Mechanism of the maintenance of sagittal trunk posture in maximal sprint running

Original Mechanism of the maintenance of sagittal trunk posture in maximal sprint running SADO Natsuki 1,2), YOSHIOKA Shinsuke 1), FUKASHIRO Senshi 1) Mechanism of the maintenance of sagittal trunk posture in maximal sprint running Sado Natsuki 1,2), Yoshioka Shinsuke 1) and Fukashiro Senshi 1) 1) Department of Life Sciences, The University of Tokyo 2) Research Fellow of the Japan Society for the Promotion of Science Abstract The purpose of the current study was to investigate the mechanism of the maintenance of sagittal trunk posture in maximal sprint running. Twelve male sprinters performed 50-m sprint running with maximal effort. Kinematic and ground reaction force data were collected at approximately 40 m from the starting point. An inverse dynamics approach was applied to calculate joint forces and torques at the hip and lumbosacral joints. The pelvic anterior-posterior tilt components from hip extension torque and from the moment of hip joint force were calculated. The lumbosacral extension torque was greater than lumbosacral lateral flexion and torsional rotation torque during stance phase. The first and second peak values of the lumbosacral extension torque were 6.58 ± 1.58 Nm/kg and 3.11 ± 0.76 Nm/ kg, respectively. The mechanical load to flex the lumbar region was due to hip extensor, and the posture was not affected by the impact force via the supporting leg. The large lumbosacral extension torque was exerted to maintain the posture while resisting this mechanical load by hip extensor. These findings suggested that lumbar extensor were instantaneously required to exert maximal torque, and sprinters needed to strengthen the lumbar extensor to decrease the load on tissues around the spine. Keywords: lumbosacral joint, pelvis, pelvic tilt, inverse dynamics (Jpn J Biomechanics Sports Exerc 20 (2) : 56-64, 2016) 1. Introduction Lower back pain is one of the most frequently occurring problems in humans. The relative incidence of lower Sado Natsuki 1,2), Yoshioka Shinsuke 1) and Fukashiro Senshi 1) 1) The Department of Life Sciences, The University of Tokyo, Tokyo, Japan 2) Research Fellow of the Japan Society for the Promotion of Science Submitted for Publication : April 1, 2016 Accepted for Publication : July 28, 2016 back pain due to running injuries has been shown to be approximately 11 13% of all injuries sustained (Bennell & Crossley, 1996; Hootman et al., 2007). Decreasing the incidence of lower back pain would enable athletes to continuously participate in sport activities. Posture is a main factor that causes lower back pain. The repeated trunk hyperextension causes impingement of the vertebral facets, which results in spondylolysis (Nyska et al., 2000; Brukner & Khan, 2006). By contrast, a flexed posture of the trunk presses the intervertebral disks, which can result in injuries such as a slipped disk (Harrison et al., 2005; Brukner & Khan, 2006). In addition, it has been shown that the relationship between the load of ligaments around the spine and the displacement is nonlinear (Chazal, et al., 1985), and this nonlinearity of the load-displacement curve suggested that the mechanical load on ligaments would increase while the posture became not the nutral. Therefore, the collapsed posture in sprint running increases the load on tissues around the spine, and sprinters need to maintain their posture in the neutral position to prevent lower back pain. It was expected that the strength of the lumbar extensor would be particularly important to maintain the posture. Saunders et al. (2005) investigated the lumbar-pelvic movement while running at a velocity of 3.0 5.0 m/s and revealed that the lumbar spine flexed relative to pelvis in mid-stance followed by extension; moreover, this flexionextension angular displacement increased with the increase in running velocity. Thorstensson et al. (1982) found that the lumbar extensor muscles (erector spinae muscles) were active in the stance phase of running. However, the lumbar kinetics during sprint running have not been fully examined. The trunk posture is thought to be affected by hip joint kinetic behaviour. Previous studies have investigated hip joint kinetics and revealed that the hip extension torque 1

