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: 1.88). 11/2003; 18(9):812-20. DOI: 10.1016/S0268-0033(03)00167-0
Source: PubMed

ABSTRACT 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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Available from: Jim R Potvin, Jul 29, 2015
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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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    • "For example, it has been shown that loss of balance can be an important cause of back injury (Manning et al., 1984, 2000; Courtney et al., 2001; Davies et al., 2001). Suddenness (of exertion or movement)—caused, for example, by an unexpected load or a sudden release—has also drawn the attention of experimental studies (Andersen et al., 2001; Lavender et al., 1989; Commissaris and Toussaint, 1997); such events may cause an overreaction, producing an intense muscular contraction and a loss of balance (Brown et al., 2003; Chow et al., 2003, 2005). Nevertheless , it is difficult, for obvious ethical reasons, to study the impact of deviations experimentally. "
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