THE INFLUENCE OF THE BACK FUNCTIONAL LINE ON LOWER EXTREMITY FRONTAL PLANE KINEMATICS AND KINEMATIC PREDICTORS OF LOADING DURING RUNNING

Abstract

Running injuries have been linked to poor lower extremity dynamic alignment, increased whole body and joint loading, and insufficient modulation of stiffness throughout stance phase. Upper body muscle activity and movement have a relationship to lower body dynamics; however, the literature has largely neglected their role during running. To date, biomechanical gait analysis has primarily focused on lower extremity mechanics and muscle activation patterns with no studies investigating the role of functional muscle synergies on stability and loading during running. Therefore, the primary objective of this project is to determine the role of the Back Functional Line (BFL), via measure of latissimus dorsi (LD), gluteus maximus (GM), and vastus lateralis (VL) muscle activity, during running and to determine their influence on lower extremity kinematics and kinematic predictors of loading that are linked to running-related injuries (RRI). We used conditions of arm swing constraint to manipulate the action of the LD and investigate the response in GM and VL muscles. Our main variables of interest include: 1) BFL muscle activity, specifically mean and peak amplitude, onset, and co-activation of the LD and GM 2) frontal plane lower extremity kinematics, and 3) kinematic predictors of kinetics, specifically foot inclination angle at initial contact and vertical COM displacement. Twenty healthy recreational runners (10 M; 10 F) participated in this study. Male runners tended to be slighter older with a higher weekly running mileage and longer running history. All participants were between the ages of 18 and 55 years old and consistently ran at least once per week. Participants ran under three arm conditions - free arm swing, unilateral arm swing constraint, and bilateral arm swing constraint. During the running trials, surface EMG and lower extremity kinematics were collected over the gait cycle. We operationally defined the primary BFL as the muscle synergy composed of the non-dominant upper extremity (i.e., constrained side during unilateral condition) LD muscle, the dominant GM muscle, and the dominant VL muscle. The secondary BFL was defined as the dominant upper extremity (i.e., unconstrained during unilateral condition) LD muscle, the non-dominant GM muscle, and the non-dominant VL muscle. Primary and secondary BFL muscle synergy activity were analyzed during two specific phases of gait - the pre-activation (PA) phase and the loading response (LR) phase. In support of the hypothesis, the primary BFL LD mean amplitude decreased during both the PA and LR phases of gait. GM and VL muscle mean amplitude demonstrated a varied response. During the PA phase, both the GM and VL muscles increased during the unilateral condition and decreased during the bilateral condition. During LR phase, GM and VL muscles increased during both arm swing constraint conditions. The highest increase in amplitude was seen during the unilateral condition. Peak amplitudes for each muscle did not change dramatically across conditions for either the PA or LR phases of gait. Secondary BFL LD and GM mean and peak amplitude increased during both the PA and LR phases of gait, with changes during the LR phase reaching significance for both muscles. Secondary BFL VL also increased in mean and peak amplitude during the bilateral constraint condition. GM and VL mean and peak muscle amplitude were significantly correlated during the LR phase, but not for the PA phase. This indicates that the lower extremity muscles of the BFL (GM and VL) may not be preparing for impact similarly but are adjusting muscle activity in a similar fashion as the lower limb is loaded. The increase in muscle amplitude for secondary BFL muscles, particularly during the LR phase of gait, may have resulted from a difference between lower limb strength or lower extremity single leg stability. Onset of muscle activity during loading response did not significantly differ across conditions for the LD, GM, or VL muscles, however, analysis of co-activation demonstrated that LD and GM were in-phase throughout the gait cycle. This suggests that this portion of the BFL may be acting together to stabilize the lumbopelvic-hip complex (LPHC) during running. LD and GM appeared to be co-activated throughout the gait cycle regardless of arm swing variation. Instability, either from asymmetrical movement patterns or poor single leg stability may contribute to the activation of the BFL muscle synergy. GM increased during the unilateral arm swing constraint during both phase and for both BFL synergies, indicating that asymmetrical movement patterns may induce a potential instability or an unstable state requiring the need for greater stability around the LPHC. Knee frontal plane kinematics changed significantly across conditions. Knee abduction angle showed the greatest increase during the unilateral arm swing constraint condition suggesting that asymmetrical movement patterns effect lower extremity mechanics more so than symmetrical patterns (i.e., bilateral arm swing restriction or free arm swing). Hip adduction and contralateral pelvic drop angles did not differ significantly across conditions. Our study did not find a significant relationship between BFL muscle activity and knee abduction angles. Participants demonstrated larger knee abduction angles on their non-dominant limb at midstance. The corresponding (secondary) BFL LD and GM demonstrated a significant increase during the LR phase. This may indicate that BFL muscle activity is engaged when the need for lower limb stability is greater, either due to poor single leg dynamic control or abnormal frontal plane mechanics. Kinematic predictors of joint and whole-body loading differed across conditions. Vertical COM displacement was significantly decreased during the bilateral arm swing constraint condition. Foot inclination angle at initial contact did not significantly change with arm swing constraint. Differences were found between right and left lower extremity foot strikes (i.e., foot inclination angle) across all conditions; the non-dominant limb demonstrated greater plantarflexion during initial contact. Knee flexion angle at initial contact and peak knee flexion during stance did not demonstrate a significant change. Muscle activity was not significantly correlated to kinematic predictors. Spatiotemporal measures altered with arm swing suppression. Stride length decreased and step rate increased significantly. Taken together, these results suggest that runners alter spatiotemporal measures more so than sagittal plane kinematics when adjusting to arm swing suppression. The role of the BFL muscle synergy during running remains unclear. Asymmetrical movement patterns and arm swing restriction appear to influence BFL muscle activity and lower extremity kinematics. Single leg stability, particularly during the LR phase, may alter BFL muscle activity due to the need for increased stabilization of the loaded limb and the LPHC. Future research is needed to determine how these variables impact BFL muscle activation and whether injured runners respond differently to arm swing constraint during running.