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Extreme Kinematics in Selected Hip Hop Dance Sequences

Article in Medical Problems of Performing Artists · September 2015 DOI: 10.21091/mppa.2015.3026

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Extreme Kinematics in Selected Hip Hop Dance Sequences

Shaw Bronner, PT, PhD, Sheyi Ojofeitimi, PT, DPT, and Helen Woo, MSc

Hip hop dance has many styles including breakdance (break- national dance phenomenon, has been relatively unrecog- ing), house, popping and locking, funk, streetdance, krump- nized by dance medicine researchers. [Note: Hip hop dance ing, Memphis jookin’, and voguing. These movements com- includes breaking (breakdance), popping, locking, house, bine the complexity of dance choreography with the challenges of gymnastics and acrobatic movements. Despite and other styles (Appendix 1).] Injury information on hip high injury rates in hip hop dance, particularly in breakdance, hop dance has been sparse, limited primarily to case stud- to date there are no published biomechanical studies in this ies that suggest injury risks may be much greater than population. The purpose of this study was to compare repre- those found in traditional dance.3–7 Recently, a retrospec- sentative hip hop steps found in breakdance (toprock and tive survey of 312 male and female hip hop dancers breaking) and house and provide descriptive statistics of the angular displacements that occurred in these sequences. Six reported an annual injury rate of 237%, similar to that of expert female hip hop dancers performed three choreo- gymnasts and greater than other dance populations.8 graphed dance sequences, top rock, breaking, and house, to Lower extremity injuries accounted for 55% of total standardized music-based tempos. Hip, knee, and ankle kine- injuries.8 Breakers had higher incidences of injuries com- matics were collected during sequences that were 18 to 30 sec pared to non-breakers (e.g., house, poppers/lockers). The long. Hip, knee, and ankle three-dimensional peak joint angles were compared in repeated measures ANOVAs with analysis revealed multiple injuries per dancer (e.g., for post hoc tests where appropriate (p<0.01). Peak angles of the breakers, there were a mean of 3.5 non-time-loss injuries/ breaking sequence, which included floorwork, exceeded the injured dancer and 2.8 time-loss injuries/injured dancer). other two sequences in the majority of planes and joints. Hip In the United States, sprains and strains account for hop maximal joint angles exceeded reported activities of daily 44% of reported musculoskeletal disorders (MSD).9 In 63% living and high injury sports such as gymnastics. Hip hop dancers work at weight-bearing joint end ranges where mus- of cases, the source of injury was attributed to worker posi- cles are at a functional disadvantage. These results may tion or motion, to events or exposures involving bending, explain why lower extremity injury rates are high in this pop- climbing, crawling, reaching, or twisting, and to overexer- ulation. Med Probl Perform Art 2015; 30(3):126–134. tion.10 Studies suggest that specific occupational exposures are associated with lower extremity musculoskeletal ncreasingly, dancers are recognized as elite athletes.1,2 pathology. Occupational groups at high risk of knee and Dance medicine epidemiology has ascertained injury hip complaints include carpenters, carpet and floor layers, I patterns unique to theatrical-concert dance forms farmers, miners, military conscripts, and elite athletes, such as ballet and modern dance. Hip hop dance, an inter- including dancers.11–17 Ergonomic analysis of work injury has cited repetitive motion, forceful exertion, and non- neutral or awkward body postures as critical risk factors Dr. Bronner is Director, Physical Therapy Services Alvin Ailey, and for MSD.18,19 Causation is very difficult to attribute due to ADAM Center, New York, NY; Dr. Ojofeitimi is Senior Therapist, Alvin Ailey, and ADAM Center, New York, NY; and Ms. Woo is the large number of variables that contribute to MSD. Manager of Performance Engineering, Reebok Int. Ltd., Canton, MA. However, in order to begin to determine risk factors in a given occupation, it is necessary to quantify and describe This study was supported in part by Nike, Inc. At the time of the study, what those motions and postures are. Ms. Woo was employed by Nike Inc, and she currently works for Reebok Int., Ltd. No individual directly benefited from the work conducted for Modern and ballet dancers i) exhibit increased physio- this research. logical flexibility compared to the general population, ii) are required to work at motion extremes, and iii) hypermo- Appendix 1, a glossary of hip hop dance styles and including video, bile dancers are at increased risk for injury.20–22 This sug- appears in the online version of this paper, available at: www.sciandmed.com/mppa (see Sept 2015 issue, vol 30, no 3). gests a possible link between dance movements and injury in ballet and modern dancers. The postures, range of Address correspondence to: Dr. Shaw Bronner, Physical Therapy Serv- motion (ROM), and velocity incurred during hip hop ices Alvin Ailey, 405 W. 55 St., New York, NY 10019, USA. Tel 347- dance are currently unmeasured. 688-9351, fax 718-841-7116. [email protected]. Basic research remains to be conducted to determine © 2015 Science & Medicine. www.sciandmed.com/mppa. the key determinants of difficulty and stress in critical hip

