Prevalence of muscle imbalance and its potential influence on injury among female acrobatic dancers

Introduction

Acrobatics is a physically demanding sport that pushes an individual’s body to unusual limits to perform a variety of tricks that can be seen as unnatural movements. Through that, the body is used in unique ways. Acrobatic dance in South Africa is seen as a style that incorporates acrobatics and floor gymnastics with classical dance techniques and contortion.1 The execution of acrobatic skills holds a place in the history of the international gymnastic discipline,2 and this style can be seen on various stages and theatrical arenas.2

Previous studies have been conducted on athletes with similar performance demands, such as gymnasts, and an immense amount of data can be found among ballet dancers.3 Training for dancing, such as acrobatics, is unlike other sports, which have seasons, and is rather an annual endeavour. Due to this frequent duration, dancers’ risk of overuse injuries increases.4 Dancers require enhanced physiological adaptations due to the great physical demands of the sport, where both aerobic and anaerobic systems are heavily relied on, and a significant amount of muscle tension is developed.5 Multiple studies have found that the repetition of movements in a variety of sports leads to adaptations of different tissues including the muscles.6 According to Coombes and Garbutt,7 muscle imbalances and bilateral muscle asymmetry are the aetiology of many injuries due to the repetitive nature of dance8 and the annual training calendar. It would, therefore, be important to identify if acrobats, too, are at risk to muscle imbalance and possible injury aetiology.

A method in which muscle imbalance can be tested and identified is through isokinetic testing. The Hamstring:quadriceps peak torque ratio is a common influence on muscle imbalances associated with sport-related injuries.9 In a study done by Kuni and Schmitt,10 isokinetic testing found a balance deficit between the plantar flexors and dorsiflexors in professional dancers. Testing the isokinetic strength of the ankle joint of female acrobats would be useful to note if similarities to this study are present and if they possibly may be linked to injury onset.

In addition, it has been suggested that flexibility imbalances have a causal relationship with dance injuries.11 In a study done by Steinberg et al,12 42.6% of dancers they screened were injured, and one of the factors associated with the injuries seen was the range of motion, specifically the presence of either hyper or hypo range of motion.

Methods

Results

The following results of the female acrobats’ (age=13.60±2.29, height=1.58±0.09 m, body mass=51.37±12.92 kg) lower limb strength, lower limb flexibility and injury history were found.

Figure 1A displays a correlation graph that compares the Isokinetic peak torque (Nm) of the right hamstrings and right quadriceps. There was a highly significant, positive linear correlation between the isokinetic peak torque of the right hamstrings and the isokinetic peak torque of the right quadriceps (r=0.848, p=0.000). There was also a significant, positive linear correlation for the left side’s hamstrings and quadriceps peak torque values (r=0.943, p=0.000), as shown in figure 1B.

The prevalence of hamstring or quadriceps dominance in the acrobats was also tested. Of the 15 acrobats tested, 9 were quadriceps dominant on both sides. However, the limited sample size of this study group did not lead to any significant findings (p=0.933). Similarly, the prevalence of dorsiflexor or plantar flexor dominance was tested. Out of the 15 acrobats tested, 14 were plantar flexor dominant on both sides. This was confirmed by the low p value (0.000). However, due to the limited sample size of this study group, whether these relationships will exist in larger sample groups cannot be assumed.

Figure 2A displays a correlation graph that compares the isokinetic peak torque (Nm) of the right and left plantar flexors. There was a highly significant, positive correlation between the isokinetic peak torque of the plantar flexors (r=0.913, p=0.000). Figure 2B displays a correlation graph that compares the Isokinetic peak torque (Nm) of both the right and left dorsiflexors. There was a highly significant, positive correlation between the isokinetic peak torque of the dorsiflexors (r=0.951, p=0.000).

When correlating dorsiflexor isokinetic peak torque with plantar flexor isokinetic peak torque, significant findings were found for both sides as seen in figure 3A,B (right: r=0.670, p=0.006, and left: r=0.648, p=0.009). Physiological profile means were also calculated for a number of the tests performed (back strength=44.9±23.24 kg, forearm plank=50.65±12.31 s/ms, lumbar flexion=7.01±1.59 cm, lumbar extension=5.21±1.45 cm and sit and reach=52.79±8.52 cm).

