How long is too long, to be in a forward, bent posture? Many of us spend hours doing house chores; weeding, DIY, working on the car etc. Not to mention the time spent leaning over a computer desk or looking down at your phone.
Your muscles play an amazing role of suspending us in these positions, but just like with exercise our muscles will reach a point of fatigue. When the postural muscles aren’t able to provide the support we then rely on “passive” structures like ligaments and fascia, which is not their primary role, eventually leading stress and increased risk of injury.
This study looked at 2 angles of the spine leaning forwards and found that within 40 seconds the participants transitioned from the support of postural muscles to the passive structures. While this was found to be a natural transition the prolonged strain on the passive structures has been shown to increase the risk of lower back pain as suggested in another study.
As mentioned in a previous blog, these positions are not “wrong” but it’s better for the overall health of the spine to regularly change position and break from sustained load on an individual structure to provide balance.
Static trunk bending is an occupational risk factor for lower back pain (LBP). When assessing relative short duration trunk bending tasks, existing studies mostly assumed unchanged spine biomechanical responses during task performance. The purpose of the current study was to assess the biomechanical changes of lumbar spine during the performance of relatively short duration, sustained trunk bending tasks. Fifteen participants performed 40-s static trunk bending tasks in two different trunk angles (30° or 60°) with two different hand load levels (0 or 6.8 kg). Results of the current study revealed significantly increased lumbar flexion and lumbar passive moment during the 40 s of trunk bending. Significantly reduced lumbar and abdominal muscle activities were also observed in most conditions. These findings suggest that, during the performance of short duration, static trunk bending tasks, a shift of loading from lumbar active tissues to passive tissues occurs naturally. This mechanism is beneficial in reducing the accumulation of lumbar muscle fatigue; however, lumbar passive tissue creep could be introduced due to prolonged or repetitive exposure.
Alessaa F. et al (2017) Changes of lumbar posture and tissue loading during static trunk bending. Human Movement Science