126 Medical Problems of Performing Artists DOWNLOADED FOR PERSONAL USE; NOT FOR DISTRIBUTION hop steps. Hip hop dance movements combine the com- TABLE 1. Hip Hop Sequences and Steps plexity of dance choreography, rhythms, syncopation, pos- Sequence Steps Length (s) tures, and forces which entail frequent changes in direc- tions and levels (e.g., jumps, twirls, spins, upside-down, Toprock Cross-step 30 right-side-up) with the challenges of gymnastics and acro- Kickstep out batic movements. In breakdance, downrock is more acro- Kickstep twist Turn and kneejerk batic and similar to gymnastics. It encompasses moves per- formed with hands, arms, or a part of the torso involving Breaking Kickstep cross 18 contact with the floor. Breakdance generally begins with a Kickstep twist toprock sequence, steps performed from a standing posi- Topswipe 6-step tion, relying upon a mixture of coordination, flexibility, Babyswipe rhythm, and style. House dance also involves steps per- Tornado formed in a standing position. It combines many dance elements such as the Lindy and bebop, African, Latin House Heel-toe kick 27 (+ sidestep, salsa trot, instep) salsa, Brazilian capoeira, jazz, tap, and modern. Both Sweepstep back toprock and house can involve jumping, stomping, and Skating squatting movements. Selection of common breakdance Hook (e.g., downrock, toprock) and house steps allowed us to Farmer focus on two styles of dance (breakdance and house) that Heel-toe were represented in a previous report on patterns of injury in hip hop dance.8 The three sequences included steps that required the dancer to perform “repetitive motion, forceful • Breaking began with several toprock steps, transitioned exertion, and non-neutral body postures,” all critical risk with a topswipe (Appendix Fig. A4), and then included factors for MSD. A better understanding of the biome- downrock steps such as the classic 6-step (Appendix Fig. chanical requirements of critical hip hop steps may assist A3): www.sciandmed.com/mppa/video/30.3.126 (panel B) clinicians involved in prevention, training, and rehabilita- or http://youtu.be/X5kUEjQBxhk. tion of hip hop dancers. • House included upright complex stepping patterns charac- The purpose of this study was to compare three dimen- teristic of the style (Appendix Fig. A6): sional (3D) angular displacements that occur in representa- www.sciandmed.com/mppa/video/30.3.126 (panel C) tive hip hop dance sequences. We hypothesized that the or http://youtu.be/4tf5LjlsXyI.* breakdance downrock sequence (breaking) would demon- strate greater angular displacements than the other Each dance sequence included multiple steps that were sequences. divided into groupings for analysis. Dancers rehearsed the METHODS sequences (each 18 to 30 sec in length) as a group on two occasions prior to data collection. Subjects conducted their Participants and Experimental Protocol own warm up and rehearsed each movement sequence with the choreographed DVD prior to data collection. Six expert female hip hop dancers, experienced in break- ing and house dancing (mean age 31.0 ± 4.3 yrs, range 25- Instrumentation 35; height 1.63 ± 0.05 m; mass 63.07 ± 4.70 kg; 22.0 ± 6.0 yrs of dance experience; 12.5 ± 6.6 yrs hip hop experience), Reflective markers were placed according to the Cleveland participated in this study. Subjects had no musculoskeletal Clinic full-body marker set (Vicon BodyLanguage, injuries in the previous 3 months that required them to Cumnor-Cleveland Model, Vicon, Oxford, UK, 1998) stop dancing. All subjects gave informed written consent with thigh and shank arrays.23,24 Each subject was instru- in correspondence to the guidelines of the Long Island mented in a Velcro suit to minimize marker loss during University Internal Review Board. data collection and wore their preferred dancing sneakers. In order to study dance steps that were as close as pos- A static standing neutral trial of each subject was collected sible to how they are usually performed, we asked these to define the anatomical coordinate system and for knee experts to choreograph sequences that contained common and ankle joint center calculation. Markers on the medial steps found in the various hip hop styles to selected music femoral condyle, medial malleolus, medial humeral with 120–130 beats/min. Conditions included two break- condyle, and heel were removed before the dynamic trials. dance sequences (toprock and breaking) and a house Each sequence was collected four times, while subjects sequence (house) (Table 1): danced the choreography to the selected music tracks. • Toprock included only standing step (Appendix Fig. 1A and 2): *Video associated with this report is available at: www.sciandmed.com/mppa/video/30.3.126 (panel A) www.sciandmed.com/mppa/video/30.3.126 or at the YouTube links or http://youtu.be/jGazI70I9T0. indicated above.

