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 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.

  • [Show abstract] [Hide abstract]
    ABSTRACT: Understanding joint stiffness and stability is beneficial for assessing injury risk. The purpose of this study was to examine joint rotational stiffness for individual muscles contributing to elbow joint stability. Fifteen male participants maintained combinations of three body orientations (standing, supine, sitting) and three hand pre-loads (no load, solid tube, fluid filled tube) while a device imposed a sudden elbow extension. Elbow angle and activity from nine muscles were inputs to a biomechanical model to determine relative contributions to elbow joint rotational stiffness, reported as percent of total stiffness. A body orientation x pre-load interaction was evident for most muscles (p < 0.001). Brachioradialis had the largest change in contribution while standing (no load, 18.5%; solid, 23.8%; fluid, 26.3%). Across trials, the greatest contributions were brachialis (30.4 ± 1.9%) and brachioradialis (21.7 ± 2.2%). Contributions from the forearm muscles and triceps were 5.5 ± 0.6% and 9.2 ± 1.9%, respectively. Contributions increased at time points closer to the perturbation (baseline to anticipatory), indicating increased neuromuscular response to resist rotation. This study quantified muscle contributions that resist elbow perturbations, found that forearm muscles contribute marginally and showed that orientation and pre-load should be considered when evaluating elbow joint stiffness and safety.
    Journal of applied biomechanics 12/2013; · 0.90 Impact Factor
  • [Show abstract] [Hide abstract]
    ABSTRACT: It has been reported that altered neuromuscular control of the trunk is associated with lower back pain. In this context reflex delays of the trunk muscles have often been assessed but the reliability of the tests has not been well established. The aim of this study was to test the reliability of measuring reflex delays of the trunk muscles after two types of postural perturbations. 24 Healthy subjects participated in the intra-session study and 13 of them repeated the test protocol within 1-3 weeks, to determine inter-session reliability. Postural reflex delays to unexpected loading and unloading of the arms were assessed in a standing unrestrained position. Each subject performed 40 trials of each test in order to evaluate muscle responses of 5 trunk muscles using surface electromyography. Overall reliability increased with higher number of the averaged trials. Good intra-session (ICC3,1>0.75) and moderate (ICC3,1>0.60) inter-session reliability were reached in most of the monitored trunk muscles. Within the performed number of trials we did not observe any significant systematic intra- or inter-session bias effect. Averaging a higher number of consecutive trials would be recommended in future research and clinical practice.
    Journal of Biomechanics 06/2014; · 2.50 Impact Factor
  • Source
    [Show abstract] [Hide abstract]
    ABSTRACT: Effective and efficient movement is fundamental to sports performance. Movement binds together all of the skills of a game into a coherent flow. Sport -specific training program can induce peculiar neuromechanical adaptations that are commonly considered as signs of aquisition and/or improvement of a specific movement skill. An athlete’s anthropometric and physical characteristics may represent important prerequisites for successful participation in any given sport. Twenty one collegiate volleyball players,(age19.85 +/- 0.83 years; height 181.67 +/- 12.03 cm; weight 72.62 +/- 12.99 kg; training experience 6.76 +/- 2.21 years), were recruited for this study. The purpose of this study was to examine the significance of the impact of body height and weight on the specific motor abilities of volleyball players. The following tests were performed: Block jump, Spike jump, Standing broad jump, Jelka test,T– test, 93639 m test, Obstacle course backwards, Arm plate tapping and Dash 20 m. Regression analysis from package SPSS 15.0 was used for data processing. The results showed that body height and weight had a statistically significant impact on the expression of specific motor abilities of volleyball players.Indeed, it can be assumed that an athlete’s anthropometric characteristics can in some way influence his/her level of performance, at the same time helping to determine a suitable physique for a certain sport.
    Exercise and Quality of Life, Novi Sad; 01/2013


Available from
May 21, 2014