The responses of leg and trunk muscles to sudden unloading of the hands: Implications for balance and spine stability

Department of Kinesiology, School of Human Kinetics, University of Windsor, Windsor, ON, Canada N9B 3P4.
Clinical Biomechanics (Impact Factor: 1.97). 11/2003; 18(9):812-20. DOI: 10.1016/S0268-0033(03)00167-0
Source: PubMed


Objective. To examine the anticipatory and responsive actions of leg and trunk muscles, and their role in whole-body and spine control in situations of sudden unloading of the hands in the sagittal plane. Design. EMG and force plate measures were used to determine the baseline, anticipatory responses and post unloading responses of selected trunk and leg muscles under different conditions of unload timing knowledge. Background. Postural muscles have been observed to increase activation in anticipation of a known loading situation to decrease the overall effect of an impulsive load delivered to the spine. It is thought that this increased activation places the spine in a more stable state, thereby reducing the likelihood of injury. Comparisons have not been made previously of the responses of postural muscles to unloading conditions where the certainty of unload timing is varied. Methods. Eleven male subjects, holding a 6.8 kg load in the hands, were subjected to three different unloading conditions: (1) voluntary load drop; (2) known timing of load release; (3) unknown timing of load release. Anterior-posterior center of pressure data, as well as EMG activity on 8 right side muscles, were collected for 10 trials in each condition. Results. Anterior-posterior center of pressure responses were significantly different (P < 0.05) between each of the three conditions. Lumbar erector spinae and thoracic erector spinae significantly decreased anticipatory activity as knowledge of the unload timing increased. Five of the eight monitored muscles demonstrated significantly decreased response levels as knowledge of the timing of unloading increased. Conclusions. When an unload is self-triggered, preparatory adjustments can be made which reduce the overall postural perturbation to the body, and the spine in particular, while minimizing the responsive activity of trunk muscles.

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    • "The effect of spine muscle fatigue on reflex latency remains unclear with studies reporting increases (Hagbarth et al., 1995; Wilder et al., 1996) or no changes (Granata et al., 2004; Herrmann et al., 2006; Dupeyron et al., 2010) in latencies. Spine perturbations are often elicited experimentally by either suddenly loading (Gregory et al., 2008; Grondin and Potvin, 2009) or unloading the hands (Brown et al., 2003; Chow et al., 2004). When perturbed through the hands, upper limb muscles are able to modulate the perturbation before it is transmitted to the spine. "
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    ABSTRACT: Sudden loads, originating at either the hands or the feet, can cause injury to spine structures. As muscles are primarily responsible for stabilization following a perturbation, the effect of spine muscle fatigue in this context has been well investigated. However, the effect of fatigue of arm muscles, which can help control perturbations originating at the hands, on the spine is unknown. The purpose of this study was to determine if the magnitude of spine flexion or the pre-activation, reflex amplitude, and reflex latency of spine muscles were altered by elbow flexor fatigue during a sudden loading (6.8 kg) perturbation at the hands. Elbow flexor fatigue was induced by an isometric 30% maximal elbow flexion moment until failure. Results demonstrate that spine kinematics were not altered in the presence of elbow flexor fatigue. Small magnitude differences in trunk muscle pre- and peak activation indicate that the presence of elbow flexor fatigue does not necessitate substantially greater spine muscle action under the tested conditions. Despite fatigued elbow flexors, the arm muscles were sufficiently able to control the perturbation. Interestingly, 5/14 participants demonstrated altered reflex latencies in all observed muscles that lasted up to 10 minutes after the fatiguing task.
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    • "Compared to the subjects without LBP, this can be manifested as delayed actions of trunk muscles to sudden mechanical perturbation also termed postural reflex responses (PRRs) of the trunk (Boudreau et al., 2011; Cholewicki et al., 2005; Radebold et al., 2001). Trunk PRRs are most often evoked by perturbations applied to the trunk (Cholewicki et al., 2005; Radebold et al., 2000, 2001) or to the hands (Brown et al., 2003; Gregory et al., 2008; van der Burg et al., 2000). Furthermore most of the authors are assessing neurophysiological properties using electromyography while some studies are focusing on mechanical properties of PRRs (Hendershot et al., 2012; Reeves et al., 2014). "
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    • "The backward COP displacements in the AP direction were found in the symmetrical stance (PARA) and the staggered stance (RIGHT and LEFT) conditions. Similar anticipatory COP displacements in the backward direction were reported previously while describing body perturbations induced by self-initiated fast arm movements (Aruin and Latash 1995) and expected load release in standing (Brown et al. 2003; Shiratori and Aruin 2007) as well as when pointing in the sagittal plane while in sitting (Le Bozec and Bouisset 2004, 2009). Stance-related changes in the muscle activity in the legs also affected the COP displacement during the compensatory phase of postural control: the COP displacements were larger in staggered stance compared to parallel stance, and the difference was statistically significant only for the COP CPA1_AP . "
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