Influence of Mirror Feedback and Ankle Joint Laxity on Dynamic Balance in Trained Ballet Dancers.
The assessment of DPS offers several advantages over more traditional measures of balance during unipedal stance, with the potential to elucidate mechanisms capable of benefiting ballet dancers. While all balance requires continuous sensorimotor integration to react to minute changes in joint stiffness and center of gravity, dynamic tasks create an increased neuromuscular demand that better represents what is experienced in dance and sport. (10) A combination of preparatory (feed-forward) and reactive (feedback) muscle activation must be precisely regulated such that stability is achieved quickly and without excessive joint excursion throughout the lower extremity. This traditionally represents an intrinsic model of neuromuscular control, whereby the individual need only assess his or her joint stiffness, position, and muscle activation in order to respond appropriately; however, in real world translation many extrinsic variables can modify DPS in positive or negative ways. For instance, landing on unstable surfaces, unanticipated acoustic or visual startles, and increased cognition have the potential to negatively impact the preparatory and reactive processes, resulting in deficient balance performance. (11-13) Conversely, such extrinsic factors as cueing with visual or auditory feedback can result in improved performance. (14),(15) The study of DPS has often been conducted among populations with joint injury, such as patients with chronic ankle instability, (14) but it is generally unclear how those studies translate to ballet dancers.
As opposed to intrinsic variables, extrinsic contributors to DPS are typically modifiable. Among ballet dancers and other performance-based athletes, visual feedback in the form of mirrors is frequently present in practice settings to provide cueing of joint position, potentially benefiting proprioception and balance (16); however, young dancers training with mirrors have been found to demonstrate minimal improvements in static balance and detrimental effects on performance. (17) No studies have described the acute effects of mirror feedback on DPS.
One final component that may modify balance is change in the static restraint of joints affecting intrinsic neuromuscular control. Dancers often demonstrate increased ankle joint laxity and subsequent decreased innate (passive) joint stiffness that may place them at risk of injury. (5),(18) Due in part to the demands of activities with large amounts of hyper-plantar flexion serving gradually to stretch structures surrounding the ankle joint, these changes in joint stiffness may similarly modify DPS. (19) Greater amounts of laxity would allow for increased joint excursion during simple tasks, potentially decreasing the afferent capacity of the ligaments and modifying reactive (feedback) loops. (20) The diminished stability from static joint restraints would increase demand on the dynamic joint restraints (muscles) to maintain stability during the task. (20) Of interest in this population is the comparatively large reliance on intrinsic and extrinsic components of joint stability during DPS tasks.
Given the neuro-mechanical complexity of DPS, and its applicability to ballet performance and injury prevention, generating a better understanding of contributors to functional balance would be of great benefit to the performance arts scientist. By identifying the role of intrinsic and extrinsic factors in affecting balance, interventions may be implemented to optimize performance and joint stability. Therefore, the aim of this study was to evaluate DPS and lower-leg muscle activation with and without mirror feedback and to determine how ankle laxity modifies DPS. It was hypothesized that mirror feedback would improve DPS, while ankle laxity would be related to poor balance.
Fifteen trained female ballet dancers were recruited from three dance studios in western North Carolina. As previous data among this population were not available, our sample size was based on a power analysis from an initial pilot of four participants (1-[beta] = 0.8, a = 0.05). Inclusion criteria were dancers who: were recreational performers between the ages of 14 and 30; had at least 5 years of ballet training with no less than 1 year en pointe; and were injury-free for the previous year. They provided self-reported anthropometrics, including leg dominance operationally defined as the leg used to kick a ball for maximum distance. Informed consent or, in the case of minors, parental consent and informed assent was attained. Participant demographics are presented in Table 1.
This study implemented a repeatedmeasures design. The independent variables were use of a mirror and ground reaction force (vertical, anteroposterior, mediolateral). The dependent variables included time-to-stabilization (TTS) after a forward hop, peak and mean activity of the tibialis anterior (TA), peroneus longus (PL), and lateral gastrocnemius (LG) muscles, and antero-posterior (AP) and inversion-eversion (IE) ankle joint laxity.
