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The biomechanics and motor control of tap dancing.

Abstract

Although tap dancing is a popular dance genre, little is known about the biomechanics and motor control of this complex motor skill. We conducted a detailed 3D kinematic analysis of movement timing, amplitude, and symmetry in three experienced female tap dancers. Kinematic analyzes of three basic tap dance steps (nerve beats, brush brush stamps, and heel ball walks) were undertaken. A 10-camera Vicon motion analysis system was used to collect the data. The results showed the feet and knees to play a major role in movement execution. Each step required at least 10[degrees] of ankle motion (range: 10[degrees] to 66.8[degrees]). Knee range of motion varied from 1.3[degrees] to 147.4[degrees]. For each of the dance steps the hips showed the smallest amplitude of movement, which was not greater than 21[degrees] in two out of the three dance steps. Analyzes of movement timing showed that each of the dance steps was fast, accurate, and well synchronized. The nerve beats took on average 0.50 seconds, the brush brush stamps 1.36 seconds, and the heel ball walks 4.03 seconds. A high degree of symmetry in total movement amplitude was evident at the ankles, knees, and hips for the nerve beats and heel ball walks. There was a mild degree of asymmetry at the hip for the brush brush stamp steps (symmetry index 90%). The results showed that experienced dancers had very high levels of proficiency in controlling movement amplitude, timing, and inter-limb coordination across the ankles, knees, and hips. This resulted in skilled, fast, and well executed dance steps.

Tap dancing is a highly complex motor skill requiring an advanced ability to control movement timing, movement amplitude, and inter-limb coordination. Despite taking many years to master, it remains a popular dance genre throughout the world. Little is known about the biomechanics and motor control of tap dancing; hence, we conducted an analysis of some basic tap dance steps using three-dimensional (3D) motion analysis. We were particularly interested in understanding the extent to which skilled control of movement timing, amplitude, and symmetry of the lower limbs is a key determinant of tap dancing proficiency.

Most studies on motor performance and control pertaining to dance have been conducted in relation to ballet, flamenco, and Irish dancing. (1-11) For example, motion analysis studies of ballet (3,5,12,13) and flamenco dancing (6,10) have shown that elite performers are highly skilled at controlling movement speed, amplitude, and inter-limb coordination throughout the entire body and especially the feet. These studies have also demonstrated proficiency in adapting motor performance to the task constraints imposed by the accompanying music and the dance routine. Tap dancing also appears to have several elements in common with Irish dancing (8,9) ; both genres place considerable emphasis on movements of the feet and legs, while the upper body is a secondary focus of attention. (14,15) Neverthless, scant 3D data are available on the biomechanics of tap dancing, and little is known about its requirements for movement speed, amplitude, and timing.

Only one study has analyzed the kinematics and kinetics of tap dance sequences. (14) In that study four technically demanding tap dance movements were evaluated in order to understand the factors predisposing to injuries in professional tap dancers. A comparatively low incidence of injuries was reported for tap dancers, and this was correlated with comparatively small ground reaction forces, joints forces, and moments that are a hallmark of skilled performance. (14)

Given the paucity of existing data, we investigated some basic tap dance steps (nerve beats, brush brush stamps, and heel ball walks) to provide a 3D kinematic analysis of movement timing, amplitude, and symmetry in experienced tap dancers. The specific aims were to:

1. Quantify the typical range of motion at the feet, knees, and hips in experienced and highly skilled tap dancers;

2. Determine the time taken to perform different tap dance steps and movement phases; and

3. Examine symmetry in amplitude and timing between the right and left legs.

Methods

Participants

Three experienced and highly skilled female tap dancers participated in this study (Table 1). This was a sample of convenience of young adults aged 20 to 30 years, with experience in dancing and teaching the Australian Tap Dance Syllabus. (16) Participants were free of musculo-skeletal injuries, co-existing medical conditions, and pain in the previous six months. They gave informed written consent in accordance with La Trobe University Australia Ethics Committee requirements.