JJBSE 20(2)2016 contributed to the increase in running velocity by rotating the supporting leg backwards (Johnson & Buckley, 2001; Bezodis et al., 2008). In other words, a larger magnitude of hip extension torque was inevitably exerted during faster running. In addition, the hip extension torque tilted the pelvis backward. Schache et al. (2011) indicated that the peak hip extension torque was greater than the hip abduction and internal rotation torque during maximal sprint running. Based on these findings, it was hypothesised that the hip extensor would exert a large mechanical load to flex the trunk. However, the extent and mechanism of the effect of hip extensor on the trunk posture in maximal sprint running has not been fully examined. To understand how to maintain the trunk posture, it is essential to investigate the kinetics related to the posture in sprint running. The purpose of the current study was to investigate the mechanism of the maintenance of sagittal trunk posture in maximal sprint running with the a priori hypothesis that 1) hip extensor would place a load to flex the lumbar region and 2) sprinters would particularly need the strength of the lumbar extensor to maintain the posture during sprint running. 2. Methods 2.1 Experimental protocol The participants included 12 male sprinters [age, 22.7 ± 1.2 years; height, 1.75 ± 0.05 m; body mass, 64.5 ± 4.6 kg; personal best for 100 m, 10.43 11.17 s (10.89 ± 0.23 s)] who were not experiencing lower back pain. The participants received an explanation of the purpose of the experimental protocol and provided informed consent. The Human Research Ethics Committee at The University of Tokyo, Japan approved the study protocol. All participants wore close-fitting clothing and their own running shoes with spikes. A total of 47 retro-reflective markers, 20 mm in diameter, were secured to the trunk and limbs for motion capture for each participant (Fig. 1). After a suitable warm-up, participants performed 50 m sprints with maximal effort until two trials in which the foot made contact with the force platform, located at the 40 m point of the sprint, were obtained. The maximum number of trials was five. Adequate recovery time was provided between trials to avoid the effects of fatigue. Of these two trials, kinetic analysis was performed on the trial with the highest running Fig. 1 The locations of retro-reflective markers velocity. A 13-camera motion capture system (Motion Analysis Corporation, Santa Rosa, CA, USA) recorded the threedimensional coordinates of the position of the reflective markers (sampling rate, 200 Hz). Ground reaction force (GRF) was recorded using a force platform (Force Plate 9281E, Kistler, Switzerland), at a sampling rate of 1000 Hz, and synchronized with the motion data. The volume of motion capture was 1.0 4.5 2.0 m, with a 0.4 0.6 m force platform. The, and axes of the global coordinate system (GCS) defined medial-lateral, anterior-posterior and superior-inferior directions, respectively. Position coordinates of the markers were smoothed using a Butterworth, low-pass, digital filter. A residual analysis (Wells & Winter, 1980; Winter, 2009) was performed to identify the optimal cut-off frequency for each of the three-dimensional positions of each marker in each trial. A range of cut-offs between 11.0 and 20.0 Hz was used for the dataset. The GRF data were smoothed using a Butterworth low-pass digital filter with a cut-off frequency of 100 Hz. This study analysed the right foot stance phase from the right foot strike on the force platform to the right foot toe-off. The timing of the right foot strike and toe-off was identified from the onset of the GRF signal. 2.2 Data analysis The whole-body kinematic model used for analysis included 15 rigid segments (head-neck, thorax-lumbar, pelvis, upper arms, forearms, hands, thighs, shanks and feet) linked 2

Mechanism of the maintenance of sagittal trunk posture in maximal sprint running Fig. 2 The link segment model The inertial parameters for each segment and the position of centre of mass of the body were estimated using anthropometric data by Dumas et al. (2007a; 2007b). The wholebody velocity vector was calculated by differentiating the position vector of the centre of mass of the body. The running velocity was defined as the average of the -axis component of the velocity vector during the stance phase. As shown in Fig. 3, joint coordinate systems (JCS- ) were fixed at the lumbosacral and hip joints (Grood & Suntay, 1983; Wu et al., 2002). Rotation about defined flexion-extension at the lumbosacral and hip joints. Rotation about defined hip abductionadduction and lumbosacral lateral flexion. Rotation about defined internal-external rotation at the hip joint and lumbosacral torsional rotation. The angular velocity of the segment was calculated as follows: (1) Fig. 3 Definition of the (hip) joint coordinate system by 14 joints (Fig. 2). The degree of freedom of each joint was three. Right-handed local coordinate systems were defined for each segment (segment coordinate system: SCS) in each frame. All joint centre of rotation positions were estimated based on the location of reflective markers. Joint centres for the elbow, wrist, knee and ankle were localised at the midpoint of a line joining the lateral and medial markers, located at the joint line. The joint centre of the hip was estimated in each frame of motion using the functional method described by Harrington et al. (2007). The joint centres of the lower neck, lumbosacral and shoulder were estimated in each frame of motion using the functional method described by Reed, Manary and Schneider (1999). where is the transformation matrix from the SCS defined at the segment to GCS, and is the unit vector along each axis of the SCS, respectively. Newton Euler equations were used to calculate the threedimensional joint forces and torques at the ankle, knee, hip and lumbosacral joints (Winter, 2009), with joint torques transformed into the JCS. The pelvic anterior-posterior tilt components were calculated as the product between the unit vector along the pelvic medial- lateral axis and the vector of joint torques and moment of joint forces at both hip joints. 2.3 Statistical Analysis Pearson s correlation coefficient test was utilised to investigate the relationship between the profiles of the lumbosacral and hip extension torque. Statistical significance was set at p < 0.01. 3. Results The running velocity was 9.40 ± 0.41 m/s, with ranges of 8.76 9.92 m/s. 3

