Fig. 5.1
a, b, c Adolescent dancers participating in resistance training in the dance studio as part of dance training
Many people feel that resistance training will not increase muscle strength in girls. Recent scientific studies have demonstrated that this is not true. Malina [30] reviewed 22 reports dealing with experimental resistance training protocols in both males and females before and during early puberty. He found that most programs used weight machines and free weights (i.e., dumbbells and barbells), 2- to 3-day protocols, and 8- to 12-week durations. Significant improvements in muscular strength during childhood and early adolescence were achieved in both sexes.
There is not one optimal combination of sets, repetitions, and exercises that will result in favorable adaptations in muscular strength, fitness performance, and body composition in all youth [39, 40]. By periodically altering the training variables over time the training stimulus will remain effective, and adaptations to the training program will be maximized [1, 41–43]. Nor is there a “minimal age” requirement for participation in a youth resistance training program provided participants are able to follow directions and pay attention to safety rules, and by 7 or 8 years of age most girls are ready for some type of resistance exercise [32]. Resistance training with free weights, medicine balls, elastic bands, and one’s own body weight is beneficial for pediatric dancers for multiple reasons. As part of an integrative neuromuscular training program resistance training may help to enhance motor skill performance and motor coordination, improve balance, and increase muscle strength and power [31, 41, 44, 45]. These same factors are critical components of dance training.
There is a paucity of published data on the strength characteristics of ballet dancers. Micheli et al. found female professional dancers (mean age 27 years) to have strong lower extremities (quadriceps, hamstrings, dorsiflexors, and plantarflexors) and relatively weak upper extremities (elbow flexors and extensors) [46]. Muscle groups about the hip and pelvis were not isolated in this study, and there was no control group for comparison. In other studies, female dancers demonstrated only 77% of weight-predicted norms when compared to other groups of athletes [47]. Relative to peak in-season performance, Kirkendall and Calabrese [48] reported the mean quadriceps strength in female ballet dancers was the lowest among the athletic groups reviewed, at around 70% of the weight-predicted normal for athletes. Gupta et al. [49] found that dancers demonstrated an adaptive training response with a shift in the strength curve toward the inner range of hip external rotation as opposed to a true difference in strength, as compared to non-dancer controls. More specific to pediatric dancers, Bennell et al. compared hip muscle strength in 8- to 11-year-old novice female ballet dancers versus non-dancing age matched controls. After controlling for body weight controls had stronger overall hip muscles than dancers, with the exception of their hip abductor strength, which was similar between groups [50]. Further study in this same age group revealed that 12 months of dance activity produced greater muscular strength improvement in female dancers compared to that of controls over the same period of time [51]. Supplementary resistance training programs for professional dancers have been shown to be beneficial in improving both muscular endurance and anaerobic power [20]. Finally, Koutedakis and Sharp [21] reported that after strength training professional dancers demonstrated enhanced knee extension and flexion torques without altering dance aesthetics.
Resistance Training During Growth and Maturation in Dancers
A basic understanding of child and adolescent development as it relates to injury profile is critical when caring for or training pediatric dancers. Overall body mass and height may increase at a dramatic rate in the preadolescent and adolescent years. The rapid increase in bone length often results in a decrease in muscle-tendon unit flexibility, leading to greater injury risk from either a single traumatic event, such as a sudden forced muscle elongation, or repetitive stress and micro-trauma [11]. The biomechanical changes that occur during growth may predispose the dancer to a host of injuries. Prime examples include coxa saltans externa or snapping hip, and posterior column stress injury or spondylolysis. These quintessential dance injuries may, in part, be attributed to strength, flexibility, and neuromuscular deficits that are typically associated with growth and often magnified during the growth process. Specifically, the combination of relatively weak abdominal and posterior chain musculature, tight overused and weak hip flexors and quadriceps, and the anterior tilt of the widened pelvis may place female pediatric dancers at risk for injury [52]. Tursz et al. [53] demonstrated that following rapid growth and development female athletes sustain a higher rate of “sprains” than males, and this trend continues into maturity.
