Neck Solutions

Effects of slouching and muscle contraction on the strain of the iliolumbar ligament

From: Man Ther. Volume 13, Issue 4, Pages 325-333 (August 2008)

The cause of lower back pain is often attributed to intolerable high intradiscal pressure. Use of the spinal compression model is often referred to for workload standards and is the starting point for spinal buckling instability models for lifting in stooped postures. The precise mechanism producing back sprain is, however, still under discussion. Therefore, we decided to explore a novel approach. In contrast to established biomechanical research we do not relate injury risk to forward trunk inclination, but take the unconstrained erect posture as a starting point.

In a previous study we developed a biomechanical model on sitting with hyperkyphosis while leaning against a high backrest. For verification of the model we measured in vitro stepwise backward tilt of the pelvis combined with forward flexion of the spine. We found that during forward flexion of the L5 vertebra the sacrum moved in the opposite direction (counternutation). During the same test we measured (indirectly) elongation of the iliolumbar ligaments. The increase of strain on the iliolumbar ligament by forward flexion of L5 was similar to that reported earlier.

Because patients suffering from acute lower back pain often present with pain at the site of the iliolumbar ligament we decided to develop a model on sudden slouching. Starting point was the absence or delay of protective muscle force. A higher incidence of lower back pain was found in athletes showing delayed muscle reflex response on a quick force release in trunk flexion, extension and lateral bending. In continuation of our earlier biomechanical model on sitting we decided to model dynamic slouching of the upright trunk. The aim of the present study was to assess failure risk of the iliolumbar ligament by means of explorative calculations and to measure in vitro if such risk could be prevented by back muscles. The following hypothesis was postulated: tension in the iliolumbar ligament increases with forward flexion of L5 and decreases by multifidus and erector spinae muscle contraction.

The unconstrained upright standing posture was the starting point of this study on slouching. In this position back muscle activity is minimal or absent because the upper body weight is centred above the spine and above or just behind the hip joints. Therefore, during sudden and fast slouching, the response of back muscles to protect the change from lumbar lordosis into lumbar hyperkyphosis may be delayed. Muscle delay can exist after a period of prolonged stretch of dorsal ligaments and muscles and muscle fatigue. The explorative calculations show that failure load of the spine and the iliolumbar ligament could be reached during sudden slouching. This change of form in the upright position is called the click clack movement and is characterized by backward tilt of the pelvis combined with forward flexion of the spine.

The present study on the strain of the iliolumbar ligament by spinal flexion may provide an explanation for acute lower back pain in situations with neglectable axial spinal load. Newman states that slouching resembles the primitive reflex mechanism including limb movement to the centre of the body; he refers to this type of movement in patients who have had an acute attack of lumbago while shaving and he attributes the acute attack of pain to bending forward in a position of flexion of the spine with the erector spinae muscles relaxed. This flexion relaxation mechanism was, however, revised by Andersson et al., who used measurements with fine wire electrodes to demonstrate that quadratus lumborum and deep lateral erector spinae were activated when the flexion relaxation phenomenon was present for the superficial medial erector spinae muscles. Therefore, we do not refer to forward trunk inclination but rather to the absence of protective dorsal muscle activity while slouching in the upright posture. We suppose that this only occurs in humans. A human, when in a static standing posture, has his centre of gravity positioned just behind the hip joints, requiring low or absent dorsal muscle activity. This enables the erect slouched posture to be adopted by humans, but not by apes. The bent legged bipedal posture of the ape is the consequence of limited hip extension. Therefore, the centre of gravity is anterior to the hip joints requiring stabilizing dorsal muscle activity.

Strain of the iliolumbar ligaments may occur in dynamic and in static situations. Deursen van et al. reported that lower back pain patients experience significantly more pain provocation in slouched standing postures (e.g. vacuuming, brushing teeth and washing dishes) than in stooped postures.

Sims and Moorman suggest that stress at the ligamento-osseous junction of the iliolumbar ligament at the ilium, and stress in the innervated iliolumbar ligament, can trigger pain from both tissues, which is the premise behind local injections of anaesthetics. Although controversial, evidence exists for the significant role of the iliolumbar ligament in lower back pain. Of interest to our study is that, of the 19 clinical parameters addressed during medical examination of a group of children reporting lower back pain, only one was significantly more prevalent: i.e. pain on palpation at the insertion site of the iliolumbar ligament on the iliac crest.

Further support for our model on slouching can be found in the loss of thoracolumbar curvature in pregnant women, in astronauts during flight and in individuals suffering from psychosocial distress. Our study on the slouched sitting was reported in a previous article, which included measurements on fresh human specimen. For the present study we used a small sample size of embalmed specimen which, however, resulted in similar load deformation patterns. This is in agreement with expectation because the orientation of the iliolumbar ligament is, although individual differences are reported in literature, in all cases in the direction of loading.

The present study provides preliminary information on the effects of muscle contraction on the strain of the iliolumbar ligament. It shows that the lateral erector spinae muscles and the sacral part of the multifidus muscles are capable of protecting the L5-S1 disc against hyperflexion and the iliolumbar ligament against excessive tension, i.e. by hollowing of the lower back. This followed from load tests with the use of embalmed specimen, which implies stiffer material than in vivo. We expect, however, that the relation between the applied load on L5 and the iliolumbar ligament elongation will show the same trend in the in vivo situation. The rotation between the sacrum and ilium remained smaller than the error of measurement, which is attributed to stiffness of the embalmed specimens.

For the effect of back muscle force on the sacroiliac joints we made a distinction between the effect of erector spinae and multifidus muscle. This is of interest because the sacral part of the multifidus muscle has the unique ability to produce the isolated action of extension of the L5-S1 intervertebral disc together with forward rotation of the sacrum with respect to the ilium (nutation). This suggests co-activation with pelvic floor muscles according to the agonist antagonist action about the sacroiliac joints. Therefore, it may be expected that lower back pain due to strain on the iliolumbar ligament may be related to pelvic floor problems. Pool-Goudzwaard et al. demonstrated a significant increase of pelvic floor activity in a population of postpartum low back and pelvic pain patients accompanied by frequency, urgency and stress incontinence.

This study lends credibility to the idea that back sprain can be the result of slouching in the upright posture. The iliolumbar ligament is designated as a possible source of pain, because this ligament was strained by flexion of L5. In line with these measurements were the explorative calculations pointing to the possibility of injury to the iliolumbar ligament and spine by slouching. Ease of the iliolumbar ligament was obtained by simulated back muscle force. Conditional for the conceptual model on back sprain is the delay or absence of muscle recruitment which has to prevent the click clack movement, i.e. backward tilt of the pelvis combined with forward flexion of the spine. This suggests that the attention for lifting in a stooped posture may be diverted to dynamic or static slouching with the trunk in the upright position as the main cause of lower back pain in daily life activities.