Effects of load carriage and high-heeled shoes on spinal motor control

Abstract

Load carriage has been identified as a potential risk factor for low back pain. Previous studies of the effects of load carriage on spine were conducted under quasi-static conditions. In upright stance, it was shown that repositioning consistency and muscle activity of the lumbar spine were significantly reduced during load carriage. Thus, the effect of load carriage on spinal motor control and its possible association with the cause of low back pain have been our concern. On the other hand, high-heeled shoes are commonly used nowadays as a fashion for ladies. It was shown that back muscle activity was increased in wearing high-heeled shoes. It was thought that whether high-heeled shoes could be used to counteract the effect load carriage on the spine or not. The objectives of the current study are to investigate the effects of load carriage on spinal motor control under dynamic condition and to explore the possibility of using high-heeled shoes to counteract the effect of load carriage. The study was divided into two phases. In phase I, dynamic system theory was applied to study the movement coordination of the lumbar spine relative to pelvis under different weights of load carriage (0, 5, 10 and 15% of body weight (BW)). In phase II, the combined effects of load carriage (0, 5, 10 and 15% BW) and high-heeled shoes (0, 2 and 5cm heel height) were investigated. Eight male and eight female healthy volunteers participated in phase I and another twelve female healthy subjects participated in phase II of the study. In both phases I and II, functional reaching distances (FRD) of each subject under different conditions (i.e. with and without load carriage at different heel heights) were determined by a standard functional reaching test. Afterwards, the subject was asked to perform three consecutive and continuous movements which consisted of symmetric forward reaching to a midline target located at shoulder height and returning to the upright standing posture with feet at shoulder width, shoulders in 90° flexion and fully extended elbow. The target distance for each subject was standardized to 50% of individual's FRD. Reflective markers were affixed to the participants' spine, pelvis and thigh and their coordinates were captured by a motion analysis system (Vicon Nexus, Oxford Metrics, Oxford, UK) during the entire motion. Kinematics of the lumbar spine, pelvis and the thigh in sagittal plane were determined. The initial upright posture, the repositioning consistency of the upright posture and lumbar movement ratio were also determined for each condition. Based on the dynamical systems theory, two parameters, namely, mean absolute relative phase (MARP) and deviation phase (DP) were calculated for studying the movement coordination between lumbar spine and pelvis. The results were analyzed using repeated measure analysis of variance (RANOVA) with level of significance set at p=0.05. It was found that the initial upright posture was not significantly affected by load carriage. In comparison with the unloaded condition, repositioning consistency of lumbar spine was found to be significantly decreased during carrying load even the weight was only 5%BW. FRD was found to be significantly decreased with increased load carriage and heel height. Load carriage was also found to induce significant increase in lumbar movement ratio, MARP and DP. However, the effects of heel-height on these three parameters were opposite, high-heeled shoes were found to decrease lumbar movement ratio, MARP and DP. Besides spinal motor control was significantly affected under quasi-static situation, dynamic spinal motor control was also significantly affected by load carriage. Though there was no interaction between loading and high-heeled shoes, the combined effects may counteract the adverse effects of load carriage to some extent. Also other pragmatic approaches should be considered to elucidate the adverse effects of load carriage on the spine.