September 2015 127 DOWNLOADED FOR PERSONAL USE; NOT FOR DISTRIBUTION Kinematic data were collected using a 6-camera motion (GLM) were used for each joint (hip, knee, ankle) for the capture system (Vicon Nexus, Oxford Metrics Ltd, dependent variables (3-D plane peak angle) in SPSS (SPSS v. Oxford, UK) at a sampling rate of 120 Hz. 21, IBM Corp, Armonk, NY). In the second analysis, we examined steps within a sequence, again using GLM Data Reduction repeated measures. For all analyses, Mauchly’s test was examined for violations of sphericity. In the case of signifi- Coordinate data were low-pass-filtered using a Woltring cance, the Huynh-Feldt correction was applied to the quintic spline (MSE 10 mm). The kinematic data were cal- degrees of freedom (DOF) and F-value if the epsilon (ε) value culated using a joint coordinate system. Three-dimensional was ≥0.75, and the Greenhouse-Geisser correction was used (3-D) joint angular displacements were calculated using if ε was <0.75. In these cases, the ε and corrected values (e.g., Vicon Bodybuilder software (Vicon, Oxford Metrics Ltd, DOF and F-values) are reported. Pairwise comparisons were Oxford, UK). Pelvic movement, with respect to the global conducted where there was a significant main effect. Given reference in x, y, and z axes, was defined as pelvic obliquity the number of dependent variables analyzed, a conservative (side bending around the x-axis), pelvic tilt (anterior-poste- level of significance was set at α = 0.01 for all tests. rior motion around the y-axis), and pelvic transverse rota- tion (rotation around the longitudinal z-axis).25 Joint RESULTS motion of the extremities was locally referenced to the body. Hip, knee, and ankle angular displacements were cal- Sequence Angular Displacements culated with respect to a local reference with Euler angle conventions using the following sequence of rotations: Sagittal Plane: The regularity of sagittal plane angular dis- flexion and extension about the y-axis of the proximal seg- placement at the right and left hip, knee, and ankle during ment; adduction and abduction about a floating x-axis; fol- performance of the house sequence by a representative sub- lowed by internal and external rotation about the z-axis of ject is seen in Figure 1. There were differences between the distal segment.26 By convention, hip and knee exten- sequences in sagittal plane peak angles at the hip [ε = 0.867, sion and ankle plantar flexion in the sagittal plane, hip and F(1.735,81.522) = 79.414, p<0.001]. Breaking peak hip flexion knee abduction and ankle eversion in the frontal plane, (102 ± 8º) exceeded that in house (77 ± 16º) and toprock (73 ± and hip and knee external rotation and ankle abduction in 12º) (pairwise comparisons p<0.001) (Fig. 2A). Similarly, the transverse plane were denoted as negative. there were differences between sequences in knee flexion [ε For each limb, hip, knee, and ankle 3-D angular dis- = 0.568, F(1.137,53.435) = 16.555, p<0.001] (Fig. 2B). There placements were determined. Each of the 4 to 6 grouped was greater peak knee flexion in breaking (113 ± 10º) com- steps within a sequence had a broad range of angular dis- pared to house (98 ± 20º) (pairwise comparisons p<0.001). placement values. Because we were interested in how much There were also differences between sequences in ankle DF hip hop dance forms push the joints to extremes, our [ε = 0.634, F(1.268,59.598) = 12.763, p<0.001], but not for PF. analysis focused on ROM greater than that found in walk- Breaking DF (27 ± 6º) was greater than toprock (24 ± 3º) (pair- ing and running:27,28 wise comparisons p≤0.005) (Fig. 2C). In summary, breaking • In the sagittal plane: hip flexion ≥50º, knee flexion ≥70º, peak angles exceeded those of the other sequences at all ankle dorsiflexion (DF) ≥10º, plantar flexion (PF) ≤–10º; joints, with the exception of ankle PF. • In the frontal plane: hip adduction (ADD) ≥20º and abduc- Frontal Plane: Differences were found between sequences tion (ABD) ≤–20º, knee ADD ≥5º and ABD ≤–5º, ankle in frontal plane peak angles at the hip in ADD and ABD inversion (INV) ≥5º and eversion (EV) ≤–5º; and [F(2,94) = 94.481, p<0.001 and F(2,94) = 109.781, p<0.001, • In the transverse plane: hip internal rotation (IR) ≥15º and respectively]. Peak hip ADD in breaking (37 ± 6º) exceeded external rotation (ER) ≤–15º, knee IR ≥5º and ER ≤–5º, that in toprock (24 ± 4º) and house (25 ± 4º) (pairwise compar- ankle ADD ≥10º and ABD ≤–10º. isons p<0.001) (Fig. 3A). Peak hip ABD in breaking (–38 ± 6º) We extracted data within these parameters and calcu- exceeded that in toprock (–28 ± 6º) and house (–25 ± 3º) (pair- lated mean ±SD peak angles for i) sequences and ii) steps wise comparisons p<0.001), and toprock peak hip ABD within a sequence, as the dependent variables (Table 1). exceeded that of house (p=0.001). There were also differences Steps in toprock included cross-step, kickstep out, kickstep between sequences in knee ADD and ABD [F(2,194) = twist, and turn and kneejerk. Breaking steps included kickstep 15.853, p<0.001 and ε = 0.777, F(1.553,72.996) = 18.034, cross, kickstep twist, topswipe, 6-step, babyswipe, and tornado. p<0.001, respectively] (Fig. 3B). Toprock knee ADD (12 ± 2º) House steps included heel-toe kick (included within this was exceeded both house (11 ± 2º) and breaking (10 ± 2º) (pairwise sidestep, salsa trot, instep), sweepstep back, skating, hook, comparisons p<0.001) (Fig. 3B). House (–12 ± 1º) and toprock (– farmer, and heel-toe. 12 ± 2º) knee ABD exceeded that of breaking (–10 ± 2º) (pair- wise comparisons p<0.001). Ankle INV and EV peak angles Statistical Analyses did not exceed our ROM criteria (INV ≥5º and EV ≤–5º). Transverse Plane: There were differences between In the first analysis for the sequence factor (toprock, breaking, sequences in hip IR [ε = 0.763, F(1.526,71.734) = 32.406, house), separate repeated measures in general linear models p<0.001] but not hip ER. Breaking (32 ± 9º) hip IR exceeded