When analysing the results of trunk lateral flexion, the following observations were found. Participants with right-hand dominance, on average, had reduced flexibility in lateral trunk flexion when testing the right side’s flexibility (23.13±5.57 cm). In contrast, on average, participants with left-hand dominance performed better in the flexibility testing of lateral flexion of the right side (26±3.46 cm) (the opposite was true for the other side). The right-hand dominant individuals performed better on average in the lateral flexion testing of the left side (25.81±4.31 cm) in comparison to the left-hand dominant individuals (24.78±0.56 cm).

It can be seen in figure 4A,B that there was a non-significant positive correlation between hamstring peak torque and hamstring goniometer flexibility on the right side (r=0.121, p=0.668) and on the left side (r=0.266, p=0.338).

In figure 5A,B, it was also seen that there was a non-significant negative correlation between quadriceps peak torque and quadriceps goniometer flexibility on the right side (r=−0.373, p=0.171) and on the left side (r=−0.106, p=0.707). Similar findings were found with the testing of the plantar flexors and dorsiflexors. There was a non-significant positive correlation between the plantar flexor peak torque and plantar flexor goniometer flexibility on the right (r=0.297, p=0.282) and a non-significant negative correlation on the left (r=−0.244, p=0.382). There was a non-significant negative correlation between the dorsiflexor peak torque and dorsiflexor goniometer flexibility on the right side (r=−0.289, p=0.297) and left side (r=−0.238, p=0.393).

Lastly, it was seen in the completed injury survey’s that the ankle was the most injured joint, with 5 of 15 participants (33.3%) reporting an ankle injury, with the knee being the second most injured joint (26.7%) of the participants. However, it is important to mention that these injuries did not exclusively occur during an acrobatic class or competition. Two out of the five ankle injuries occurred during acrobatic participation, and only one of the four knee injuries occurred during acrobatic participation, along with one back injury (26.7%). The remaining injuries mentioned in table 1 occurred in a different setting (73.3%).

Discussion

This study provides insight into the physiological profile of young female acrobats in Johannesburg, South Africa. Acrobats’ agonist and antagonist muscle groups tested via the Cybex Norm machine (quadriceps–hamstrings and plantar flexors–dorsiflexors) were seen to have a similar trend in all individuals. This observed trend strongly suggested that as an acrobat’s agonist peak torque increased, so would their antagonist peak torque. Despite the results being non-significant, it is still important to note that most of the acrobatic dancers were quadriceps and plantar flexor dominant. Quadriceps dominance was also seen in a study among trained dancers by Vogelpohl et al.22 It could be due to dancers’ reliance on classical dance movements that include knee extension and fewer movements incorporating knee flexion. Quadriceps dominance also suggests reduced hamstring strength compared with quadriceps strength and is implicated in increasing the risk for lower extremity injuries.23 Plantar flexor dominance may be a result of the dancers spending a lot of time on their toes in extreme plantarflexion,24 such as in relevé (action of standing on the ball of your foot), performing tricks and dance movements. Furthermore, plantar flexor dominance suggests a greater plantar flexor strength in comparison to the dorsiflexors. According to Fong et al,25 this increased plantar flexion strength can be a risk factor for an ankle injury, specifically sprains.

Dancers need to have sufficient lower limb muscle strength and adequate trunk stabilisation strength as well,26 which justifies the need for testing sufficient back and abdominal strength. No previous literature on reference values for forearm plank duration and lower back strength in dancer-specific populations could be found. However, a study conducted by Strand et al16 among college students found that the average forearm plank duration for female students was 83±63 s. Compared with the current study, the acrobats’ average forearm plank was noticeably below this value. It could be deduced that this was due to the age of the participants. Lastly, the average back strength results of the acrobats in the current study were 44.9±23.24. When compared with adolescents of a similar age group in a study conducted by Ten Hoor et al,27 it was found that the acrobats had a lower back strength result overall. The acrobats falling below for both these measures could warrant further research into the trunk stabilisation abilities of acrobats and if their lack thereof may affect their performance or increase their risk of injury.

