The transfer of load from the active tissues (muscle) to passive tissues (ligament, fascia, disc, etc.) in the lumbar spine as the human spine approaches full trunk flexion has implications for spinal loading and has been the focus of much research (e.g., Floyd and Silver, 1955, McGill and Kippers, 1994). Since these passive tissues are viscoelastic in nature, they have been shown to exhibit both creep (e.g., McGill & Brown, 1992) and stress-relaxation responses (e.g., Shin & Mirka, 2007). Previous studies that have examined these creep and/or stress-relaxation responses have typically done so in “maximum” trunk flexion posture (e.g., McGill and Brown, 1992, Shin and Mirka, 2007, Larson et al., 2020). The results of these studies have shown that the developed creep led to increased peak lumbar flexion angle (McGill and Brown, 1992, Shin and Mirka, 2007, Larson et al., 2020), increased EMG-off lumbar flexion angle (i.e., increased angle at which the trunk extensor muscle activity becomes indistinguishable from that seen in full flexion postures) (Solomonow et al., 2003), increased lumbar muscle activity (Shin and Mirka, 2007, Shin et al., 2009) and increased lumbar muscle fatigue (Shin et al., 2009). Previous studies have suggested that as creep in the passive tissues occurs, increasing levels of contraction force of the lumbar extensor muscles may be required to compensate for the loss of passive stiffness (Shin & Mirka, 2007), denoting a compensatory biomechanical response to maintain the spine stability. However, our understanding of whether submaximal trunk flexion postures cause time-dependent viscoelastic deformation of the lumbar spine is still far from complete.

Some evidence has been presented that holding submaximal trunk flexion postures can alter lumbar angle (Alessa & Ning, 2018), a result that may imply that submaximal trunk flexion postures may also generate creep/flexion-relaxation in the passive tissues. In many in-vivo experiments, the lumbar flexion angle was regarded to be unchanging at a given trunk flexion posture (e.g., Arjmand and Shirazi-Adl, 2005, Kahrizi et al., 2007). However, a recent empirical study evaluated lumbar biomechanics while holding static trunk flexion postures of 30° and 60° with varied hand-held loads (0 kg, 6.8 kg) for only 40 s revealed that lumbar flexion angle and passive moment significantly increased while lumbar extensor muscle activity decreased (Alessa & Ning, 2018) providing evidence that the extensor moment gradually shifted from active to passive tissues even while the trunk flexion angles were maintained. These differences highlight the notion that changes in the local coordinate systems (lumbar angle) can occur even if the global system (trunk angle) is held constant and lead us to the hypothesis that the creep can occur at trunk flexion angles less than full trunk flexion.

The goal of this study was to examine the effect of submaximal trunk flexion on creep deformation of viscoelastic tissue in the lumbar spine. It was hypothesized that holding submaximal trunk postures for extended periods of time would result in fatigue in lumbar extensor muscles which would increase the lumbar flexion angle and result in significant creep deformation of the viscoelastic soft tissue even as the trunk flexion posture is held constant.