Experimental Protocol

Each participant performed the three basic tap dance steps mentioned previously, chosen from the Australian Tap Dance Syllabus (17) levels 1 and 2, executed in random order. These steps are frequently combined to create complex routines. All dance steps were analyzed with respect to three movement phases: preparation, movement, and recovery. The movement phase of each step was also subdivided, as will be explained in detail for each dance step below. Table 2 summarizes the elements of each step.

The dancers were instructed to begin each step with the right foot followed by the left foot, and to perform the sequence as required by the rhythm of the selected music, "Black Cabaret," with 118 bpm (0.51 seconds between each beat). Each movement sequence was performed and analyzed separately. Although familiar with the dance steps, dancers were instructed to practice each of the steps twice before testing.

Data Capture

Thirty-six retro-reflective markers, placed by a trained and experienced evaluator, were used to identify lower limb and pelvic movement patterns in the three planes of motion. The markers were attached to the following anatomical landmarks: left and right anterior superior iliac spines, left and right posterior superior iliac spines, left and right mid-superior aspect of the left iliac crests, lateral and medial condyle of the left and right knees, lateral and medial malleolus of the left and right ankles, lateral aspect of head of left and right fifth metatarsals, over the left and right second metatarsal heads, and the left and right calcaneus bones. During the tap dance routines the raw position-time coordinates for the lower-limb markers were sampled continuously at 100 Hz, using a 10-camera Vicon motion analysis system (Vicon, Oxford Metrics Ltd., Oxford, UK).

Additional reflective markers were cemented to rigid thermoplastic shells to construct rigid-body marker clusters that were firmly attached to the thighs and shanks using Velcro straps. A standing trial was initially captured to identify and calculate the joints' centers. The medial condyle and medial malleolus markers were then removed and were not used during the movement trials to avoid interference with the performance.

Data Processing

Raw position-time signals were first interpolated to compensate for occluded markers using a window of up to 10 frames, followed by a 4th order zero-lag Butterworth Filter with a cut-off frequency of 6 Hz. Following data sampling, VISUAL3D (c-motion Inc., Germantown, Maryland, USA) software was used to reconstruct the position-time coordinates for each of the marker and segment orientations.

Data Analysis

Data points of interest were extracted from each individual trial for each dancer. These individual data were used to investigate consistency between repetitions. When a specific movement was repeated more than once, these data were combined to derive an average value for each dancer. Descriptive statistical analyses also included the range of motion of each joint and medians and ranges in general.

The total amplitude of movements at the ankles, knees, and hips was analyzed for each movement phase of each dance step. The total amplitude of movement for each of the joints was determined by calculating the difference between the peak angle of flexion and the peak angle of extension for each movement phase. The time taken to complete each of the movement phases for each limb for each dance step was also measured.

The symmetry in performance between left and right limbs was calculated for both amplitude and timing using the Gait Asymmetry Index (GA) (18,19) according to the following formula: GA= (Xr/X1) x 100%. This formula assumes that Xr is the variable recorded for the right limb and Xl for the left limb.

The first and last nerve beats were excluded from the analysis to avoid interference from the preparation and recovery phases. Therefore, nine nerve beats were evaluated. To explore the time to completion of the nerve beats the movement phase of the nerve beat step was sub-divided into two sub-phases: the upward phase and the downward phase (Table 2). Variation in the timing between these phases was analyzed by calculating the consistency in timing across each sub-phase of the nine nerve beats.

The movement phase for the brush brush stamps was analyzed for the three different sub-phases: brush forward, brush backward, and stamp (Table 2). Heel ball walks were divided into first stance-first swing, and second stance-second swing phases (Table 2).