Participants attended a single testsession. After they provided informed consent and written assurance that inclusion criteria were met, a coin was flipped to randomly determine the test limb. Ankle laxity was assessed using an instrumented ankle arthrometer (Blue Bay Research, Milton, Florida, USA). The arthrometer was affixed to the ankle with the participant supine on a padded table. It consists of a six-degrees-of-freedom kinematic linkage system, footplate, and handle that measures the AP displacement and IE rotation of the ankle joint complex. (21) A trained investigator applied an AP load from -30 N posteriorly to 130 N anteriorly five times, followed by five total IE loads from -4.2 to 4.2 Nm. These loads have been previously validated to assess ligamentous complex laxity of the ankle joint, and strong intra- and inter-rater reliability has been demonstrated (ICC = 0.80-0.99). (21-23) The investigator received realtime feedback in the form of a moving cursor in custom LabVIEW software (National Instruments, Austin, Texas) to ensure a loading rate of 50 N/sec and 1 Nm/sec for AP and IE loads, respectively, with 3 seconds between each load application. Separate LabVIEW software was used to inspect trials for consistent force application, remove trials without consistent loaddisplacement responses, and partition trials from beginning to end of the load application. The LabVIEW software was used to extract the peak AP displacement, inversion rotation, and eversion rotation, defined as the maximal excursion of each trial. Peaks were averaged across the five trials for each participant.
Following laxity assessment, participants were fitted with electromyography (EMG) electrodes (99.99% Ag, single differential, 10 mm spacing) over the lower leg muscles of the test leg. The TA, PL, and LG were each palpated, shaved (if necessary), cleaned, and lightly abraded prior to attaching a bipolar EMG electrode over each muscle. A ground was placed over the medial malleolus. Electrodes were connected to a preamplifier (10 VN, -92 dB CMRR) worn on the participant's waistband, and to the main amplifier (Bagnoli-4, Delsys Inc., Boston, Massachusetts) by a 7.5 m cable. Amplifier gain was typically set at 1,000 Hz and adjusted to maximize signal-to-noise ratio. These muscles were selected due to their established role in regulating static and dynamic postural stability about the ankle joint. (3)
A suspended dance floor (American Harlequin Corp., Moorestown, New Jersey, USA) was set up in line with an instrumented force plate (Kistler Instrument Corp., Amherst, New York, USA), with a 10 cm hurdle placed a distance equal to the participant's test leg length (measured from the anterior superior iliac spine to the apex of the medial malleolus) from the end of the force plate. Participants were instructed in a two-step hop-to-stabilization progression in which they first stepped with the test limb, then pushed off with the non-test leg to jump over the hurdle, and landed on the force plate with the test leg (Fig. 1). After landing, participants were told to place their hands on hips and slightly flex the non-test hip while maintaining unipedal balance for 10 seconds or until instructed to relax by the tester. (24),(25) Trials were discounted if participants were unable to maintain unipedal stance. Each participant was given as many trials as needed to familiarize themselves with the task, including determining the most comfortable distance from which to begin their approach. All testing was performed barefoot.
Once comfortable with the task, participants were tested for a total of ten hops, five "mirror" and five "no mirror." For mirrored trials, a vertically oriented mirror was placed 2 m from the end of the force plate, while for non-mirrored trials, the mirror was rotated away from the participant. To determine the order of trials (mirror vs. no mirror), the investigator flipped a coin up to 10 times until five trials for each condition were reached. For all trials, force plate and EMG data were collected at 1,000 Hz in LabVIEW software for a period of 0.25 seconds prior to force plate contact to 10 seconds after force plate contact. If participants were unable to maintain unipedal stance for the duration of 10 seconds, that trial was marked as a failure and was repeated.
All procedures were approved by the Appalachian State University Institutional Review Board.
