Compensatory behavior of the postural control system to flexion-relaxation phenomena.

SUMMARY: Laxity of the passive tissues of the spine during prolonged spinal flexion has been shown to disturb spinal stability. This study investigated the effects of short periods of static lumbar flexion and short rest periods on the flexion-relaxation angle for the erector spinae muscles in 36 healthy female college students. The surface electromyographic activity of the erector spinae muscles was measured in three states before the onset of creep, immediately after 7 min of static lumbar flexion, and after a 10 min rest. The results showed that 7 min of static lumbar flexion will produce relaxation of the erector spinae muscles that occurs at greater absolute lumbar and trunk angles, during the forward bending activity (P<0.05), while the corresponding relative angles did not change before and after creep. The results also indicate that postural compensations are dominant over the muscular compensations for load sharing in flexion-relaxation phenomena of asymptomatic healthy participants. This further highlights the importance of postural modulation in the control of movement and preservation of skeletal stability. CLINICAL RELEVANCE: Considering spinal posture in the upright condition, and its changes by phenomena such as creep, can reduce postural injuries by instructing subjects to approach a more vertical posture, after periods of bending, to compensate the stretching effects of the tissues and thus regaining the normal muscular activity pattern.

The relationship between flexibility and EMG activity pattern of the erector spinae muscles during trunk flexion-extension.

BACKGROUND: Movements in the lumbar spine, including flexion and extension are governed by a complex neuromuscular system involving both active and passive units. Several biomechanical and clinical studies have shown the myoelectric activity reduction of the lumbar extensor muscles (flexion-relaxation phenomenon) during lumbar flexion from the upright standing posture. The relationship between flexibility and EMG activity pattern of the erector spinae during dynamic trunk flexion-extension task has not yet been completely discovered. OBJECTIVE: The purpose of this study was to investigate the relationship between general and lumbar spine flexibility and EMG activity pattern of the erector spinae during the trunk flexion-extension task. METHODS: Thirty healthy female college students were recruited in this study. General and lumbar spine flexibilities were measured by toe-touch and modified schober tests, respectively. During trunk flexion-extension, the surface electromyography (EMG) from the lumbar erector spinae muscles as well as flexion angles of the trunk, hip, lumbar spine and lumbar curvature were simultaneously recorded using a digital camera. The angle at which muscle activity diminished during flexion and initiated during extension was determined and subjected to linear regression analysis to detect the relationship between flexibility and EMG activity pattern of the erector spinae during trunk flexion-extension. RESULTS: During flexion, the erector spinae muscles in individuals with higher toe-touch scores were relaxed in larger trunk and hip angles and reactivated earlier during extension according to these angles (P<0.001) while in individuals with higher modified schober scores this muscle group was relaxed later and reactivated sooner in accordance with lumbar angle and curvature (P<0.05). Toe-touch test were significantly correlated with trunk and hip angles while modified schober test showed a significant correlation with lumbar angle and curvature variables. CONCLUSION: The findings of this study indicate that flexibility plays an important role in trunk muscular recruitment pattern and the strategy of the CNS to provide stability. The results reinforce the possible role of flexibility alterations as a contributing factor to the motor control impairments. This study also shows that flexibility changes behavior is not unique among different regions of the body.