Results

Movement Amplitudes for the Feet, Knees, and Hips

The amplitude of movement varied across movement phases and dance steps. Nevertheless, for each of the three tap dance steps the total movement amplitude was found to be greater at the ankle and knee than at the hip (Table 3, Fig. 1). During performance of the nerve beat steps, the ankle joints showed greater movement amplitude (range: 41.2[degrees] to 57.1[degrees]) than the hips (range: 0.1[degrees] to 3.3[degrees]) and knees (range: 1.3[degrees] to 16.4[degrees]).

For the brush brush stamps, the knee joints had the highest movement amplitude in all movement phases. The greatest motion at all lower limb joints occurred during the brush backward phase of these movements. During this particular phase the range of knee motion was between 126.9[degrees] and 147.4[degrees], the range of ankle motion was between 28.8[degrees] and 42.3[degrees], while the range of hip motion was between 5.2[degrees] and 14.9[degrees]. Although the control of knee flexion and extension was the key determinant of control for the brush brush stamps, motion at the ankles was also important, presumably in an effort to create the well-known tap sound.

For the heel ball walks, the greatest range of motion for all lower limb joints occurred during the swing phase. During this phase the knee joints contributed more to movement amplitude (range: 113.0[degrees] to 145.9[degrees]) than the ankles (range: 58.6[degrees] to 66.8[degrees]) and hips (range: 52.4[degrees] to 71.5[degrees]).

Movement Timing

All tap dancing steps were performed very quickly by these elite performers (Table 4, Fig. 2). Tap steps were fast, accurate, and well synchronized with respect to the different movement phases. The nerve beats were completed quickly by all dancers, taking only 0.49 to 0.50 seconds, which corresponds to one beat per second of the accompanying music. The brush brush stamps were completed within 1.25 to 1.70 seconds, and the heel ball walks took 4.02 to 4.08 seconds to be performed.

For the nerve beats, the time taken to perform the upward movement phase (range: 0.33 to 0.37 seconds) was approximately twice as long as the time taken to execute the downward movement phases (0.13 to 0.16 seconds). There was only 0.03 seconds variation between the nine downward phases for the nerve beats, and less than 0.17 seconds difference between the upward phases, showing how consistent the movements were across the sequences. The time to complete the forward phases and the backward phases of the brush brush stamps steps was comparable. However, for this step the time taken to complete the stamp phase was quicker on the left side (Table 5, Fig. 2). Stance phase (1.29 seconds [1.21 to 1.37seconds]) of heel ball walks was longer than the swing phase (0.74 seconds [0.66 to 0.81 seconds]), (Table 5 and Fig. 2).

In interpreting these timing results the concurrent amplitude regulation needs to be taken into account. For the nerve beats, the ankle joint moved the most, with approximately 50[degrees] of total movement amplitude for each phase. For the brush brush stamps, the knee joint moved the most, with approximately 140[degrees] total amplitude for each movement phase (Tables 4 and 5 and Fig. 2)

Amplitude and Timing Symmetry

A high degree of symmetry (> 97%) in total movement amplitude for the nerve beats and heel ball walks was found between the left and right sides for the ankles and knees, and slightly less for the hips (between 90% and 96%; Table 3). For the brush brush stamps, there was a high degree of symmetry (> 94%) between the left and right knees and ankle total amplitudes. In contrast, the total hip amplitude was moderately asymmetrical between sides (67% and 86%). Symmetry was less for the brush backward phase (Table 3).

The time taken to complete the upward and downward movement phases of the nerve beats was consistent between sides, with a timing symmetry close to 100% in both phases. The time variations across the nine nerve beats were more marked between sides, especially in relation to the upward movement phase, with only 60% timing symmetry (Table 4).

A high degree of symmetry (> 92%) between sides could be seen for the time taken to complete the brush forward and brush backward phases of the brush brush stamps (Table 5). However, the stamp phase showed lesser symmetry, as the time to complete this phase was faster on the left side than on the right side (symmetry index = 72%).

For the heel ball walks, high levels of symmetry (> 96%) between sides were observed with respect to the time taken to complete each movement phase of the sequence, with timing symmetry ranging from 96% to 98% (Table 5).

