bioRxiv preprint doi: https://doi.org/10.1101/2020.08.30.274423; this version posted November 17, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 1 Original Article 2 3 Trunk Control during Gait: Walking with Wide and Narrow Step Widths Present Distinct 4 Challenges 5 6 Hai-Jung Steffi Shih, James Gordon, Kornelia Kulig 7 Division of Biokinesiology and Physical Therapy, University of Southern California, Los Angeles, 8 CA, USA 9 10 11 Corresponding Author: 12 Hai-Jung Steffi Shih 13 Address: 1540 E. Alcazar St, CHP 155, Los Angeles, CA, 90033 14 Telephone: +1 (323)442-2089 15 Fax: +1 (323)442-1515 16 Email: [email protected] 17 18 19 Keywords: Gait stability, Lateral stability, Trunk coordination, Muscle activation, Foot placement 20 Word count (intro-discussion): 3519 21 1 bioRxiv preprint doi: https://doi.org/10.1101/2020.08.30.274423; this version posted November 17, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 22 Abstract 23 The active control of the trunk plays an important role in frontal plane gait stability. We 24 characterized trunk control in response to different step widths using a novel feedback system 25 and examined the different effects of wide and narrow step widths as they each present unique 26 task demands. Twenty healthy young adults walked on a treadmill at 1.25 m/s at five prescribed 27 step widths: 0.33, 1.67, 1, 1.33, 1.67 times preferred step width. Motion capture was used to 28 record trunk kinematics, and surface electromyography was used to record longissimus muscle 29 activation bilaterally. Vector coding was used to analyze coordination between pelvis and thorax 30 segments of the trunk. Results showed that while center of mass only varied across step width 31 in the mediolateral direction, trunk kinematics in all three planes were affected by changes in 32 step width. Angular excursions of the trunk segments increased only with wider widths in the 33 transverse plane. Thorax-pelvis kinematic coordination was affected more by wider widths in 34 transverse plane and by narrower widths in the frontal plane. Peak longissimus activation and 35 bilateral co-activation increased as step widths became narrower. As a control task, walking 36 with varied step widths is not simply a continuum of adjustments from narrow to wide. Rather, 37 narrowing step width and widening step width from the preferred width represent distinct control 38 challenges that are managed in different ways. This study provides foundation for future 39 investigations on the trunk during gait in different populations. 40 41 2 bioRxiv preprint doi: https://doi.org/10.1101/2020.08.30.274423; this version posted November 17, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 42 Introduction 43 Maintaining stability is one of the primary goals in human locomotion. In contrast with 44 sagittal plane stability, which is largely achieved through passive mechanisms, frontal plane 45 stability during gait requires active control (Bauby and Kuo, 2000; Kuo and Donelan, 2010). 46 Frontal plane stability during walking is achieved through a combination of different strategies, 47 including adjusting mediolateral motion of the body’s center of mass (CoM) (Arvin et al., 2016a), 48 manipulating mediolateral foot placement (Bruijn and van Dieën, 2018) or external rotation of 49 the foot (Rebula et al., 2017). The trunk accounts for 48% of the body mass and is the largest 50 contributor to the CoM (MacKinnon and Winter, 1993; Prince et al., 1994). Therefore, how the 51 trunk is controlled will directly influence the CoM, and in some cases adjustment of the CoM 52 motion is preferred over modifying foot placement (Best et al., 2019). The trunk should not be 53 considered a rigid body, since structurally it involves multiple linked segments and degrees of 54 freedom that needs to be fine-tuned. When these degrees of freedom within the trunk are 55 constrained with an orthosis, an impact on CoM excursion and step width is observed (Arvin et 56 al., 2016b). 57 Although the relationship between CoM and lateral foot placement has been extensively 58 studied (Arvin et al., 2016a; Bruijn and van Dieën, 2018; Hurt et al., 2010; McAndrew Young 59 and Dingwell, 2012; Perry and Srinivasan, 2017; Stimpson et al., 2018; Wang and Srinivasan, 60 2014), the within-trunk control in response to step width has not been well-investigated. 