JJBSE 20(2)2016 Fig. 4 The ensemble averages of the pelvic cardan angle during stance phase Fig. 5 The ensemble averages of the lumbosacral joint torque during stance phase Fig. 4 shows the ensemble averages of the pelvic cardan angles. The angular displacements of the pelvic anteriorposterior angle, pelvic hike angle and pelvic rotation were 4.1 ± 1.3 degrees, 9.0 ± 2.3 degrees and 11.5 ± 3.2 degrees, respectively. The pelvic angular displacement was smaller than the pelvic hike and rotation angle. Fig. 5 shows the ensemble averages of the lumbosacral extension (Fig. 5a), lateral flexion (Fig. 5b) and torsional rotation torque (Fig. 5c). The lumbosacral extension torque developed for a short period immediately after foot strike and the middle of the stance phase. The first and second peak values of the lumbosacral extension torque were 6.58 ± 1.58 Nm/kg (Fig. 5a EF1) and 3.11 ± 0.76 Nm/kg (Fig. 5a EF2), respectively. The first and second peak values of the 4

Mechanism of the maintenance of sagittal trunk posture in maximal sprint running Fig. 6 The ensemble averages of the pelvic lateral-medial axis component of the hip extension-flexion torque and the moment of hip joint force during stance phase. Fig. 7 An example of the torque-torque plot displaying the correlations between the profiles of lumbosacral and hip extension-flexion torque Table 1 The correlation coefficients between the profiles of lumbosacral and hip extension-flexion torques for all participants Participants Correlation coefficient (r) p-value 1 0.71 <0.001 2 0.90 <0.001 3 0.84 <0.001 4 0.79 <0.001 5 0.92 <0.001 6 0.83 <0.001 7 0.88 <0.001 8 0.88 <0.001 9 0.77 <0.001 10 0.78 <0.001 11 0.91 <0.001 12 0.93 <0.001 Average 0.84 S.D. 0.07 lumbosacral lateral flexion torque were 2.66 ± 1.21 Nm/kg and 1.54 ± 0.87 Nm/kg, respectively. The peak value of the lumbosacral torsional rotation torque was 2.52 ± 1.71 Nm/kg. Fig. 6 shows the ensemble averages of the pelvic anteriorposterior tilt components of the moment of joint force and the hip extension torque exerted on the pelvis at the right (Fig. 6a) and left (Fig. 6b) hip, respectively. The component of the hip extension torque at the right hip was larger than any other component. Fig. 7 shows an example of the torque-torque plots displaying the correlation between the hip and lumbosacral extension torque. Table 1 shows the correlation coefficients between the profiles of lumbosacral and hip extensionflexion torques for all participants. The profiles of the hip 5

JJBSE 20(2)2016 and lumbosacral extension torque had a strong correlation for all subjects (Table 1). 4. Discussion The purpose of the current study was to investigate the mechanism of the maintenance of sagittal trunk posture in maximal sprint running. The kinematic and ground reaction force data at approximately 40 m from the starting point were collected from 12 participants. Three-dimensional kinematic and kinetic variables and the distribution of pelvic anterior-posterior tilt axis components during the stance phase were calculated. The principal findings of the current study were that the mechanical load to disturb the posture was due to the hip extensor, which was not due to the impact force via the supporting leg, and that the peak value of the lumbosacral extension torque was slightly higher than the maximal strength (5.30 Nm/kg) estimated from previous studies (Van der Burg et al. 2005; Blache & Monteil, 2014). The three-dimensional pelvic angular kinematics derived from the current study (Fig. 4) was consistent with that obtained in previous studies (Schache et al., 1999; Schache et al., 2002; MacWilliams et al., 2014). The pattern and amplitude of the pelvic medial-lateral axis component of the right hip extension torque (Fig. 6) was also consistent with the hip extension torque indicated by previous studies (Bezodis et al. 2008; Schache et al., 2011). These consistencies support the validity of the current study. This study found that the lumbosacral extension torque was larger than the lateral flexion and torsional rotation torque (Fig. 5). The first and second peak values of the lumbosacral extension torque were 6.58 ± 1.58 Nm/kg (Fig. 5a EF1) and 3.11 ± 0.76 Nm/kg (Fig. 5a EF2), respectively. The estimated maximal value of the lumbosacral extension torque reported in previous studies (Van der Burg et al. 2005; Blache & Monteil, 2014) was 5.30 Nm/kg (= 7000 N 0.053 m / 70 kg). The first peak value of the lumbosacral extension torque of the current study was slightly higher than this estimated value. Therefore, these results suggest that the lumbar extensor was instantaneously re- Fig. 8 Explanation of the maintenance of sagittal trunk posture. Note: 6.58 ± 1.58 Nm/kg, The peak value of the lumbosacral extension torque in the current study. 6

