Comparison of Landing Biomechanics Between Dancers and Team Sport Athletes via a BTS Bioengineering Motion Capture System

Introduction: Motion capture, electromyography(EMG), and force data were collected on student and professional dancers, and athletes for the purpose of attempting to identify differences in EMG activity with kinetics and kinematics in relation to stiffness between professional dancers and athletes. Methods: We examined differences in the order of muscle activation between subject groups comparing pre and post fatigue data. The study involved 20 female professional dancers, 5 female student dancers, 20 female professional collegiate athletes, 20 male professional dancers, and 20 male collegiate athletes. The motion capture system tracked movement and collected force data using infrared cameras, video cameras, and force plates. The dancers and athletes both performed 3 single legged drop landings from a 30-cm platform. Landings were defined as the period of time from initial contact with the force plate to the point at which the maximum amount of knee flexion was achieved during each trial. Results: By comparing the landing biomechanics between several subject groups, it was determined that women land with different biomechanics than men. Women showed a greater range of motion than men, as well as lower ankle stiffness. More importantly, we saw differences in ankle mechanics and muscle activation during landing between preprofessional dancers and athletes, with student dancers often falling between the two. Professional dancers landed in a position of greater plantarflexion than did athletes (p = 0.015) and absorbed the impact with a less-stiff ankle (p < 0.001). Differences in activity of the medial gastrocnemius and peroneus muscles reflected many of these differences. Fatigue resulted in changes to ankle kinematics and kinetics that were largely similar across groups and genders, though fatigue did seem to affect ankle plantarflexion at initial contact with the ground to a greater degree in dancers as opposed to athletes (p < 0.001). 1.0 Review of Literature Sports medicine deals with physical fitness, and the treatment and prevention of injuries related to sports and exercise. In this branch of medicine, the term biomechanics is defined as the study of the mechanical laws relating to the movement or structure of living organisms. Biomechanics can be applied to the movement of the athlete’s body, specifically kinetics (forces acting upon and generated by the body) and kinematics (motions of the body). Dancers may be considered elite athletes based on the training they undergo and the level of skill required; the performance demands of ballet, for example, have been compared to those of American football (Nicholas, 1975). The annual rate of injury for professional dancers is reported to be between 67%–95% (Bronner et al., 2003) with the ankle being the most often injured joint (Liederbach et al., 2008). The high occurrence of these injuries can be explained by the dancer’s high degree of repetition, high level of physical demands, and environmental risk factors (Koutedakis and Jamurtas, 2004). In some cases (e.g., anterior cruciate ligament injury), dancers show much lower injury rates than do athletes (Liederbach et al., 2008) as well as a longer time to fatigue when performing a similar task (Liederbach et al., 2014). Thus, it is informative to study biomechanical differences between athletes and dancers as well as the muscle activity that results in and from these motions, which can be analyzed by electromyography (EMG). Several studies have suggested that kinetic and kinematic variables of the lower extremities are altered when the foot is positioned on an inclined surface; dancers often perform on inclined stages, especially in larger theaters. Floor inclination has been shown to alter kinetic and kinematic variables during gait analysis studies (Kawamura et al., 1991, Vogt and Banzer, 1999, Sun et al., 1996, McIntosh et al., 2006). Even though floor conditions had a significant effect on kinematic and kinetic variables, they did not have a significant effect on muscle activity, as measured by EMG in dancers (Hagins et al., 2009). Early detection of malalignment in the lower extremities is also important. The assessment of lower extremity alignment can be accurately performed using 3D motion analysis in a laboratory setup (Orishimo et al., 2009, Ross et al., 2016). Poor alignment is constituted as deviation from standard anatomic measures (Poggini et al., 1999) and is a risk factor for injury. Studying whether or not the athlete has poor control of their lower extremities may help detect and prevent alignment related injuries (Stensrud et al., 2011). Understanding leg and joint stiffness during a hopping task can provide information for developing more effective training methods for dancers and other athletes. For these purposes, the lower extremity of an athlete or dancer can be modeled as a simple linear spring, and the stiffness of this “leg spring” computed based on measurements carried out in a biomechanics lab. Similarly, joints in the body can be modeled as torsional springs (i.e., the moment or torque required to cause the joint to rotate is proportional to the amount of rotation; the constant of proportionality is the stiffness of the joint). It has been shown that ankle stiffness has a major influence on spring-mass dynamics during hop tests (Mrdakovic et al., 2014). The connection between the musculoskeletal system and the environment plays a key role in landing biomechanics. Research has shown that leg spring stiffness adapts to surface stiffness such that the combined stiffness of the leg and the surface remains constant (Ferris, 1997). In 2016, Ross et al., studied ankle stiffness specifically in relation to time to stability. They found that subjects with lower ankle stiffness values took longer to stabilize after a drop landing than those with increased ankle stiffness. Both kinetics and kinematics can demonstrate the strategies used by the subjects in an attempt to maintain joint stability during activity (Ross et al., 2005). Caulfield (2002) analyzed patterns of lower extremity motion prior to and following the landing of a single leg jump in subjects with functional instability of the ankle and uninjured subjects. Their study showed that subjects with functional ankle instability performed the landing task in a unique way that contributed to the different results. If an individual is in a position where he or she cannot stay upright (attempting to stand or maneuver on a Swiss ball), the amount of resistance that can be applied to the muscle will be negligible because all focus is on balance (Behm et., al 2002). Landing tasks are a valid metric in the examination of various landing techniques used by dancers and athletes. Jumping and landing are common tasks and provide detail on the use of lower extremities (Liederbach, 2000). The ability to stabilize quickly is a desirable characteristic for many dancers because they often have to change from one balance task to another without falling. Orishimo et al., (2014) showed that dancers with greater experience demonstrated more biomechanical characteristics that are protective for ACL injury during landing. This research aims to build on these studies and compares the order of muscle activation and its relation to fatigue between dancers and athletes at the same stage in their career. If the order of muscle activation does differ between subject groups, this could provide information to help reduce the risk of injury. 2.0 Research Questions and Hypotheses Knowing that dancers have a lower rate of specific injuries, means that if a dancer activates his/her muscles in a different order than athletes, than it is likely an athlete could be taught to land similarly to a dancer and avoid those same injuries. This study hypothesized that during a landing task, professional dancers would display different landing mechanics than those of team-sport athletes, who have not undergone the same thorough training in jumping and landing as the dancers; additionally, student-level dancers would display these differences, but not to the same degree as professionals. Specifically, we believe that there will be changes in the time and amount of muscle activation, between dancers and athletes; and that these differences will become more extreme when fatigue is a factor. Both male and female subjects were tested and data was processed based on time, vocation, and gender. 3.0 Methods 3.1 Participants This study had five groups of participants including 20 female professional dancers, 20 male professional dancers, 20 female professional athletes, 20 male professional athletes, and 5 student dancers. All subjects completed a medical history questionnaire and signed a consent form to participate in the study. Demographic information including height, weight, and age