Article

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: 1.87). 11/2003; 18(9):812-20. DOI: 10.1016/S0268-0033(03)00167-0
Source: PubMed

ABSTRACT 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.
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.
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.
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.
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.
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.
Spinal instability has been identified as a risk factor for low back injury during trunk loading. This study demonstrates that, in situations of sudden unloading, knowledge of the timing of the unloading may lead to anticipatory actions of postural muscles which actually decrease spinal stability, thereby increasing the risk of injury were an unexpected perturbation to occur.

0 Bookmarks
 · 
54 Views
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Objective: To investigate neck muscle activity and postural control in patients with whiplash-associated disorder compared with healthy controls. Design: Cross-sectional study with convenience sampling. Subjects: Ten females with whiplash-associated disorder (age 37.7 years (21-58), neck pain > 2 years and Neck Disability Index (NDI) > 10) and 10 healthy female controls (age 35.9 years (21-53), NDI < 6). Methods: Surface electromyography measured muscle activity of the anterior scalene, sternocleidomastoid, neck extensors and upper trapezius muscles, expressed as mean relative activity related to maximum voluntary electromyography (%MVE). On a force plate, 3 balance tasks (Romberg stance with open and closed eyes, 1-legged stance) and a perturbation task with sudden unloading, were performed. The total area, areas from slow and fast components, and range of displacements were calculated from decomposed centre of pressure anterior-posterior and medial-lateral signals. Results: During balance tasks with closed eyes and one-legged stance, the relative mean activity of all 4 muscles was significantly increased in whiplash-associated disorder compared with healthy controls. Postural sway was also significantly increased. Conclusion: Increased neck muscle activity and increased postural sway during simple balance tasks indicate disturbed sensory feedback patterns in people with whiplash-associated disorder, which may have negative consequences when performing daily activities.
    Journal of rehabilitation medicine: official journal of the UEMS European Board of Physical and Rehabilitation Medicine 03/2013; · 1.88 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: A number of occupational and leisure activities that involve pushing are performed in symmetrical or asymmetrical stance. The goal of this study was to investigate early postural adjustments (EPAs), anticipatory postural adjustments (APAs), and compensatory postural adjustments (CPAs) during pushing performed while standing. Ten healthy volunteers stood in symmetrical stance (with feet parallel) or in asymmetrical stance (staggered stance with one foot forward) and were instructed to use both hands to push forward the handle of a pendulum attached to the ceiling. Bilateral EMG activity of the trunk and leg muscles and the center of pressure (COP) displacements in the anterior-posterior (AP) and medial-lateral (ML) directions were recorded and analyzed during the EPAs, APAs, and CPAs. The EMG activity and the COP displacement were different between the symmetrical and asymmetrical stance conditions. The COP displacements in the ML direction were significantly larger in staggered stance than in symmetrical stance. In staggered stance, the EPAs and APAs in the thigh muscles of the backward leg were significantly larger, and the CPAs were smaller than in the forward leg. There was no difference in the EMG activity of the trunk muscles between the stance conditions. The study outcome confirmed the existence of the three components of postural control (EPAs, APAs, and CPAs) in pushing. Moreover, standing asymmetrically was associated with asymmetrical patterns of EMG activity in the lower extremities reflecting the stance-related postural control during pushing. The study outcome provides a basis for studying postural control during other daily activities involving pushing.
    Experimental Brain Research 06/2013; · 2.22 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: To investigate effects of symmetric and asymmetric stance and pushing movement on anticipatory and compensatory postural adjustments (APAs and CPAs). Ten healthy volunteers stood symmetrically (feet parallel) or asymmetrically (one foot forward and the other backward) and pushed a handle with both hands or right or left hand. Bilateral EMG activity of the trunk and leg muscles and center of pressure (COP) displacements in the anterior-posterior (AP) and medial-lateral (ML) directions were recorded and analyzed during the APAs and CPAs. Isolated asymmetry of stance was associated with larger muscle activity of the backward leg while isolated asymmetry of pushing movement induced larger trunk muscle activity on the contralateral side. A combined asymmetry of stance and pushing movement resulted in the increase or decrease of the thigh muscle activity and ML COP displacement depending on whether both asymmetries were induced on the same side of the body or on opposite sides. Both isolated and combined asymmetries affect APAs and CPAs in pushing. Using combined asymmetry of stance and arm movement might be beneficial in performing pushing activity. The outcome of the study provides a basis for studying postural control in individuals with unilateral impairment while performing daily tasks involving pushing.
    Clinical neurophysiology: official journal of the International Federation of Clinical Neurophysiology 10/2013; · 3.12 Impact Factor

Full-text

View
32 Downloads
Available from
May 21, 2014