fukubayashi, 2007). Indeed, video analysis has confirmed that the most frequent position at the time of an ACL injury is knee flexion <30°, knee valgus, and external rotation of the foot with respect to the knee (Teitz, 2001). Therefore, one of the essential elements within Sportsmetrics™ is to instruct female athletes how to increase knee and hip flexion and reduce knee valgus upon ground contact in an effort to subsequently reduce landing forces (i.e., achieve a ‘softer’ landing) (Hewett et al., 1996). In fact, most studies (Hewett et al., 1996; Irmischer, Harris, Pfeiffer, DeBeliso, Adams, & Shea, 2004; Myer, Ford, Brent, & Hewett, 2006), but not all (Lephart, Abt, Ferris, Sell, Nagai, Myers, et al., 2005), have reported a reduction in vertical ground reaction forces (VGRF) ranging from 7% to 26% after completing 6–9 weeks of training with plyometric exercises.
The use of plyometric exercises with athletic populations as a means to enhance performance is a well-accepted practice amongst sports performance professionals (Durell, Pujol, & Barnes, 2003; Ebben, Carroll, & Simenz, 2004; Ebben, Hintz, & Simenz, 2005; Simenz, Dugan, & Ebben, 2005); however, research examining the effects of plyometrics on performance indices with women has received minimal attention (Chimera et al., 2004; Gehri, Ricard, Kleiner, & Kirkendall, 1998). In their initial report on the effects of the Sportsmetrics™ program, Hewett et al. (1996) indicated that in addition to the 22% reduction in VGRF during a volleyball block jump, they observed a mean improvement in vertical jump height of 3.81 cm (9%) in female volleyball players following 6 weeks of training. However, the increase in mean jump height may have been strongly influenced by one participant showing an improvement of approximately 15 cm. More recently, Myer, Ford, Brent, and Hewett (2006) demonstrated an improvement in vertical jump performance, but observed no change in VGRF in a group of female athletes after completing 7 weeks of a subsequent version of Sportsmetrics™, which used instructional cues focused on performing the plyometric drills with maximal effort. Taken together, these data highlight the difficulty of simultaneously improving jump performance and reducing VGRF with a single training program.
A confounding factor to the studies utilizing variations of the Sportsmetrics™ program is the inclusion of a resistance training component (Hewett, Lindenfeld, Riccobene, & Noyes (1999), Hewett, Stroupe, Nance, & Noyes (1996); Myer et al., 2006; Myer, Ford, Palumbo, & Hewett, 2005). Adams, O’Shea, O’Shea, and Climstein (1992) demonstrated that improvements in vertical jump performance were greater when resistance training was combined with plyometric exercises compared to either training modality alone. Therefore, it is difficult to ascertain which mode of training resulted in the improvements in jumping ability or reductions in landing forces when investigating the effects of Sportsmetrics™. The purpose of the current investigation was to examine the effects of the plyometric portion of the Sportsmetrics™ program on peak VGRF as well as kinetic jumping characteristics in recreationally athletic college women.
Section snippets
Study design
A 6-week prospective intervention was used to assess the effects of the plyometric portion of the Sportsmetrics™ program on jumping performance and VGRF in college women participating in recreational athletics. Participants were randomly assigned to either an intervention (plyometric training) or control group. The intervention group performed the plyometric program three times per week for 6 weeks, whereas the control group received no training. Prior to and following the 6-week intervention
Landing performance
Table 3 displays the baseline and absolute change values for landing and jumping performance in the training and control groups. Mean baseline values for VGRF were not different between the training (2583.6±505.8 N) and the control (2543.1±788.1 N) groups (z=−0.444, p=0.696). The mean absolute change value in VGRF after 6 weeks of the plyometric training program was not statistically different between the training (−222.8±610.9 N) and control (54.6±257.6 N) groups (z=−1.60, p=0.122), however the
Discussion
Women tend to land with a more erect posture (Decker, Torry, Wyland, Sterett, & Richard Steadman, 2003), have less knee flexion upon landing (Chappell et al., 2007; Lephart, Ferris, Riemann, Myers, & Fu (2002a), Lephart, Ferris, & Fu (2002b)), and have greater landing forces compared to men (Schmitz, Kulas, Perrin, Riemann, & Shultz, 2007), which are potential risk factors for non-contact ACL injuries in female athletes (Yu & Garrett, 2007). Plyometric drills are commonly incorporated into ACL
Conclusions
The current study is limited by the small sample size. We recognize that the analysis would likely lack the statistical power necessary to detect significant differences between the training and control groups unless the treatment effect was larger than 20% and 10% for VGRF and countermovement jump height, respectively. Nevertheless, the reductions in VGRF observed in the current study, while not statistically significant, are clinically meaningful. Clinicians working with female athletes
Ethical Approval
The University of Connecticut Institutional Review Board.
Funding
None.
Conflict of Interest
None of the authors have any competing interests.
Acknowledgement
We would like to acknowledge Carl Maresh, David Tiberio, and William Kraemer for their expert advice throughout the process of this research.
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