128 Medical Problems of Performing Artists DOWNLOADED FOR PERSONAL USE; NOT FOR DISTRIBUTION FIGURE 1. Angular displacement of a representative subject performing the house sequence: A, left and right hip; B, left and right knee; C, left and right ankle. both house (23 ± 6º) and toprock (28 ± 4º) (pairwise compar- flexion reached values of 111 to 125º (cross-step, kickstep out, isons p≤0.001), and toprock exceeded that of house (pairwise kickstep twist, turn and kneejerk) (Fig. 2B). comparisons p≤0.001) (Fig. 4A). There were no differences There were differences between steps in ankle DF [ε = in knee IR but there were differences in knee ER [ε = 0.556, 0.444, F(1.331,14.639) = 44.504, p<0.001]. Turn and kneejerk F(1.113,52.310) = 11.815, p=0.001] (Fig. 4B). Toprock knee ER peak ankle DF (26 ± 2º) exceeded cross-step (23 ± 2º) and kick- (–27 ± 2º) exceeded that of house (–25 ± 3º) and breaking (–23 step out (23 ± 2º) (pairwise comparisons p<0.001), and kickstep ± 6º) (pairwise comparisons p<0.001). Finally, there were dif- twist (25 ± 3º) exceeded cross-step and kickstep out (pairwise ferences in ankle ADD [ε = 0.629, F(1.259,59.153) = 41.841, comparisons p<0.001). There were also differences between p<0.001] and ankle ABD [ε = 0.566, F(1.132,153.198) = steps in ankle PF [ε = 0.851, F(2.554,28.094) = 38.058, 49.341, p<0.001]. Toprock ankle ADD (25 ± 3º) exceeded that p<0.001]. Turn and kneejerk peak ankle PF (–27 ± 3º) exceeded of house (22 ± 3º) and breaking (21 ± 5º) (pairwise compar- cross-step (–25 ± 2º), kickstep out (–23 ± 3º), and kickstep twist (– isons p<0.001) (Fig. 4C). Both toprock (–28 ± 1º) and house (– 24 ± 4º) (pairwise comparisons p<0.001) (Fig. 2C). 26 ± 2º) exceeded ankle ABD in breaking (–24 ± 4º) (pairwise Frontal Plane: Differences between steps in the frontal comparisons p<0.001). plane were found in peak hip ADD [ε = 0.545, F(1.635, 17.980) = 10.250, p=0.002], as well as hip ABD [F(3,33) = Step Angular Displacements 8.575, p<0.001] (Fig. 3A). Pairwise comparisons of hip ADD steps were not significant. Turn and kneejerk peak hip ABD The peak angles of steps within each of the three (–33 ± 5º) exceeded kickstep out (–26 ± 7º) (pairwise compar- sequences are reported below. isons p=0.010). There were no differences between steps in knee ADD or ABD. Ankle INV and EV peak angles did Top Rock not exceed our ROM criteria (INV ≥5º and EV ≤–5º). Sagittal Plane: Toprock was broken into four steps for fur- Transverse Plane: In the transverse plane, there were no ther analysis. There were differences between Toprock steps differences between toprock steps in hip and knee IR or hip in hip flexion [ε = 0.383, F(1.150,12.650) = 987.151, p<0.001). ER. There were differences in knee ER [F(3,33) = 11.237, Turn and kneejerk hip flexion (92 ± 4º) (Appendix Fig. A2) p<0.001). Cross-step knee ER (–25 ± 3º) exceeded kickstep out exceeded that of cross-step (70 ± 5º), kickstep out (65 ± 3º), and ER (–17 ± 4º) (pairwise comparisons p=0.002) (Fig. 4B). There kickstep twist (63 ± 2º) (pairwise comparisons p<0.001) (Fig. were no differences between steps in ankle ADD or ABD. 2A, Appendix Fig. A1). Differences between steps were also Breaking found in knee flexion [ε = 0.481, F(1.445,15.868) = 192.979, p<0.001). Turn and kneejerk peak knee flexion (115 ± 6º) Sagittal Plane: Breaking was broken into six steps for fur- exceeded cross-step (102 ± 6º), kickstep out (112 ± 5º), and kick- ther analysis. There were differences between breaking step twist (104 ± 4º) (pairwise comparisons p<0.001); and kick- steps in hip flexion [ε = 0.350, F(1.752,19.277) = 150.813, step out exceeded cross-step and kickstep twist (pairwise com- p<0.001]. Babyswipe peak hip flexion (107 ± 10º) (Appendix parisons p<0.001). In some individuals, mean peak knee Fig. A5), 6-step (101 ± 6º) (Appendix Fig. A3), topswipe (102

September 2015 129 DOWNLOADED FOR PERSONAL USE; NOT FOR DISTRIBUTION FIGURE 2. Maximal and mean (SE) sequence and step comparisons of sagittal plane angular displacements: A, hip flexion; B, knee flexion; C, ankle dorsiflexion and plantar flexion.