Discussion

Tap dancing is a popular yet demanding dance genre from a technical point of view. Our study showed that in elite tap dancers there were exceptional levels of skill in regulating lower limb movement size, movement timing, and inter-limb coordination. One of the key aims of this study was to understand what proportion of joint motion occurs at the ankles compared with knees and hips for each of the three core steps in tap dancing. The results showed that for the performance of nerve beats, ankle range of motion was on average approximately 50[degrees], which was three times that of the knees (average maximum of 15[degrees]) and almost 50 times that of the hips (on average 1.3[degrees]). In contrast, brush-brush-stamps movements emphasized knee range of motion (maximum of 130[degrees] on average), being three times that of the ankles (maximum of 35[degrees] on average) and 10 times that of the hips (average 12[degrees]). Heel ball walks presented the most even distribution of motion down the lower limb joint chain, with approximately equal range at ankles and hips (around 50[degrees] on average), being half the range of motion at the knees (around 100[degrees]). Thus, a notable finding was that successful motor performance relied on a high degree of amplitude control, especially at the ankles and knees. The role of the hip for some of the steps may well be to provide stability allowing the ankles and knees to move fast with high amplitude, creating the tap dance sound.

Tap dancing movements were performed very quickly. The timing to complete each tap dancing step was closely associated with the total movement amplitude. As the timing of a step and the music beat influence the range of motion, faster steps, such as nerve beats, were associated with a smaller movement amplitude than brush brush stamps and heel ball walks, as described above. The timing for heel ball walks had some similarities to normal gait. During this dance step the stance phase was twice as long as the swing phase, which was similar to straight line walking during which 60% of the cycle duration is in the stance phase. (20)

In most cases, movements were highly symmetrical in these elite performers. Despite the small degree of irregularity in hip movement amplitude for the brush brush stamps, the dancers were able to maintain high levels of hip symmetry for the other dance steps. High levels of symmetry close inter-limb coordination, and efficient muscle activation are characteristics that distinguish highly skilled dancers from novices. (1,21,22)

Each person is unique and finds his or her own biomechanical solutions to performing a skilled dance movement. (23) In this study, even highly skilled dancers did not show exactly the same performance for each movement sequence. There were small differences within and between limbs and across performers. The dance steps were generally executed using a similar approach by the three dancers, although individuals had subtle characteristics that were a signature of their individuality. The capacity for variability in performance rather than adherence to a stereotyped mode is a characteristic of highly accomplished and skilled dancers. (1,21,22) Such subtle differences in motor execution are best identified using the 3D biomechanics analysis, which allows for precise measurement of skilled performance. A limitation of this study was the small sample size and restriction of data collection to experienced and highly proficient performers. Future studies should evaluate the kinematics and kinetics of a range of tap dance steps in novices, community dancers, and elite performers. The current results do not generalize to older adults, children, or those with disabilities. In addition, all of the analyses were performed with the dancers wearing tap dancing shoes. Because the weight of the shoes and the sound produced by the metal taps influence performance, future trials could compare performance with and without footwear.

Conclusion

Tap dancing is an advanced motor skill that requires high levels of control of movement amplitude, timing, and inter-limb coordination to generate symmetrical and highly reproducible steps. The findings of this study emphasis the need for control of large amplitudes of movement, particularly of the ankles and knees, within short time periods. The hip appears to be used predominantly in a supporting or stabilizing role during these tap dancing steps. The high degree of side-to-side movement symmetry in these participants is likely to be a common feature of professional dancers; however, future studies could compare the performance of elite dancers with novices, across the age span, in health and disease.

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Priscila Rocha, M.Sc, Jodie McClelland, Ph.D., Tony Sparrow, Ph.D., and Meg E. Morris, Ph.D.