Mechanism of the maintenance of sagittal trunk posture in maximal sprint running quired to exert maximum strength. The current study found that the mechanical load to disturb the stability of posture was due to the hip extensor. The lumbosacral extension torque has an anatomical function to tilt the pelvis forward. Although the lumbosacral extension torque was greater than the lateral flexion and torsional rotation torque (Fig. 5), the amplitude of the angular displacement of pelvic anterior tilt was smaller than that of the pelvic hike and rotation (Fig. 4). These observations suggest that the lumbosacral extension torque was exerted to maintain the pelvic anterior tilt resisting the mechanical load around the pelvic anterior-posterior tilt axis. We investigated the components of the torque rotating the pelvis around the anterior-posterior tilt axis. The component from right (supporting leg) hip extension torque was the greatest, and that from the moment of right hip joint force was little (Fig. 6a), suggesting that the effect of the impact force via the supporting leg on sagittal trunk posture was minimal. The right hip extensor muscle exerted the torque causing the pelvic posterior tilt (Fig. 6a), which resulted in lumbar flexion. In order to maintain the sagittal trunk posture, the lumbosacral extension torque, having the function to tilt the pelvis forward, needs to cancel out the hip extension torque causing posterior pelvic tilt, as shown in Fig. 8. Accordingly, the current study investigated the relationship of the profiles between hip and lumbosacral extension torques, and the profiles of them were quite similar with each other (Fig. 5, Fig. 6, Fig. 7, and Table 1). Therefore, the lumbosacral and hip extension torque were exerted almost equally. This phenomenon suggested that the lumbosacral and hip extension torques on pelvis cancelled out each other, which resulted in the maintenance of sagittal trunk posture. It was revealed that the hip extensor contributed to running velocity by rotating the supporting leg backward (Johnson & Buckley, 2001; Bezodis et al., 2008), suggesting that the mechanical load to disturb the stability of the lumbar region due to the hip extensor muscles inevitably occurs during sprint running. The current study suggested that sprinters need to strengthen the maximal strength of lumbar extensor to prevent lower back pain. The first peak of the lumbosacral extension torque in sprint running was close to the maximal strength. The mechanical load that disturbs the stability of the posture was due to hip extension torque (Fig. 6), which was inevitably exerted during faster running (Johnson & Buckley, 2001; Bezodis et al., 2008). The collapse of the equality of the hip and lumbosacral extensors would result in instability of the running posture. The flexed trunk posture increased the risk of injuries such as a slipped disk (Harrison et al., 2005; Brukner & Khan, 2006) and ligament damage around the spine (Chazal et al., 1985). In addition, if lumbar extensor muscles cannot exert the torque enough to maintain the posture, the tissues besides these muscles, such as joint capsule and ligaments, compensate for this deficiency. Therefore, it is neccesary for sprinters to strengthen lumbar extensor muscles to prevent lower back pain during maximal sprint running. 5. Summary In summary, the purpose of the current study was to investigate the mechanism of the maintenance of sagittal trunk posture in maximal sprint running. This study calculated three-dimensional kinematic and kinetic variables from 12 participants during maximal sprint running. The principal results are shown below: 1. The lumbosacral extension torque was greater than the lumbosacral lateral flexion and torsional rotation torque during maximal sprint running. However, the angular displacement of the pelvic anterior-posterior tilt was lower than that of the pelvic hike and rotation. 2. The first and second peak values of the lumbosacral extension torque were 6.58 ± 1.58 Nm/kg and 3.11 ± 0.76 Nm/kg, respectively. 3. The hip extension torque was the greatest among the components around the pelvic anterior-posterior tilt axis, and the moment of hip joint force around the axis was almost zero. 4. The hip and lumbosacral extension torques showed a strong correlation (r = 0.84 ± 0.07). The current study found that the mechanical load to disturb the posture during sprint running was due to the hip extensor muscles and is not affected by the impact force via the supporting leg. In addition, maximal strength of the lumbar extensor is required to maintain the posture. Therefore, sprinters need to maximally strengthen lumbar extensor muscles to prevent the lower back pain. 7

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Mechanism of the maintenance of sagittal trunk posture in maximal sprint running Profile Sado Natsuki Doctor s course of the Department of Life Sciences, The University of Tokyo; Research Fellow of the Japan Society for the Promotion of Science (DC 1) He received his bachelor s degree in 2014 from University of Tsukuba and his master s degree in 2016 from The University of Tokyo. The major research interest is the contribution of the trunk to sprinting and jumping. 9