Data Reduction and Analysis
The vertical (Fz), anteroposterior (Fx), and mediolateral (Fy) components of ground reaction force were assessed from the point of foot contact until the end of balancing time. Time-to-stabilization was calculated using the sequential estimation technique described by Colby et al., (26) where a point-by-point sequential mean of each component of the ground reaction force is used to find the point at which it falls within 0.25 standard deviations of the series mean. Time-to-stabilization was calculated for each directional component of the ground reaction force and interpreted such that a longer TTS indicated poorer DPS within the respective plane of the ground reaction force. Electromyography activity for each muscle was bandpass filtered (20 to 400 Hz), rectified, and lowpass filtered (5 Hz) to create a complete linear envelope. Peak EMG activity for each trial, as well as mean EMG activity in a period of 250 ms prior to force plate contact (PRE), 0 to 250 ms after force plate contact (POST), and 250 to 500 ms after force plate contact (POST-2), were extracted for analysis.
To assess differences in TTS across conditions (mirror vs. no mirror) and direction (anteroposterior, mediolateral, and vertical), a two-way analysis of variance (ANOVA) with two within-subjects factors (condition, 2 levels; direction, 3 levels) was implemented. To assess differences in peak EMG activity across conditions (mirror vs. no mirror), a two-way factorial ANOVA with two within-subjects factors (condition, 2 levels; muscle, 3 levels) was used. To assess differences in mean EMG activity across conditions (mirror vs. no mirror) and times (PRE, POST, and POST-2), a three-way ANOVA with three within-subject factors (condition, 2 levels; muscle, 3 levels; time, 3 levels) was used. In cases of significant main or interaction effects, Fisher's least significant difference pairwise comparisons were used to determine differences. Although main effects of muscle were not inherent to the research question, the three muscles were included as a within-subjects factor to minimize the number of statistical procedures and decrease the likelihood of Type I error. To understand the effect of laxity on balance scores and muscle activation, Pearson product-moment correlation coefficients were analyzed between laxity variables (AP displacement and IE rotation) and jump performance (TTS and EMG activity). Correlational effect sizes were interpreted as weak (r > 0.10), moderate (r > 0.30), or strong (r > 0.50). An a priori level of significance was set at 0.05. All data were analyzed in IBM SPSS Statistics software version 24 (IBM, Armonk, New York, USA).
Time-to-stabilization values are presented in Table 2. No significant condition-by-direction interaction was observed [F(2, 28) = 0.105, p = 0.900], nor was there a significant effect of mirror [F(1, 14) = 0.600, p = 0.452]. A significant main effect of Direction [F(2, 28) = 44.076, p < 0.001] was observed. Under all conditions, TTS was longest in the anteroposterior plane (Fx) and fastest in the vertical plane (Fz), with significant differences between Fz and both Fx and Fy (p < 0.001), but not between Fx and Fy (p = 0.090). No significant condition-by-muscle interaction [F(2, 28) = 1.433, p = 0.256] or main effect of condition [F(1, 14) = 0.274, p = 0.609] was observed for peak EMG, although there was a significant main effect of muscle [F(2, 28) = 8.114, p = 0.002]. Electromyography was highest in the TA and least in the LG, with significant differences only between TA and LG (p < 0.001).
For mean EMG, no significant interaction of condition, muscle, or time was observed [F(4, 56) = 0.817, p = 0.520]; however, significant effects were found for condition-by-time [F(2, 28) = 5.006, p = 0.014] and muscle-by-time [F(4, 56) = 28.527, p < 0.001; Table 3]. Post hoc pairwise comparisons revealed no significant differences between mirror and no-mirror conditions (p > 0.05). No significant differences between muscles were observed prior to landing; however, at both time points after landing TA activation was significantly greater than PL (p < 0.022) and LG (p < 0.001), and PL activation was greater than LG (p < 0.004).
Correlations between laxity and TTS measures revealed a significant correlation between AP laxity and mediolateral TTS with the mirror (r = 0.655, p = 0.008) that was not present without the mirror (r = 0.251, p = 0.366; Fig. 2). Vertical TTS correlated with AP laxity without the mirror (r = 0.858, p < 0.001) but not with the mirror (r = 0.502, p = 0.057; Fig. 3). For all significant correlations, higher laxity was associated with a longer TTS (or poorer dynamic balance). Near-significant medium-sized correlations were observed between peak inversion laxity and vertical TTS both with (r = 0.512, p = 0.051) and without (r = 0.499, p = 0.059) the mirror. No significant correlations were observed for eversion rotation (p > 0.05).