Priscila Rocha, M.Sc, School Allied Health, Department of Physiotherapy, College Science, Health & Engineering, La Trobe University, Bundoora, Australia; and CAPES Foundation, Ministry of Education of Brazil, Brasilia, Brazil. Jodie McClelland, Ph.D., andTony Sparrow, Ph.D., School Allied Health, Department of Physiotherapy, College Science, Health & Engineering, La Trobe University, Bundoora, Australia. Meg E. Morris, Ph.D., School Allied Health, Department of Physiotherapy, College Science, Health & Engineering, La Trobe University; and Healthscope, Northpark Private Hospital, Bundoora, Australia.

Correspondence: Priscila Rocha, M.Sc, School Allied Health, Department of Physiotherapy, La Trobe University, Bundoora, Melbourne, Victoria 3086, Australia; rocha.physio@yahoo.com.au.

Caption: Figure 1 Movement amplitude for different tap dance steps. GA: Gait Asymmetry Index.

Caption: Figure 2 Time to complete movement phases.
Table 1 Participant Characteristics

                            Experience
              Age      Dancing  Teaching
Participants  (years)  (years)  (years)   Injuries  Comorbidities

A             24       21        7        0         0
B             27       22       10        0         0
C             27       22       10        0         0

Participants  Medication

A             0
B             0
C             0

Table 2 Tap Dance Step Elements

Steps   Description

Nerve   Lift the moving leg forward without touching the floor.
Beats   Keep the standing knee flexed. The ball of the foot of the
        moving leg strikes the floor 12 times. Only the ball of the
        foot touches the floor. Each nerve beat movement phase is
        composed of dorsiflexion, progressing to plantar flexion,
        and finishes with dorsiflexion.
Brush   Keep the knee of the supporting leg flexed during the
Brush   whole sequence. Brush forward: Lift the moving leg backward
Stamps  while flexing the knee. Swing the moving leg forward,
        hitting only the ball of the foot on the floor. Continue the
        movement until the foot is off the floor and the moving
        knee is extended. Brush backward: Swing the moving leg
        backward, hitting only the ball of the foot on the floor.
        Continue the movement until the foot is off the floor and
        the knee of the moving leg is flexed. Stamp: Return the
        moving leg to the standing position, stamping a flat foot
        down on the floor with both knees bent. Each brush brush
        stamp movement phase is composed of one brush forward,
        a brush backward, and a stamp.
Heel    Flex the knee of the supporting leg. Stance: Lift the moving
Ball    leg forward, striking the back edge of the heel on the
Walks   floor while keeping the toe lifted (maximum dorsiflexion).
        Drop the ball of the foot to the floor transferring weight
        quickly, moving forward. Repeat the movements with the
        other leg. Swing: The moving leg is behind the support-
        ing leg. Lift the toe off the floor and move the leg forward
        until the heel strikes the floor, initiating the stance phase
        again. Each movement phase is composed of one stance
        and a swing component.

Steps   Instruction       Movement Phase

Nerve   The ball of the   Down phase: maximum peak
Beats   foot strikes the  of dorsiflexion to maximum
        floor 12 times,   peak of plantar flexion
        completing 11     Up phase: maximum peak of
        nerve beats with  plantar flexion to maximum
        each leg          peak of dorsiflexion
Brush   One movement
Brush   phase each leg    Brush forward: first peak angle
Stamps                    of knee flexion to first peak
                          angle of knee extension
                          Brush backward: first peak
                          angle of knee extension to
                          the second peak angle of knee
                          flexion
                          Stamp: second peak angle of
                          knee flexion to third peak angle
                          of knee flexion
Heel    Two movement      Stance: heel contact on the
Ball    phases each leg,  floor to heel lift and then toe
Walks   alternating the   off
        feet              Swing: toe off to heel contact

Table 3 Movement Amplitude for Different Tap Dancing Steps

                                      Brush Brush Stamps
                                      Brush          Brush
                         Nerve Beats  Forward        Backward