Dancers require extreme ranges of motion to prevent injury and achieve their sport’s demands.11 According to Ruggieri and Costa,9 ballet dancers were reported to have a sit-and-reach flexibility of 22.8±4.1 cm. Gymnasts were reported to have sit-and-reach flexibility of 36.4±4.2 cm, indicating that the acrobats in this current study had a noticeably greater sit-and-reach flexibility than ballet dancers and gymnasts. However, it is important to note that the SD of the acrobats’ results was larger than the other two dance styles, showing more variation in the results. Lateral trunk flexion flexibility results were also seen to be higher on the acrobats’ non-dominant sides compared with their dominant sides. Opposite findings were true for flexibility and dominance in a study conducted by Neto et al28 on healthy subjects. Increased ranges of motion within a joint reduce muscle injury risk.11 However, hypermobility and flexibility imbalances between muscle groups could be associated with higher injury risk.29 It would also benefit from more research, particularly in acrobatic dance.

As previously mentioned, sit-and-reach flexibility was noticeably better than other dance styles, and it could be speculated that their lumbar forward flexion could be of a similar result as the movements are similar and recruit the same musculature.30 Acrobats perform various tricks that require extreme ranges of back extensions, such as backbends, forward walkovers and backward walkovers31; this could suggest why their trunk extension flexibility results were relatively high compared with a normal population.17 According to a study conducted by McKay et al,32 the acrobats’ quadriceps flexibility fell below the norm of their age group, suggesting possible tightness in their quadriceps. Following further flexibility, it was seen that there were no significant findings between the acrobats’ muscle peak torque and their muscle flexibility. It could be speculated that greater strength on one side most likely will not influence that same side’s flexibility.33 A study conducted by Agopyan further supported this finding,34 in which similar findings within hamstring isokinetic results and hamstring goniometry results were found.

Out of the limited sample size that this study tested, only seven participants experienced a dance-related injury. Only three of those injuries occurred in an acrobatic setting. The most common injury site among acrobats was the ankle, with the knee following closely. This is in contrast to what was found in the literature that reported that knee injuries were the most common injury, with the ankle being far less.35 These findings could suggest that even with possible muscle imbalances among the acrobats, it cannot be concluded that it was a result of acrobatic training alone.

A consensus could be reached that female dancers experience a significant number of injuries,9 specifically of the lower limb,2 and female acrobats in South Africa are seen to possibly show a similar finding. However, it must be noted that further investigation with larger sample size groups of injury prevalence in acrobatic dancers is needed before decisive claims can be made. These injuries have been seen to have a variety of possible causes, including muscle imbalance in strength7 and range of motion.11 Well-rounded exercise training programmes could aid acrobats in meeting the physiological demands of their sport by reducing muscle imbalance in strength and flexibility and consequently may aid in reducing injury prevalence.

Conclusion

Quadriceps dominance along with plantar flexor dominance was seen within this limited acrobatic sample population. Dancers (particularly with gymnastics and ballet) are at risk of lower extremity injuries, and these muscle imbalances noticeable in the acrobatic community could potentially be seen as risk factors for lower extremity injury with further research required. Increased ranges of motion in sit-and-reach results were noted in this study of acrobats compared with other dance styles. This increased range of motion could protect the acrobats from injury. However, more research on the effect of injury risk caused by hypermobility and imbalances of flexibility would be of great benefit, particularly in acrobatic dance. No significant findings were found between the acrobats’ muscle peak torque and their muscle flexibility. It could be suggested that greater strength on one side most likely will not influence flexibility on the same side. Due to the limited sample size of the current study, few significant findings were reported regarding muscle imbalance and its potential influence on an injury. Despite this, new insight was added to the physiological make-up of young acrobats in South Africa, which has not been previously conducted and could potentially encourage more research to be spent on this population along with less researched dance styles. Research with the inclusion of exercise interventions focusing on holistic training programmes to reduce muscle imbalances and ultimately identify if the reduction in muscle imbalance influences injury prevalence could be extremely insightful and should be encouraged.