± 8º) (Appendix Fig. A4), and tornado (99 ± 7º) exceeded Transverse Plane: There were no differences between kickstep cross (57 ± 5º) and kickstep twist (73 ± 8º) (all pairwise breaking steps in hip IR or ER or ankle ADD or ABD. Dif- comparisons p<0.001) (Fig. 2A). ferences were found in knee ER [F5,55) = 7.215, p<0.001] There were also differences between steps in knee flex- but not IR (Fig. 4B). Babyswipe knee ER (–14 ± 4º) was less ion [F(5,55) = 66.795, p<0.001]. Topswipe peak knee flexion than kickstep cross (–23 ± 3º) and tornado (–20 ± 4º) (pairwise (120 ± 9º) exceeded kickstep cross (111 ± 11º), kickstep twist comparisons p≤0.005). (105 ± 7º), 6-step (113 ± 7º), babyswipe (114 ± 10º), and tor- nado (105 ± 8º) (all pairwise comparisons p<0.001); while 6- House step, babyswipe, and kickstep cross exceeded kickstep twist and tornado (pairwise comparisons p≤0.009) (Fig. 2B). Sagittal Plane: House was broken into six steps for further Finally, steps differed in ankle peak DF [ε = 0.285, analysis. There were differences between house steps in hip F(1.423,15.648) = 13.083, p=0.001] as well as ankle PF [ε = flexion [ε = 0.337, F(1.685,18.537) = 259.195, p<0.001] (Fig. 0.282, F(1.409,15.496) = 17.234, p<0.001] (Fig. 2C). Topswipe 2A). Farmer hip peak flexion (100 ± 5º) (Appendix Fig. A6a) (28 ± 4º) and 6-step (30 ± 3º) ankle DF exceeded kickstep cross exceeded that of heel-toe kick (71 ± 6º), sweepstep back (67 ± 5º) (24 ± 2º), kickstep twist (25 ± 2º), and babyswipe (25 ± 4º) (Appendix Fig. A6c), skating (72 ± 4º), hook (77 ± 10º) (Appen- (pairwise comparisons p≤0.010). In PF, 6-step peak ankle dix Fig. A6b), and heel-toe (71 ± 6º) (Appendix Fig. A6d) (pair- PF (–28 ± 4º) exceeded kickstep cross (–27 ± 3º), kickstep twist wise comparisons p<0.001), and hook exceeded heel-toe kick (–24 ± 4º), and tornado (–19 ± 7º) (all pairwise comparisons and sweepstep back (pairwise comparisons p≤0.001). p<0.001); while tornado PF (–28 ± 5º) exceeded kickstep twist There were differences between steps in knee flexion [ε and tornado (pairwise comparisons p≤0.003). = 0.358, F(1.788,19.637) = 670.520, p<0.001] (Fig. 2B). Farmer Frontal Plane: There were differences between breaking (118 ± 6º) and hook (116 ± 7º) peak knee flexion exceeded steps in hip ADD and ABD [F(5,55) = 9.232, p<0.001 and heel-toe kick (79 ± 2º), sweepstep back (78 ± 7º), skating (84 ± F(5,55) = 6.279, p<0.001]. Babyswipe (38 ± 4º) and 6-step (41 ± 3º), and heel-toe (80 ± 4º) (pairwise comparisons p<0.001). 5º) hip ADD exceeded that of kickstep cross (30 ± 4º) and Individual subject mean peak knee flexion reached 126º kickstep twist (27 ± 6º) (pairwise comparisons p≤0.007) (Fig. (hook and farmer). 3A). Hip ABD in 6-step (–44 ± 5º) exceeded kickstep twist (– There also were differences between steps in ankle DF 31 ± 6º) (pairwise comparisons p<0.001). Steps differed in [ε= 0.521, F(2.605,28.651) = 21.597, p<0.001] (Fig. 2C) as well knee ABD [ε = 0.793, F(3.958,43.541) = 5.163, p=0.002] but as in ankle PF [ε = 0.464, F(2.318,25.498) = 88.102, p<0.001]. not ADD. Pairwise comparisons of knee ABD steps were Farmer (26 ± 3º) and sweepstep back (26 ± 3º) ankle DF not significant. Ankle INV and EV peak angles did not exceeded heel-toe kick (26 ± 1º), hook (24 ± 3º), and heel-toe (23 exceed our ROM criteria (INV ≥5º and EV ≤–5º). ± 3º) (pairwise comparisons p≤0.008). Skating PF (–28 ± 3º)