The main finding of the current investigation is that TTS after a hopping maneuver was not affected by mirror feedback in ballet dancers. However, increased AP joint laxity negatively affected dynamic balance in both mirrored and non-mirrored conditions. Interestingly, there was a strong relationship between AP ankle joint laxity and both mediolateral and vertical TTS with and without a mirror. While muscle activity was not affected by the different conditions (mirror vs. no mirror), tibialis anterior (TA) activation was significantly greater than both the peroneus longus (PL) and lateral gastrocnemius (LG) from 0 to 250 ms and 250 to 500 ms after landing. The balance required to successfully perform a leap, such as grand jete, leading into other movement combinations is a prime example of the DPS that is integral to dance. These results highlight different DPS strategies with and without extrinsic feedback, based on intrinsic properties related to joint stability in ballet dancers.
Effects of Mirror Feedback
The novel result obtained from this investigation is that a specific dynamic task was not impacted by use of a mirror in a trained ballet dancer population, and therefore achieving the task apparently was not affected by visual feedback. A long history of training with mirror feedback is native to most dance studios and conservatories. The emphasis placed on "line," extremity placement, and overall fluidity of movement during dance should in theory be improved with visual feedback via increased targeting of the primary motor cortex to adjust neuromuscular activation and ultimately synchronize whole body movement. (27-29) Golomer et al., (30) in fact, established through several investigations that dancers rely more on proprioceptive input than visual input during a postural sway task. However, a unique feature of dance is that aesthetic achievement cannot necessarily be quantified, and thus the appeal of a movement is difficult to ascertain when biomechanically or physiologically measuring dynamic tasks. (27)
while inconclusive data have been reported on the effect of training with and without mirrors, (6),(28) the single dynamic task explored in this study appeared not to be influenced by mirror feedback. Multiple forms of feedback exist for encouraging improved task performance, such as external feedback (focusing on applying external force), internal feedback (focusing on achieving body position or activation), use of mirrors, and task-oriented feedback. Evidence suggests that mirror feedback during performance tasks (e.g., vertical jumps) is best using external focus, whereas mirror feedback is consistent with task-oriented feedback. (31) It is worth noting that these feedback-based changes are based on instructions and verbal cueing by instructors, rather than direct attention to the external feedback. In this investigation, participants were instructed to utilize the mirror to optimize their landing ability and maintain aesthetics, but it could not be determined how many of these experienced ballet dancers actually used the mirror to improve aesthetics compared to those who found it a distraction or ignored it entirely. Furthermore, we could not be certain whether the aesthetics of the landing were improved regardless of the ground reaction force profile, as kinematic assessment techniques were not employed in this study.
The mirror produced minor effects on muscle activation, indicating
small changes to the neuromuscular regulation of balance control. Consistently greater activation was observed in the TA compared to the PL and LG, likely due to the inherent necessity of this muscle for achieving balance regulation as well as the forward travel required in this task. (19) The TA has been previously acknowledged as a stabilizing muscle during demiand grand-plie, given its major role in dorsiflexing the ankle. (32),(33) Minor increased adjustments in activation of the TA and PL were observed at POST-2 without a mirror that were not seen with a mirror. As the no-mirror condition increased the demand on vestibular and proprioceptive feedback, this activation may have served to increase [alpha]-[gamma] co-activation and subsequently muscle spindle sensitivity surrounding the ankle joint. (34) While afferent mechanoreceptors arising from the capsuloligamentous structures of the ankle would provide levels of feedback as they are stretched, muscle spindle afferents are able to modify sensitivity to provide feedback throughout motion. Diminished muscle spindle sensitivity has been previously implicated as a causative factor in patients with joint instability resulting from ligamentous injury. (35) Further exploration of muscle spindle activity and how it is influenced by internal and external feedback should be of interest; however, technological limitations make it difficult to determine how this would affect dynamic tasks.
