Wednesday, October 14, 2009

The flexion relaxation phenomenon

I. Lumbar erector spinae flexion-relaxation phenomenon (FRP):

A number of studies have shown differences in the FRP between patients with chronic low back pain and healthy individuals. Presence of the FRP during trunk flexion represents myoelectric silence (1).

This leads to increased load sharing on passive structures and further these tissues have been found to fail under excessive loading conditions and are a source of low back pain. Persistent activation of the lumbar erector spinae musculature among patients with back pain may represent the body's attempt to stabilize injured or diseased spinal structures via reflexogenic ligamentomuscular activation thereby protecting them from further injury and avoiding pain (1). However, flexion relaxation phenomenon (FRP) is an interesting model to study the modulation of lumbar stability (2).

II. Presence of fatigue of the erector spinae (ES) muscles modifies the FRP

Descarreaux et al studied to identify the effect of erector spinae (ES) muscle fatigue and spine loading on myoelectric silence onset and cessation in healthy individuals during a flexion-extension task. Trunk and pelvis angles and surface EMG of the erector spinae (ES) L2 and L5 were recorded during flexion-extension task.

They found:

1. Onset of myoelectric silence during the flexion motion appeared earlier after the fatigue task + cessation of myoelectric silence was observed later during the extension after the fatigue task.

2. Persistence of erector spinae (ES) myoelectric activity in flexion during the load condition.

These findings indicate presence of fatigue of the erector spinae (ES) muscles modifies the FRP. This study also confirmed earlier findings that superficial back muscle fatigue seems to induce a shift in load-sharing towards passive stabilizing structures. The loss of muscle contribution together with or without laxity in the viscoelastic tissues may have a substantial impact on post fatigue stability (2).

III. FRP in seated forward flexion or slumped postures: differences in thoracic & lumbar levels

Callaghan et al did SEMG on 22 subjects (both males & females) to examine the myoelectric activity of the erector spinae muscles (thoracic and lumbar) of the back if the FRP occurs in seated forward flexion or slumped postures.

The study shows:

1. Slumped sitting posture yield FRP of the thoracic erector spinae muscles, whereas the lumbar erector spinae muscle group remained at relatively constant activation levels regardless of seated posture.

2. Thoracic erector spinae silence occurred at a smaller angle of lumbar flexion during sitting than the flexion relaxation angle observed during standing flexion relaxation.

Hence myoelectric activity of the lumbar erector spinae did not increase, in seated forward flexion or slumped postures. This indicates that the passive tissues of the vertebral column were loaded to support the moment at L4/L5. Ligaments contain a large number of free nerve endings which act as pain receptors and therefore could be a potential source of low back pain during seated work.

This study has a great relevance as examination of flexion relaxation during seated postures may provide insight into the association between low back pain and seated work.

IV. FRP in lifting

During the full flexion phase of the back lift movement the lumbar part of the erector spinae muscle exhibits a reduced activity level (flexion relaxation).

But the question is how the required extension torque in the lumbo-sacral joint (L5/S1 joint) is balanced during the period in which apparently the lumbar erector spinae ceases to take its share?

Toussaint et al reported:

1. While flexion it is true that there is flexion relaxation but this coincide with a 25% increase in lumbar length. The change in length-force relationship & passive tissue strain provide part of the required extension torque.

2. With lifting of barbell there is a significant increase in EMG level of the thoracic part of the erector spinae occurr just before the flexion relaxation at the lumbar level. Apparently, the extensor function of the lumbar part is then taken over by the thoracic part of the erector spinae muscle.

3. This suggests that an intricate coordinating mechanism operate that apportions the load to be balanced over active--(lumbar and thoracic part of the erector spinae) and passive structures (post vertebral ligaments).

V. Repeated spinal flexions modulate FRP

Repeated flexion causes muscular fatigue, creep of passive tissues and diminished protective reflexes.

Dickey et al reported:

1. Majority of people show flexion-relaxation throughout the repeated trunk flexion.

2. With flexion-relaxation and maximum flexion angles increase.

3. The flexion-relaxation angle relative to the maximum flexion angle also increases. This effect depended on the load condition; the flexion-relaxation and maximum flexion angles show a greater increase in the unloaded than loaded condition.

Dickey et al concluded that FRP change due to repeated trunk flexions because of changes to the neuromuscular control system. The deactivation of the erector muscles near full flexion angle result in a greater spinal flexion following repeated spinal flexion. This may be related to the increased risk of injury associated with repeated flexion.

VI. FRP in relation to load & speed

The relative spine motion time differ depending on the direction of movement, being longer during trunk flexion and shorter during extension. Sarti et al (6) tried to find out whether variable speed and loading during trunk flexion-extension.

They found:

1. Increase in speed of movement increase the relative lumbar flexion time and significantly reduced the relative lumbar extension time.

2. Increased speed delays the appearance of the electrical silence or the flexion relaxation.
Adding FRP in assessment of LBA

The studies that show differences in the presence of the FRP among patients and control subjects are encouraging for this type of clinical assessment and suggest that assessment of the FRP is a valuable objective clinical tool to aid in the diagnosis and treatment of patients with low back pain (1).


1. Colloca CJ et al, The biomechanical and clinical significance of the lumbar erector spinae flexion-relaxation phenomenon: a review of literature; J Manipulative Physiol Ther. 2005 Oct;28(8):623-31.
2. Descarreaux M et al, Changes in the flexion relaxation response induced by lumbar muscle fatigue; BMC Musculoskelet Disord. 2008 Jan 24;9:10.
3. Callaghan JP et al, Examination of the flexion relaxation phenomenon in erector spinae muscles during short duration slumped sitting; Clin Biomech (Bristol, Avon). 2002 Jun;17(5):353-60.
4. Toussaint HM et al, Flexion relaxation during lifting: implications for torque production by muscle activity and tissue strain at the lumbo-sacral joint; J Biomech. 1995 Feb;28(2):199-210.
5. Dickey JP et al, Repeated spinal flexion modulates the flexion-relaxation phenomenon; Clin Biomech (Bristol, Avon). 2003 Nov;18(9):783-9.
6. Sarti MA et al, Response of the flexion-relaxation phenomenon relative to the lumbar motion to load and speed; Spine (Phila Pa 1976). 2001 Sep 15;26(18):E421-6.

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