There is a tendency toward an increasingly early age of menarche for girls across the USA, with the average age of 12.6 years [54], and girls tend to reach their peak height and body mass at approximately 15 years of age [55]. Notably, the onset of puberty in the child dancer may be delayed, extending both the dancer’s growth period and duration of increased vulnerability to injury [56, 57]. In general, female dancers appear to have greater ligamentous laxity than their male counterparts following the onset of puberty [58, 59]. It is hypothesized that this increased laxity (alterations in passive joint restraints) combined with the physiological changes (i.e., neuromuscular control) that occur during puberty may affect the type, severity, and incidence of injuries in the maturing female adolescent population [60, 61]. It has also been suggested that cyclic changes in the hormonal milieu of females may play a role in injury patterns [62–66]. Parents, teachers, and the dancers themselves need to be aware of the physiological changes, psychological issues, nutritional considerations, and the need for training modifications during this vulnerable time in the dancer’s career.
Strength deficits unique to the discipline of dance need to be aggressively addressed with individualized training protocols that target actively growing females [46, 49]. Anatomical changes during growth and development include the aforementioned widening of the pelvis, increase in the Q angle, and changes in center of gravity [67]. Researchers have hypothesized that following the onset of puberty and the initiation of peak height velocity, increases in tibia and femur length, overall body mass, and height of the center of mass lead to decreased core stability. A key tenet of this decrease in core stability is that this change occurs in the absence of increases in strength and muscle recruitment at the hip and trunk [68, 69]. As female athletes reach maturity, decreased core stability and deficient core strength may underlie their tendency to demonstrate increased dynamic lower extremity valgus load during sport specific tasks, placing them at increased risk for injury [70–72].
Intervention efforts with targeted training that includes increasing strength and improving neuromuscular control while incorporating skill development can be implemented in a timely and effective manner [43]. Quatman-Yates and colleagues investigated the longitudinal trajectories of lower extremity strength across maturational stages for females. They found that hip abduction and hamstring-quadriceps strength ratio decreased from prepubertal to pubertal stages, lending further support to the need for strength training during preadolescence with the goal of injury prevention [73]. Evidence currently supports the implementation of injury prevention programs that focus on strength and neuromuscular control of the lower extremity [74–82]. The window of opportunity is open to maximizing training during preadolescence, when children are more responsive to interventions that promote strength, flexibility, and peri-pelvic stabilization. This is especially important before neuromuscular deficits become engrained, and before the pediatric dancer succumbs to injury [83]. Finally, and importantly, new research has shown that intervention with an individualized conditioning program based on injury history and functional movement screening is effective in preventing dance injuries [84].
Resistance Training and Bone Mineral Density (BMD)
A recent meta-analysis by Amorin et al. [85] evaluating 35 studies of BMD in dancers concluded that based on the current available research it is unclear whether low BMD is prevalent among female dancers. Childhood through late adolescence is the most critical time period for bone mass accrual [86]. The literature on BMD and sports participation continues to evolve. Physical activities characterized by considerable loading have been shown to have the greatest osteogenic effects on the growing skeleton [87]. Resistance training for females during this time period may be an important component of exercise when considering future bone health and risk for stress related bone injury [88]. Bass et al. [89] have reported that prepubertal female gymnasts, whose training mainly involves high-impact and resistance training, had significantly higher lumbar spine bone mass, volume, and volumetric BMD than a control group. Morris et al. explored lean mass, strength, and bone mineral response to a 10-month, high-impact, strength-building exercise program in premenarchal girls. The exercise group gained significantly more lean mass, strength, and BMD versus controls, and had less body fat content. Although a large proportion of bone mineral accrual in the premenarchal skeleton was related to growth, an osteogenic effect was also found to be associated with the exercise protocol in this young female cohort [90].