130 Medical Problems of Performing Artists DOWNLOADED FOR PERSONAL USE; NOT FOR DISTRIBUTION FIGURE 3. Maximal and mean (SE) sequence and step comparisons of frontal plane angular displacements: A, hip adduction- abduction (ADD-ABD); B, knee ADD-ABD. exceeded heel-toe kick (–22 ± 2º), sweepstep back (–25 ± 2º), extremes. The majority of the extreme movements in the hook (–23 ± 2º), farmer (–26 ± 2º), and heel-toe (–24 ± 3º) (pair- breaking sequence were weight-bearing in deep crouched wise comparisons p<0.001). squats (e.g., topswipe, babyswipe, 6-step, tornado). In toprock Frontal Plane: In house, there were no differences and house, on the other hand, most weight-bearing move- between steps in the frontal plane at the hip or knee. ments were relatively vertical (standing), and extreme Ankle INV and EV peak angles did not exceed our ROM movements were gestural kicking-type movements, such as criteria (INV ≥5º and EV ≤–5º). kickstep twist in toprock, and hook and farmer in house. Transverse Plane: In house, there were differences Previous analyses of dance and gymnastics kinematics between steps in both hip IR and ER [ε = 0.511, have demonstrated large angular displacements of the F(2.557,28.132) = 9.781, p<0.001 and F(5,55) = 6.780, lower extremity.20,29 We compiled a list of gymnastics, p<0.001, respectively]. Heel-toe kick (25 ± 5º), sweepstep back ballet, hip hop, and ADL movements (e.g., squatting, (27 ± 2º), and farmer (26 ± 3º) peak hip IR exceeded hook (16 kneeling, sitting cross-legged), reported peak angles, and ± 1º) (pairwise comparisons p≤0.001). Hook (–36 ± 4º) and combined these with lower extremity active range of farmer (–35 ± 5º) peak hip ER were greater than skating (–29 motion (AROM) measurements in healthy young adults ± 3º) (pairwise comparisons p≤0.002) (Fig. 4A). There were (Fig. 5).30–32 The amount of AROM that is used in func- no differences between steps in either knee IR or ER or tional movements (e.g., ADL, ballet, and gymnastics) helps ankle ADD or ABD. to interpret the potential stresses of functional hip hop movements. In the sagittal plane, squatting,33,34 probably DISCUSSION the most extreme of non-athletic activities, required the greatest amount of hip flexion, approximately 130º (107% To our knowledge, this is the first report of the kinematics of AROM), with breaking babyswipe and gymnastics back of selected hip hop movements. Breaking involved greater salto requiring 127º (104% of AROM) and 70º (58%), respec- peak angles than those of the other sequences. In order to tively. No hip extension was measured in any of the hip better understand these findings, we review our results in hop sequences. Squatting, kneeling,33,34 and ballet grand the context of other sports and activities of daily living plié (a weight-bearing turned-out “squat”)20 movements (ADL). Many of the landings in the breaking “power” moves involved 157º (117% of AROM), 155º (116%), and 145º (e.g., topswipe and babyswipe which twist the body into the (108%) of knee flexion, while gymnastics front salto and air and land on the hands followed by the feet in a squat) breaking babyswipe movements required 128º (96%) and resemble those seen in gymnastics. Therefore, where avail- 134º (99%), respectively. It is important to bear in mind able, we compared our findings to those of female gymnasts. that squatting and kneeling are static postures, ballet The sagittal plane lower extremity peak angles found in grand plié is a slow sustained movement, while the gym- the three hip hop sequences suggest that hip hop dancers, nastics salto and breaking babyswipes are dynamic high- like many dancers and athletes, frequently work at joint velocity movements with potentially greater sustained

September 2015 131 DOWNLOADED FOR PERSONAL USE; NOT FOR DISTRIBUTION FIGURE 4. Maximal and mean (SE) sequence and step comparisons of transverse plane angular displacements: A hip internal rotation—external rotation (IR-ER); B, knee IR-ER; C, ankle adduction-abduction (ADD-ABD). landing forces. All hip and knee flexion values greatly did not measure muscle activity or study the effect of exceeded those reported in walking, running, and stair- fatigue on these movements, one theory explaining a factor climbing.35 Ankle DF ranged from 38º (112% of AROM), in the development of exercise-related joint problems and 40º (118%), and 28º (82%) in squatting, kneeling, and gym- osteoarthritis is the muscle dysfunction hypothesis.39 This nastics front salto compared to 36º DF (109% of AROM) in hypothesis is based on research findings that muscle fatigue breaking babyswipe. Greatest PF values are found in the increases the impact forces crossing the joint. With the ballet pointed foot (in gestural postures) at –45º (100% of muscles already at functional disadvantage at joint physio- AROM),20 with gymnastics front salto requiring –26º (58%) logical end range, force absorption may be compromised or and breaking babyswipe –36º (80%). muscle fatigue may occur more rapidly. In addition to over- In the frontal plane, hip ABD ranged from –27º (60% of use injuries, hip hop dancers reported landing, twisting, AROM) in kneeling, to –34º (75%) in sitting cross-legged,33 and slipping as the most common mechanisms of injury.8 while breaking 6-step was –35º (78% of AROM) (Fig. 6B). Hip One of the primary causes of injuries was cited as fatigue. ER was reported to be –37º (82% of AROM) in sitting cross- Hip hop battles may involve repeated bouts of rapid danc- legged, while breaking 6-step was –48º (107% of AROM). At ing with repetitive steps. With increased muscle fatigue, the knee, breaking 6-step ADD (15º and 150% of AROM) impact forces crossing the joint can cause joint trauma or and ER (–27º and 67% of AROM) motion exceeded those there may be diminished control in sustaining ergonomi- reported for kneeling (ADD 11º and ER –12º), squatting cally “safe” landings. Landing and twisting most frequently (ADD 8º and ER –10º), sitting cross-legged (ADD 14º and occur during performance of breaking power moves. ER –15º),33,36 and spin turns (ADD <1º and ER –14º)37,38 in In summary, many hip hop movements are working at the frontal and transverse planes. Ankle ABD (–30º and joint ranges where muscles are at a functional disadvantage 100% of AROM) in breaking 6-step exceeded those reported to protect joints.40 As a consequence, these movements for spin turns (–28º and 93% of AROM).37 place an increased amount of stress on the joints. Although Rapid and repeated motions in non-neutral postures are velocity is not reported in this study, hip hop dancers flip risk factors for MSD.18,19 In particular, kneeling and squat- and land in these weight-bearing positions at high velocities ting are linked causally to lower extremity disorders.18 and without the benefit of a cushioning mat used in gym- Within a 20- to 30-sec sequence of multiple steps, dancers nastics. Therefore, the ROM reported for sedentary pos- hit these joint excursions repeatedly. For example in the tures is less stressful than hitting a position forcefully and breaking sequence, all steps within the 6-step are performed rapidly as in hip hop steps. Customarily, breaking is per- in a crouched position, the tornado is similarly crouched formed in gymnasiums or on the street. Breakers are wear- throughout, and the babyswipe starts and ends in a crouch, ing sneakers, which may serve a cushioning function similar resulting in 25 steps or more within the sequence. While we to the gymnastic mat with respect to the lower extremity.

