Activité EMG, cinématique et forces au sol lors de flexions-extensions du rachis à différentes vitesses

In Canada, the most widespread form of disability in adult population of working-age is associated with pain in general. Such problems impede productivity and deteriorate quality of life in affected population. For low back disorders, diagnosis is often complex or unknown: making treatment procedure difficult. Asymptomatic individuals can be identified from those with low back pain using patterns of muscular activity during the flexion-relaxation phenomenon (FRP). However, factors modulating FRP need yet to be clarified. Therefore, the objective of this study is set to measure trunk/pelvic kinematics, muscular recruitment, and ground reaction forces in unconstrained flexion-extension tasks at different speeds. Speed of movement in the sagittal plane has been suggested to be associated with back musculoskeletal disorders. Moreover, the effect of a manipulation called neuro-proprioceptive facilitation (FNP), which aims to gain lumbar flexibility, on results is investigated.

Measurements were carried out on fourteen healthy young male subjects performing free flexion-extension tasks at three different velocities with distinct time periods (6 sec, 3 sec and minimal at approximately 1 sec carried out randomly). During these tasks, the electromyography activity of five pairs of superficial back and abdominal muscles, trunk and pelvis rotations, and ground reaction forces were simultaneously measured. For the sake of normalization of muscular activities, maximum voluntary contractions (MVC) were also measured. One way ANOVAs with α=0.05 were performed followed by Tukey's post hoc multiple comparisons with p<0.05 as significance level. </p>

Abdominal activities showed a very different pattern among subjects. Maximum activity of dorsal muscles, however, occurred at significant larger angles according to statistics during flexion phase when compared with the extension phase. At the time maximum activity of dorsal muscles happened, contribution of the lumbar spine to generate trunk rotation was larger than that of the pelvis. The FRP was mainly observed in muscles at the lumbar level for the case of moderate and slow velocities: presenting a very significant drop of the back muscular activities by about 84%. Trunk angle in which muscles were re-activated following the FRP was smaller (closer to the upright posture) than the angle in which silence initially begins during the flexion phase.

The upper body inertia increased at maximum speed, reaching 1.4 times of the value for slow and moderate speeds during a flexion-extension task. Accordingly, a significant increase in trunk mobility and mean maximum EMG signals of IC (ρ<0.002 between slow and maximum speeds ρ<0.008 between the moderate and maximum speeds) was observed. Activity of RA increased more significantly between slow/moderate and maximum speeds (ρ<0.002) than that of EO (ρ<0.005). Peak activity of back muscles in flexion and extension phases were likely to happen at relatively smaller trunk angle in moderate velocity when compared to the slow one (no statistical significance). As for the IC, FRP happened at smaller pelvis rotation for the tasks performed with moderate velocity compared to those performed at slow velocity (ρ<0.04). In extension phase, the FRP silence ended at smaller pelvis angle for faster movement (IC: ρ<0.05 and ER: ρ<0 03).

As the velocity of movement increased, the pelvis angle at which FRP silence started (for lumbar muscles during flexion phase) and finished (for thoracic and lumbar muscles for extension phase) tended to diminish; resulting in larger range of motion with FRP. A modification of the lumbar-pelvic rhythm would occur according to the speed, mainly because of the pelvis dynamic. The FNP manipulation did not cause any significant effect on the measured variables.

In the light of these findings, it appears that the speed of movement influences the trunk muscular activities, kinematics and kinetics of the spine during free flexion-extension tasks. A very fast execution of this type of task would not be, thus, suitable since certain protective dorsal muscular behaviors are inhibited. A slow to moderate speed, while being representative of trunk natural dynamics, is recommended during trunk flexion-extension tasks.

In future studies, different groups of subjects (symptomatic, females or having extreme morphologies like thin or obese persons) can be recruited to study the effect of different parameters such as gender, anthropometric dimensions, fatigue and resting periods on results. Moreover, knee angle as well as external loads (at different magnitudes and elevations) can be considered in order to model tasks that are more representative of those preformed at workplace or in daily life activities.

Key words: Biomechanics, electromyography; kinematics; forces platform; trunk; flexion-relaxation phenomenon.