Limited studies exist regarding the effects of mirror feedback on balance performance, with one study demonstrating slight improvement in static balance among stroke patients. (36) The effects of training with and without mirrors has received greater attention, including improved performance observed with mirror feedback among those training in Olympic weightlifting techniques (37) and a Pilates star maneuver. (38) Questions remain as to what degree training with mirror feedback translates to performing without visual feedback, as is typical in ballet. Additional research that aims to identify both technique and task achievement in dancers should produce better insight into both the purpose and effect of training with mirror feedback and its impact on performance. Furthermore, quantification of the effects of mirror feedback on the aesthetics of the hop-to-stabilization may be warranted.
Relationship Between Laxity and Balance
As indicated by our findings, the intrinsic and extrinsic factors that contribute to DPS are complexly interrelated. Generally, this supports our hypothesis and may be explained from a neuromechanical perspective, whereby increasing ligamentous laxity in turn increases the degree of ankle joint excursion, for which the individual must intrinsically negotiate, and potentially diminishes proprioceptive acuity, as stretch receptors in the ligament may not fire until the end-range of joint excursion. (20) These data are novel, as a recent investigation using static measures of postural stability indicated hypermobility had no impact on balance. (39) However, an interesting finding was the direction in which increased AP laxity modified DPS with and without mirror feedback. With mirror feedback, AP laxity correlated with frontal plane (mediolateral) TTS, while AP laxity correlated with vertical TTS without mirror feedback. The former finding may be explained in terms of the role played by the anterior talofibular ligament, which both regulates anterior translation and provides frontal plane stability of the ankle joint. (40) Therefore, increased laxity of this ligament may contribute to greater frontal plane instability as observed in participants with the mirror. (41) Without mirror feedback, on the other hand, this relationship was no longer present; rather, an increase in vertical TTS was observed. Visual feedback may have allowed users to receive external cueing of joint position and subsequently manipulate ankle positioning and thus ankle angular stiffness. (42) However, without mirror feedback, participants with less innate stiffness potentially utilized a more "protective" mechanism of landing that exaggerates vertical forces. Such a strategy would allow for greater mechanical protection by generating a close-packed position in the ankle or increasing hip and knee flexion to provide stability for an uncertain landing. (43) A similar positive relationship between laxity and vertical TTS was observed with peak inversion rotation, but this relationship was not different with and without the mirror, indicating that the accessory joint motion rather than physiologic rotation impacted DPS strategies. While the lack of kinematic analyses in the present investigation is a limitation, these results suggest a more vertical and protective style of landing to minimize risk of injury, as has been seen in patients with varying degrees of ankle joint instability. (44)
During any sort of leap, the ballet performer is commonly focused on ankle and foot positioning. Stylistic demands during some ballet landings necessitate a full point (maximal toe and ankle plantar flexion) upon contact with the ground. With increased awareness of the aesthetic appeal of the ankle-foot complex, it is possible that mirror feedback would cause a dancer to activate more pronounced plantar flexion of the ankle during the flight phase of a leap, consequently increasing abnormal joint forces and anterior talofibular ligament strain. (32) This could increase the risk of lateral ankle injury in this population. To better understand the effect of ankle stiffness on balance ability and injury risk, it would also be helpful to assess dynamic joint stiffness aside from passive stiffness as well as the kinematics of performance throughout the DPS task.
The purpose of the current investigation was to evaluate the effect of mirror feedback and ankle laxity on DPS and muscle activation in trained ballet dancers utilizing a hop-to-stabilization task with and without mirror feedback. The lack of difference between mirror and no-mirror conditions may suggest that ballet dancers have conditioned movement patterns and therefore coordinate loading strategies with musculature recruitment based on ankle laxity and task regardless of visual input. Another consideration is that ballet dancers train and perform in environments with varying surfaces, lighting, spatial dimensions, etc., and a requirement of such demands adaptability. More basic research that deciphers the configurations of extrinsic and intrinsic factors that influence DPS in dance populations is needed to promote optimal training and performance.
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Hannah N. Miller, ATC, Paige E. Rice, MS, Zachary J. Felpel, MS, Alyssa M. Stirling, MS, ATC, Eric N. Bengtson, MS, ATC, and Alan R. Needle, PhD, ATC
Hannah N. Miller, ATC, Zachary J. Felpel, MS, Alyssa M. Stirling, MS, ATC, and Alan R. Needle, PhD, ATC, Injury Neuromechanics Laboratory, Appalachian State University, Boone, North Carolina, USA. Paige E. Rice, MS, Neuromuscular and Biomechanics Laboratory, Appalachian State University, Boone, North Carolina, USA. Eric N. Bengtson, MS, ATC, Department of Physical Therapy, Belmont University, Nashville, Tennessee, USA.