132 Medical Problems of Performing Artists DOWNLOADED FOR PERSONAL USE; NOT FOR DISTRIBUTION FIGURE 5. Functional joint ROM requirements: A, Sagittal plane—references: AROM,1–3 squatting,4,5 kneeling,4,5 ballet demi plié, grand plié, battement front (non-weight-bearing gesture limb kick to the front),6 and female gymnastics front and back salto.7 B, Frontal and transverse planes—references: AROM,1,8–11 squatting,4,5,12 kneeling,4,5 sit cross,4,5 spin turn.13,14 Abbreviations: ADL, activities of daily living; Gym, gymnastics; AROM, active range of motion; Sit cross leg, sitting cross-legged.

Limitations and Future Research healthcare practitioner in understanding the rehabilitation needs and functional requirements of hip hop dancers fol- This study was limited to female hip hop dancers. Female lowing musculoskeletal injury. With annual injury rates athletes have demonstrated greater hip and ankle joint comparable to those reported for gymnastics, further analy- ROMs and joint angular velocities compared to males sis of hip, knee, and ankle joint kinetics may shed addi- during landing activities in ways that predispose them to tional light on the stresses encountered during these move- injury.41 Some of the movements required subjects to be in a ments and help us to better understand the high injury crouched position with occasional marker occlusion. Inter- rates reported. Further investigations into training regimes polation was performed to minimize lost kinematic data. that may reduce injuries are warranted. Hip hop dancers Future work analyzing the hip hop kinetics data may are a neglected population, sustaining high rates of injuries provide new insights into the forces sustained throughout that may result from these biomechanical stressors. They the sequences. Analyses of peak velocities and accelera- represent a culture with unmet training and medical needs. tions related to peak forces during impact activities will provide insight into the types of accelerations that may Author Contributions: Bronner, Ojofeitimi, and Woo designed the study; Bronner and Ojofeitimi participated in data collection and analysis; contribute to injury risk. Additionally, kinetic analysis of Bronner, Ojofeitimi, and Woo participated in data interpretation. joint moments will help us to understand peak forces sus- The authors thank Laura Becica, DPT, who assisted in data collec- tained in hip hop dance. tion, Yu-Chien Cheng who assisted in data processing, and the dancers who participated in this study. Our gratitude goes to Robin Dunn, who CONCLUSION assisted in data collection and mentored us in learning everything we could about hip hop dance and its culture, and to Ereina “Honey Rock- well” Valencia and Ephrat “Bounce” Asherie for allowing us to use their This is the first report of the kinematics of a “street” dance video clips. form that appeals to today’s youth. To realistically capture the dance form in an ecologically valid manner, dancers REFERENCES performed sequences of multiple steps. The selected steps required many changes in levels and forces. The excessive 1. Bronner S, Ojofeitimi S, Rose D. Injuries in a modern dance company: effect of comprehensive management on injury inci- joint angles measured in this study may provide a prelimi- dence and time loss. Am J Sports Med. 2003;31(3):365–73. Pub nary explanation for the high lower-extremity injury rates Med PMID: 12750128. reported by these dancers. These results will assist the 2. Solomon R, Solomon J, Micheli LJ, McGray E. The “cost” of

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Joint laxity and the benign joint hypermobility syndrome in student and profes- Appendix 1, a glossary of hip hop dance styles, appears in the online sional ballet dancers. J Rheumatol. 2004;31(1):173–8. version of this paper, available at www.sciandmed.com/mppa (see 22. Briggs J, McCormack M, Hakim A, Grahame R. Injury and joint Sept 2015 issue, vol 30, no 3). hypermobility syndrome in ballet dancers—a 5-year follow-up. Rheumatology. 2009;48:1613–20. Video associated with this report is available at: 23. Manal K, McClay I, Stanhope S, et al. Comparison of surface www.sciandmed.com/mppa/video/30.3.126.

134 Medical Problems of Performing Artists DOWNLOADED FOR PERSONAL USE; NOT FOR DISTRIBUTION APPENDIX 1: GLOSSARY OF HIP HOP DANCE TERMINOLOGY

BBoy (break-boy) or BGirl (break-girl) is a term for a person who is devoted to hip hop culture, and more specifically to dancers who participate in breaking. These stylized movements are commonly known as Breaking or Break dancing and dancers are often called Breakers.