Hip range of  Right        0.4          12.0            6.1
motion        (degrees)   (0.2-2.5)     (6.3-17.5)     (5.2-13.4)
              Left         1.4          13.5           12.7
              (degrees)   (0.1-3.3)    (10.1-21.2)     (9.3-14.9)
              GA (%)      92            80             67
Knee range    Right        2.8         126.8          145.3
of motion     (degrees)   (1.3-15.3)  (109.2-129.8)  (126.9-147.4)
              Left         2.7         121.0          143.9
              (degrees)   (2.6-16.4)  (120.5-143.1)  (138.3-145.3)
              GA (%)      99            95             98
Ankle range   Right       45.0          29.2           33.2
of motion     (degrees)  (44.3-50.8)   (28.2-35.4)    (32.8-42.3)
              Left        50.7          31.3           34.6
              (degrees)  (41.2-57.1)   (29.6-34.4)    (28.8-38.8)
              GA (%)      97            97             94

               Brush Brush Stamps     Heel Ball Walks
               Stamp               Stance       Swing

Hip range of    11.2                29.4          68.7
motion          (6.4-19.8)         (26.4-34.3)   (52.4-69.9)
                14.3                36.7          68.6
               (13.3-15.8)         (20.7-40.5)   (61.4-71.5)
                86                  90            96
Knee range      90.2                47.6         125.6
of motion      (88.9-99.5)         (46.3-48.9)  (123.5-129.7)
                95.9                49.1         130.5
               (86.5-103.4)        (48.3-49.9)  (113.0-145.9)
                98                  96            99
Ankle range     14.7                39.5          62.2
of motion      (10.0-27.7)         (25.8-59.0)   (61.3-66.8)
                14.0                37.3          64.9
               (11.0-29.8)         (34.2-41.5)   (58.6-65.7)
                97                  97           100

Median (range); GA = Gait Asymmetry Index.

Table 4 Time to Complete Movement Phase of Nerve Beats

                                           Right             Left
                                   Downward     Upward       Downward
                                   Phase        Phase        Phase
                                   (seconds)    (seconds)    (seconds)

Time to complete movement phase     0.15         0.35         0.15
                                   (0.14-0.16)  (0.33-0.36)  (0.13-0.16)
Time variation across nerve beats   0.03         0.06         0.03
                                   (0.02-0.04)  (0.04-0.08)  (0.02-0.03)

                                   Left         GA (%)
                                   Upward
                                   Phase        Downward  Upward
                                   (seconds)    Phase     Phase

Time to complete movement phase     0.34        97        100
                                   (0.34-0.37)
Time variation across nerve beats   0.07        89         60
                                   (0.06-0.17)

Median (range); GA = Gait Asymmetry Index.

Table 5 Time to Complete Movement Phase of Brush Brush Stamps and Heel
Ball Walks

                                    Timing to Complete Movement Phase
                                    (seconds)
                                    Right             Left

Brush Brush Stamps  Brush Brush     1.33 (1.32-1.70)  1.39 (1.25-1.40)
                    Stamps
                    Brush Forward   0.47(0.37-0.51)   0.55 (0.35-0.57)
                    Brush Backward  0.43 (0.40-0.53)  0.37 (0.35-0.67)
                    Stamp           0.46 (0.42-0.76)  0.38 (0.33-0.47)
Heel Ball Walks     Heel Ball       4.03 (4.02-4.08)  4.04 (4.02-4.13)
                    Walks
                    Stance          1.27(1.26-1.33)   1.35(1.21-1.37)
                    Swing           0.77(0.68-0.81)   0.71 (0.66-0.79)

                    GA (%)

                    93
Brush Brush Stamps  92
                    98
                    72
                    99
Heel Ball Walks     98
                    96

Median (range); GA = Gait Asymmetry Index.
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Author:Rocha, Priscila; McClelland, Jodie; Sparrow, Tony; Morris, Meg E.
Publication:Journal of Dance Medicine & Science
Article Type:Report
Date:Jul 1, 2017
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