61 Furthermore, the use of strings or tape to prescribe step width in previous studies required 62 participants to look down, affecting their trunk motion (Arvin et al., 2016a; Perry and Srinivasan, 63 2017). We therefore designed a study using visual feedback at eye-level to investigate trunk 64 control at different step widths. This study will provide insight into walking mechanics and a 65 basis for understanding and training trunk motion in different populations, such as persons with 66 spinal pathology, balance issues, or older adults. 3 bioRxiv preprint doi: https://doi.org/10.1101/2020.08.30.274423; this version posted November 17, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 67 During gait, the mass of the trunk is controlled in the frontal plane mainly by the spinal 68 musculature and the hip abductors (MacKinnon and Winter, 1993). Studies have primarily 69 focused on the contribution of hip abductors to modifying foot placement (Rankin et al., 2014) 70 and stability during stance phase (Kubinski et al., 2015). However, the role of paraspinal muscle 71 activation has not received much research attention. A study on typical walking demonstrated 72 that paraspinal muscles at the lumbar region were the most highly activated among the C7 to L4 73 paraspinal muscles (Prince et al., 1994), but how the muscles’ activation patterns are modulated 74 for different step widths is still unknown. 75 Walking with wider and narrower step widths places unique demands on the motor 76 system. The preferred step width is likely to be selected to minimize energy cost without 77 compromising stability (Donelan et al., 2001). Moving away from the preferred step width affects 78 the inverted pendulum-like motion of the CoM and influences energy demands (Kuo et al., 79 2005). A previous study demonstrated that walking with wider step widths requires greater 80 mechanical work to redirect the CoM, while walking with narrower step widths increases work 81 associated with swinging the leg laterally to avoid the stance limb (Donelan et al., 2001). 82 Walking with a narrower width also presents greater challenges to stability and increases 83 demand on active postural control as the base of support decreases (Donelan et al., 2004; 84 MacKinnon and Winter, 1993; Perry and Srinivasan, 2017). The current study will investigate 85 whether trunk control varies continuously across step widths or presents as separate motor 86 patterns in wide and narrow widths. If the latter is true, walking with wide and narrow widths can 87 be viewed as two distinct tasks. 88 The purpose of this study was to characterize changes in trunk control during different 89 prescribed step widths, and to compare the effects of wide and narrow step widths on trunk 90 kinematics and muscle activation. We hypothesized that narrower widths will present greater 4 bioRxiv preprint doi: https://doi.org/10.1101/2020.08.30.274423; this version posted November 17, 2020. The copyright holder for this preprint (which was not certified by peer review) is the author/funder, who has granted bioRxiv a license to display the preprint in perpetuity. It is made available under aCC-BY-NC-ND 4.0 International license. 91 stability demands and result in an increase in muscle co-activation, while wider widths will 92 present greater mechanical demands and influence trunk kinematics. 93 94 Methods 95 Participants 96 Twenty healthy young adults participated in this study (14 females, 6 males; 26.25 ± 97 3.31 years; 165.54 ± 9.93 cm; 61.39 ± 12.71 kg; BMI = 22.21 ± 2.84 kg/m2). Participants were 98 included if they were between 18 to 45 years old and excluded if they had a history of lower 99 extremity or spine pathology or surgery. Participants gave written informed consent that was 100 approved by the university’s institutional review board. 101 Instrumentation 102 Participants were instrumented with a lower extremity marker set and additional markers 103 on bilateral acromion, sternal notch, and T1. Kinematic data were recorded by a 11-camera 104 Qualisys motion capture system (Qualisys, Gothenburg, Sweden) at 125 Hz. Surface electrodes 105 were placed on bilateral longissimus (2 finger widths lateral from the spinous process of L3). 106 Electromyography (EMG) data were collected using Noraxon wireless EMG system (Noraxon 107 U.S.A, Scottsdale, AZ, USA) at 1500 Hz. A portable treadmill (ICON Health & Fitness, Logan, 108 UT, USA) was used for the walking trials.