Little information is found analyzing how dancers use their muscles to perform highly trained movements such as leaps and jumps. Instead, most studies focus on the treatment of injuries sustained by dancers (Trepman et al., 1998). Some injuries, according to Hobby and Hoffmaster (1986), involve “muscle imbalances” resulting from dance training that “places specific demands on . . . bodies” (p. 39). Incorrect training can, in other words, produce underdeveloped or overdeveloped muscle groups. A study by Simpson and Kanter (1997) indicated that injury to lower extremities is common among dancers pursuing various forms of dance, for instance modern dance, jazz dance, and ballet. It linked chronic dance injuries to improper landing when jumping.
Many of the skills required in dance are also used in sports like figure skating and gymnastics (McQueen, 1986). Certain sport training techniques, therefore, can be useful to dancers (McQueen, 1986). Fahey (2000) noted that, “Jumping exercises and plyometrics enhance performance in strength-speed sports because they increase leg power and train the nervous system to activate large muscle groups when you move” (p. 76). Hutchinson and colleagues’ study of elite gymnasts suggested that leap training utilizing a swimming pool as well as Pilates safely enhanced leaping ability (Hutchinson, Tremain, Christiansen, & Beitzel, 1998). In the study, after one month of training, gymnasts improved their explosive power by 220%, their ground reaction time by 50%, and the height of their leaps by 16.2%.
The objective of plyometrics is to generate the greatest amount of force in the shortest amount of time (Seabourne, 2000). Plyometrics trains the nervous system and metabolic pathways to increase explosiveness, giving the athlete an extra push to move higher and faster. Plyometrics requires acceleration through a complete range of motion, followed by relaxation into a full stretch. The quick stretch applied to the muscle by the athlete during initial push-off is thought to increase muscle contraction, in turn increasing power. The Cincinnati SportsMedicine and Orthopaedic Center has developed a plyometrics-based program called Sportsmetrics, which has been shown to increase jump height and decrease harmful landings (Hewett & Noyes, 1998). Hewett, Stroupe, and Riccobene (1999) analyzed the effects of 6 weeks of Sportsmetrics training in female athletes, finding that, after completing the program, the athletes’ peak landing forces decreased by 22%, lateral and medial forces at the knee dropped by 50%, and the height of jumps increased 10%. Furthermore, hamstring-to-quadriceps strength ratio rose from 50% to 66%, creating “a more favorable condition for the ACL [anterior cruciate ligament]” (Boden, Griffin, & Garrett, 2000, p. 57). Plyometrics training has been shown to generate greater strength output with fewer injuries, and the present study’s purpose was to assess the effects of a 7-week plyometrics program on the vertical jumps and leaps executed by collegiate dancers.
With approval of the appropriate human subjects review board, a sample of 12 female members of a Division I college dance team participated in a plyometrics training program. The specific program used was the Cincinnati SportsMedicine and Orthopaedic Center’s Sportsmetrics program, in which the dancers participated for 7 weeks. Vertical jumps were measured using a Vertec vertical height measuring device. Strength measurements were made using a CYBEX II isokinetic testing and rehabilitation system and HUMAC software for CYBEX by CSMI.
Initially, a meeting was convened during which the Sportsmetrics program was explained in detail to the 12 participants. They were told that the program would be used 3 times a week for 7 weeks. The program featured approximately 40 min of various jumping exercises. Every week, the amount of time devoted to each exercise increased. The participants kept records of how many repetitions of each they completed. After completing the session, the participants continued with a rehearsal lasting 1–2 hr. Every two weeks, the participants were taught a new program of increased difficulty. The plyometrics program carried the dancers into the beginning of their regular season workouts and game performances.
The 12 participants completed a pretest consisting of a 5-min warm-up and 5-min stretch. Height and weight of each participant were recorded. For each participant a standing reach measurement was also obtained, as the participant stood with feet hip-width apart, eyes forward, and reached vertically, the dominant hand on top of the other hand, using the Vertec vertical height measuring device. Using the Vertec vertical height measuring device, each participant executed a standing two-leg jump; the best of three efforts was recorded.
Using the same device, a two-step leap off of the right leg and a two-step leap off of the left leg were evaluated. Participants stood behind the Vertec and attempted a run, run, leap off of the right leg, with the left leg flexed at the knee and the right hand reaching up. The foot was plantar flexed and placed against the medial side of the knee in passe position. The same leap was executed off of the left leg, with the right leg flexed at the knee.
To obtain strength measurements, the participants were evaluated in a sports medicine laboratory. Each dancer was first of all familiarized with the CYBEX II equipment. Standard protocols for measuring thigh strength with the CYBEX II were used. All posttest measurements were taken after the participants had completed 7 weeks of training. Pre- and posttest data were analyzed using a paired t test, with alpha set at 0.05.
]Results and Discussion[
There were five freshmen, one sophomore, four juniors, and two seniors on the dance team from which the study participants were drawn. The participants’ biometric data were as follows: age in years, 19.7 + 1.5; height in meters, 1.65 + 0.06; and weight in kilograms, 57.4 + 6.38. In posttests after 7 weeks of plyometrics training, the right quadriceps peak torque at 180 deg/s (M = 57.9 ft lb) was significantly higher than that from the pretest (M = 54.3 ft lb), t (11) = -2.435, p < .05. Furthermore, although the difference was not statistically significant, the change between pretest means for the left quadriceps peak torque at 180 deg/s (M = 54.2 ft lb) and posttest means (M = 57.8 ft lb) did indicate improvement, t (11) = -1.904, p > .05. Vertical jump measures taken after 7 weeks of plyometrics training indicated a significant difference, t (11) = -4.59, p < .05. Also noted was significant improvement in the two-step jump off the right foot, t (11) = -2.5, p < .05. No such improvement was noted for the two-step jump off the left foot, t (11) = -1.05, p > .05.
Thus after 7 weeks of plyometrics training, there were increases in strength in the right leg at 180 deg/s. Strength in the left leg also showed improvement in peak torque performance at 180 deg/s, although not at the level of significance. Significant improvement was seen for the vertical jump and the two-step jump off the right foot.
Most dance teachers teach leaps off of both feet, off the left foot, and off the right foot. However, because many dancers jump off the left foot when executing leaps in classroom combinations at center or in performance, many if not most dancers may exhibit an imbalance in lower limb strength. The 7-week plyometrics program employed in this study may have diminished any imbalance of strength in these dancers.
Further investigation with other dancers is warranted on this topic. It may prove useful to test dancers in middle school, high school, and college. In addition, it may be beneficial not only to take isokinetic strength measures, but a measure of isometric strength as well. The possibility that dance training may develop lower-limb muscle imbalances in dancers should be investigated, as should the usefulness of plyometrics training for younger dancers to prevent any such imbalances.
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Brenda G. Griner, Department of Health and Kinesiology and Department of Music, Theater, and Dance, Lamar University; Douglas Boatwright, Department of Health and Kinesiology, Lamar University; Dan Howell, Department of Health and Kinesiology, Lamar University, and Beaumont (Texas) Bone and Joint.