BREAKING

Breaking/breakdancing is a generally unstructured and a. Go-downs/Drop: this is the act of transitioning from highly improvisational street dance style which borrows a standing toprock to the floor to start floorwork. Go- variety of forms, motions, and maneuvers found in martial downs include knee drop, sweep drop, reverse sweep arts (especially capoeira), gymnastics, and popular funk drop, topswipe, front swipes, back swipe, dips and dance. corkscrews. The smoother the drop, the better. There are four foundation styles of breaking. The first is toprock, a term referring to the upright dancing that break- 2. Downrock/footwork, which is more acrobatic and simi- ers do when they enter a circle. The second style, downrock lar to gymnastics, encompasses moves performed with or footwork, refers to dancing performed on the floor. The hands, arms, or a part of the torso involving contact third style, known as freeze, refers to poses that add punc- with the floor. The foundational move of downrock is tuation to certain beats and shock the crowd. The fourth the 6-step (Fig. A3): element, known as power moves, refers to extreme gymnas- • www.sciandmed.com/mppa/video/30.3.126 B or tic moves that involve spinning or fast rotations. Steps • http://youtu.be/X5kUEjQBxhk include 1990, 2000, UFO, Halo, jackhammer. and combinations often transition from it into more ath- 1. Toprock refers to any string of steps performed from a letic moves known as power moves. Other steps include standing position, relying upon a mixture of coordina- helicopters, kick outs, CCs, babyswipe, and tornado. tion, flexibility, rhythm, and style. It is the opening dis- a. 6-step: The 6-step is foundational to breaking, not play of style, warming-up for transitions into more only because it is the first footwork sequence break- acrobatic maneuvers of downrock. Examples of steps ers often learn, but also because it remains the move include kickstep, cross-step, kickstep twist (Fig. A1), around which many sets are structured. The basic 6- Indian step, turn and kneejerk (Fig. A2): step resembles walking in a circle on the ground, • www.sciandmed.com/mppa/video/30.3.126 A or using both the feet and hands. The move sets up the • http://youtu.be/jGazI70I9T0 direction of rotation and builds momentum while Uprock is a competitively oriented type of toprock con- imparting body control. The breaker stays low and sisting of foot shuffles, spins, turns, and creative move- in contact with the ground, which places him in ments that mimic combat. It was developed from the position for performing other moves. Conversely, “jinga,” a footwork pattern used in capoeira, which was any move which ends on the ground can be transi- taught to youth in African-American community cen- tioned smoothly back into the 6-step. Variants ters as a method of developing self-esteem and character. include the 2-step up to 12-step.

FIGURE A1. Kickstep twist. FIGURE A2. Turn and kneejerk.

September 2015 134a DOWNLOADED FOR PERSONAL USE; NOT FOR DISTRIBUTION FIGURE A3. 6-step.

b. CCs/Cicis (abbreviation of “crazy commandos”): A Because power moves are physically demanding, break- footwork sequence that twists from side to side on ers use them as a display of upper body strength and one foot and then the other. It comes from a step stamina. They include windmills, swipes, flares, and called the Russian tap. headspins. Many moves are borrowed from gymnastics.

3. Freeze involves halting all body motion, often in an a. Topswipe (Fig. A4) is a power move in which the interesting or balance-intensive position. It is implied breaker leans back, whips his or her arms to one side that the position is hit and held from motion as if freez- to touch the ground, and his or her legs follow ing in motion, or into ice. Freezes include baby, chair, closely behind, twisting 360° to land on the ground elbow Nike, headstand, and hollowback. once again. Baby Swipe (Fig. A5) is often launched 4. Power moves refer to moves that require momentum and from the 6-step, employing a twist of the hips from a physical power to execute. They rely more on upper squatting position. body strength to dance, using the hands to do moves.

HOUSE

House dance was developed in the clubs at the end of the One of the primary elements in house dancing is a tech- disco era. House dance combines many dance elements nique called jacking that came from Chicago and involves such as the Lindy and bebop, African, Latin salsa, Brazil- moving the torso forward and backward in a rippling ian capoeira, jazz, tap, and modern. Contrary to popular motion, as if a wave were passing through it. This move- belief, house dance is not a descendant of hip hop dance ment is repeated and sped up to match the beat of a song. and did not come out of hip hop culture. However, many Jacking is the foundation that initiates more complex hip hop dancers have crossed into house dance and movements and footwork. All footwork in house dancing brought in their sequences of steps. is said to initiate from the way the jack moves the center of

FIGURE A4. Topswipe.

134b Medical Problems of Performing Artists DOWNLOADED FOR PERSONAL USE; NOT FOR DISTRIBUTION FIGURE A5. Babyswipe. gravity through space. Other than footwork, jacking, and • www.sciandmed.com/mppa/video/30.3.126 C or lofting, house dance has grown to include other related • http://youtu.be/4tf5LjlsXyI styles such as vogue, waacking, and hustle. cross-stepping, kicks, taps, slides, and spins to create a com- In house dancing there is an emphasis on the subtle plete system addressing body mechanics and postures and rhythms and riffs of the music, and the footwork follows the syncopation in the modern day house music. Other them closely. Stomping (farmer) (Fig. A6a), hook (Fig. A6b), steps include tip-tap-toe, loose legs, hurdle, pas de bourree skating, and sweepstep back (Fig. A6c), shuffling, and jack- (with a twist). ing are interlaced with classic footwork such as heel toe (Fig. A6d):

FIGURE A6. Farmer (a), hook (b), sweepstep back (c), heel-toe (d).

September 2015 134c DOWNLOADED FOR PERSONAL USE; NOT FOR DISTRIBUTION

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