Correspondence: Alan R. Needle, PhD, ATC, Department of Health and Exercise Science, Appalachian State University, ASU Box 32071, Boone, North Carolina 28608, USA; firstname.lastname@example.org.
Table 1 Subject Demographics Age (years) 24.4 [+ or -] 6.5 Height (cm) 164.6 [+ or -] 8.2 Mass (kg) 59.6 [+ or -] 7.3 Test Limb 10 Dominant / 5 Non-dominant Training in Ballet (years) 17.0 [+ or -] 7.8 Anteroposterior Laxity (mm) 9.59 [+ or -] 2.93 Inversion rotation (degrees) 23.41 [+ or -] 9.07 Eversion rotation (degrees) 22.00 [+ or -] 7.51 Values are means and standard deviations. Table 2 Time to Stabilization (TTS) in Vertical, Anteroposterior, and Mediolateral Planes With and Without Mirror Feedback Plane With Mirror Feedback Without Mirror Feedback Vertical 2.35 [+ or -] 0.45 2.40 [+ or -] 0.48 Anteroposterior 4.03 [+ or -] 0.12 4.04 [+ or -] 0.12 Mediolateral 3.56 [+ or -] 0.90 3.67 [+ or -] 1.08 Values are in seconds and represent means and standard deviations. Table 3 Average Muscle Activation Prior to and Following Hop-to-Stabilization in the Tibialis Anterior (TA), Peroneus Longus (PL), and Lateral Gastrocnemius (LG) Muscles Time Relative to Landing Muscle Condition PRE POST-1 TA With mirror 5.60 [+ or -] 1.70 8.97 [+ or -] 3.33*[dagger] Without mirror 5.76 [+ or -] 1.92 9.01 [+ or -] 3.23*[dagger] PL With mirror 4.76 [+ or -] 2.07 6.25 [+ or -] 2.98[dagger] Without mirror 4.81 [+ or -] 2.04 6.16 [+ or -] 2.84[dagger] LG With mirror 5.84 [+ or -] 1.78 3.27 [+ or -] 0.88 Without mirror 5.27 [+ or -] 1.84 3.33 [+ or -] 0.89 Time Relative to Landing Muscle Condition POST-2 TA With mirror 7.17 [+ or -] 2.83*[dagger] Without mirror 8.05 [+ or -] 3.69*[dagger] PL With mirror 5.08 [+ or -] 2.98[dagger] Without mirror 5.61 [+ or -] 3.51[dagger] LG With mirror 2.89 [+ or -] 1.00 Without mirror 2.97 [+ or -] 1.04 PRE represents 250 ms prior to contact; POST-1 represents 0-250 ms after contact; POST-2 represents 250-500 ms after contact. Values are in [micro]V and represent means and standard deviations. *Significant difference from PL (p < 0.05); [dagger]Significant difference from LG (p < 0.05).
Caption: Figure 1 Experimental set-up. Participants began at end of suspended flooring and performed a two-step approach before hopping over 10 cm barrier onto force plate in front of movable mirror. The barrier was placed one leg length from force plate.
Caption: Figure 2 Correlations between peak antero-posterior (AP) displacement and mediolateral time-to-stabilization with and without mirror feedback. Mirror feedback presented in triangles with dashed line; without mirror feedback presented in circles with dotted line.
Caption: Figure 3 Correlations between peak antero-posterior (AP) displacement and vertical time-to-stabilization with and without mirror feedback. Mirror feedback presented in triangles with dashed line; without mirror feedback presented in circles with dotted line.
Please Note: Illustration(s) are not available due to copyright restrictions.
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|Author:||Miller, Hannah N.; Rice, Paige E.; Felpel, Zachary J.; Stirling, Alyssa M.; Bengtson, Eric N.; Needl|
|Publication:||Journal of Dance Medicine & Science|
|Date:||Oct 1, 2018|
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