It is important to note, especially for the dancer population, that resistance training begun at a young age is associated with a decreased risk of osteoporotic fractures later in life [91]. Evaluation of bone mass relative to muscle strength and reproductive hormone concentration in elite endurance athletes supports the theory that muscle forces are significant osteogenic factors. Research findings also indicate that in female athletes muscle forces acting on bone potentially counteract a negative effect of reproductive hormonal disorders on BMD [92]. To et al. sought to compare BMD between regularly exercising dancers and age matched non-exercising females between the ages of 17 and 19 years to assess the impact of weight-bearing exercises and menstrual status on bone density. Results from this study were notable in that young women undergoing regular intensive weight-bearing exercise, as in the dancers under study, had higher BMD than the non-exercising females of the same age, but only if they remained eumenorrheic during their training. This advantage was lost when they developed oligo/amenorrhea [93]. Furthermore, Benson and colleagues reported that very lean dancers are more prone to injury than their less lean counterparts [94]. The evidence now appears strong in support of resistance training and weight-bearing physical activities that promote strength development to ensure the greatest osteogenic potential in young females [95]. This may be particularly critical for female athletes at high risk for bony injury, including dancers.
Resistance Training Programs for the Pediatric Female Dancer
Resistance training for pediatric dancers may be particularly challenging given what may appear to be opposing goals in the dancers’ eyes. The need to build strength may seem to contrast with the aesthetic requirements of the discipline, specifically the desire to appear thin and sleek. In this context, designing resistance training protocols that are conducted in environments familiar to the dancer (e.g., the dance studio) while dispelling the myths often associated with lifting weights is encouraged. Training protocols should be designed to correct strength imbalances unique to the discipline. For the ballet dancer this may mean conducting resistance training in the parallel, as opposed to the turned out, position. This technique encourages improving the strength of the relatively weaker adductor and internal rotator muscles of the lower extremity. Resistance training in this population should include dynamic concentric and eccentric muscle actions, and be performed through full ranges of motion in keeping with the unique demands of dance.
Liederbach and colleagues evaluated the incidence of anterior cruciate ligament (ACL) injuries in dancers and found that dancers suffer considerably fewer ACL injuries than athletes participating in team ball sports. Notably, the incidence of ACL injury was not statistically different between sexes. The authors hypothesize that the training dancers utilize to perfect lower extremity alignment, jump, and balance skills may serve to protect them against ACL injury [96]. As a result, the training protocol for the dancer may not need to target balance and landing as intensely as athletes who play soccer and basketball. More recent research on ACL injury biomechanics in dancers has shown that although they take longer to reach fatigue of the lower extremities when compared to team athletes, once fatigue is reached for both dancers and team athletes their landing mechanics place them at greater risk for ACL injuries than before the onset of fatigue [97]. This research supports the need for resistance training for dancers to improve muscle endurance, as well as strength, to reduce injury risk. More specifically, pelvic stabilization (targeting strength and neuromuscular control of the deep hip stabilizers) and strengthening of the abdominal and gluteal muscles should be a cornerstone of training for this active group of females.
Conclusion
Resistance training in pediatric dancers is safe and effective for improving muscle strength, power, and endurance without effecting dance aesthetics. Research continues to evolve supporting the multitude of benefits to be derived from resistance training for female dancers, along with a well-balanced diet and adequate rest and recovery time. The call to action for all those caring for pediatric dancers is shifting focus from injury treatment to injury prevention. A discipline-wide approach to injury prevention is needed, and might well begin with educating the dance community about the benefits, safety, and effectiveness of resistance training programs that are designed, supervised, and taught by qualified professionals who understand the physical and psychosocial uniqueness of active children and adolescents.
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Screening the Young Dancer: Summarizing Thirty Years of Screening
Preventing Degenerative Hip Injuries from a Dance Technique Perspective
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