THE EFFECTS OF TRAINING ON SPINE-HIP RATIO IN DANCERS DURING A REACHING TASK Erica L. Dickinson, and James S. Thomas School of Physical Therapy, Ohio University, Athens, OH Introduction Coupling of the spine and hip joint, or spine-hip ratio, has been described as an important aspect of clinical examination in patients with low back pain. Numerous studies have analyzed the movement between the spine and hip during a forward bend in healthy subjects (Lee et al. 2002, Porter et al. 1997, McClure et al. 1997, Esola et al. 1996, Paquet et al. 1994). In these studies, agreement exists as to the gross pattern of movement between the spine and hips. Studies analyzing the spine-hip ratio during forward bending have concluded that spine movement is predominate in the initial phases of forward flexion, with the hip contributing more in the latter part. During the return to upright posture, hip movement predominates the initial movement, with the spine contributing more towards the latter part of extension (Lee et al. 2002, Porter et al. 1997, McClure et al. 1997, Esola et al. 1996, Paquet et al. 1994). While agreement exists as to the general pattern of movement, onset timing of the movement between the spine and hips is not clear. Several studies report simultaneous onset of the spine and hips during forward bending (Paquet et al. 1994, Esola et al. 1996, Lee et al. 2002, and Porter et al. 1997). However, only Lee et al specifically analyzed onset timing. In contrast to these findings, Nelson et al reported timing between the spine and hips to be simultaneous during spine flexion, but sequential during return to upright posture with hip movement preceding that of the spine. The previous studies are limited in that the task performed (i.e. a forward bend movement) to analyze spine-hip ratio was constrained, therefore limiting the available degrees of freedom to perform the task. However, in a less constrained reaching task, individuals can perform movements in a variety of patterns due to the inherent kinematic redundancy. Thomas et al utilized a full body reaching task to analyze spine-hip ratio in healthy men and women. Although significant gender differences were found, Thomas’ study did not analyze the timing of movement, nor did it analyze the return to upright posture from the target. A follow up study performed in this lab used the same full body reaching task to analyze spine-hip ratio. Unlike Thomas et al., the follow up study analyzed timing, as well as the return to upright posture from the target. This study revealed no gender differences, yet showed clear differences in onset of spine and hip joint motions in these reaching tasks. No investigations of coordination of the spine-hip ratio, have looked at the effects of dance training on the movement pattern. Mouchnino et al reported that highly trained dancers had different kinematic patterns than their untrained counterparts while performing a lateral leg raise. Thus, we hypothesize that dance training may also have an effect on the relationship between the spine and hip. Accordingly, the purpose of this study is to examine the effects of dance training on the coordination and movement pattern between the spine and hips during a reaching task. Dancers generally have increased flexibility, and are taught to keep their spines erect during training and performances for aesthetic purposes. As a result of increased hamstring flexibility and specific movement training, it is proposed that movement at the hips will precede that of the spine while reaching to and from the target, and that the average spine-hip ratio will be less than than those of normal subjects analyzed in previous studies. Methods Subjects existed of 9 women with a minimum of 5 years dance experience. Hamstring length was measured for each subject prior to data collection. Subjects performed full body reaching tasks in which time-series changes in orientation to the forearm, humerus, trunk, pelvis, thigh, and shank were recorded. Targets were positioned such that, in theory, a subject could reach them by flexing the hips 40º, 60º, and 80º with the shoulder flexed 90º and the elbow extended. Trials at each target position were performed at two different speeds (self-selected and fast paced). Methods, cont. During each trial, joint angles at the spine and hip were measured throughout the time series. Joint motion onsets, angular velocities, angular displacements, and spine/hip ratios were determined using custom algorithms created in Matlab™. Data Analysis Separate Paired t-tests were performed to compare the onset and peak velocities during the reach towards the target and return to upright posture for each target height and movement speed. Mixed-model ANOVAs were performed to determine effects of movement speed and target height on the excursions of the spine and hip. Results Figure 1 shows the mean onset and peak latencies between the spine and the hip for each target height and movement speed. At all three target heights and at both speeds, the spine was shown to precede the hip during onset of movement and at peak velocity, where as the hip preceded the spine during both onset and peak velocity during the return (figure 1). At the 60 degree target, spine movement preceded the hip by ms at a self-selected pace (t=- 4.72, p<0.05), and by ms at a fast pace (t=-4.43, p<0.05). The spine preceded hip movement at the 80 degree target by an average of ms at a self-selected pace (t=- 4.72, p<0.05), and an average of ms at a fast pace (t=-3.52, p<0.05). During the return to upright posture from the 80 degree target, hip movement preceded the spine by ms at a self-selected speed (t=3.60, p<0.05), and by ms during a fast speed (t=3.60, p<0.05). Changes in total movement of the hip and spine, as well as changes in the spine-hip ratio were calculated during reaches towards the targets. Figure 2 shows the mean changes in joint angle for both the spine and hips for each speed and at each target height. Within subject ANOVAs revealed that target height had a significant effect on changes in joint angle for both the spine and hip as was expected, but speed only had a significant effect on the change in hip angle (F=32.29, p<0.05). Figure 3 shows the mean spine-hip ratio at each target height and movement speed averaged from initial posture to target contact. Spine-hip ratios at the 40, 60, and 80 degree targets were 3.12, 2.16, and 2.17 at the self-selected pace, and 2.20, 1.69, and 1.69 at the fast speed respectively. Conclusions The dancers in this study had significant hamstring flexibility ( mean= 97.2 degrees straight leg raise) which, in theory, would allow greater range of motion at the hip during a reaching task. Classical dance training also emphasizes the importance of an erect spine throughout movement for aesthetic purposes. Due to increased available hamstring flexibility and classical movement training, it was hypothesized that movement onset would be initiated at the hips in dancers, both during the reach towards the target and the return. In contrast, results revealed that movement was initiated at the spine during the reach towards the target, and at the hip only during the return to upright posture. The timing pattern found in these dancers correlates with the results revealed in Thomas’ follow up study using untrained subjects. Therefore, it is concluded that dance training does not have an impact on the timing between the spine and hip during a full body reaching task. Gross movement patterns utilized by the dancers were also similar to those used by untrained subjects in previous studies analyzing spine-hip ratio (Thomas et al. 1998, Paquet et al. 1994, Esola et al. 1996, Lee et al. 2002, and Porter et al. 1997). If available flexibility contributed to motor planning then it would be expected that spine-hip ratios would be less in dancers secondary to their increased hamstring length. On the contrary, the average spine- hip ratios were greater than one at every target height and movement speed, which contradicts the hypothesis that spine-hip ratios would be less than those of normal subjects. These findings support the suggestion that motor planning is dependent primarily on the central nervous system, and that training and soft tissue constraints may not be a key factor in the organization of movement patterns. This research was supported by The National Institutes of Health Grant R01-HD to J.S. Thomas Figure 2. Mean changes from initial posture to target contact for self-selected and fast-paced trials in A) spine joint angle, B) hip joint angle. Figure 1. Mean motion latencies, both forward bend and return to upright for A) 40 degrees at a Comfortable pace, B) 40 degrees at a Fast pace, C) 60 degrees at a Comfortable pace, D) 60 degrees at a Fast pace, E) 80 degrees at a Comfortable pace, and F) 80 degrees at a Fast pace. Figure 4. The spine hip ratio was averaged from initial posture to target contact and the mean values for all subjects are presented for the different target heights and movement speeds.