Prolonged Intermittent Trunk Flexion Increases Trunk Muscles Reflex Gains and Trunk Stiffness

Abstract

The goal of the present study was to determine the effects of prolonged, intermittent flexion on trunk neuromuscular control. Furthermore, the potential beneficial effects of passive upper body support during flexion were investigated. Twenty one healthy young volunteers participated during two separate visits in which they performed 1 hour of intermittent 60 seconds flexion and 30 seconds rest cycles. Flexion was set at 80% lumbar flexion and was performed with or without upper body support. Before and after intermittent flexion exposure, lumbar range of motion was measured using inertial measurement units and trunk stability was assessed during perturbations applied in the forward direction with a force controlled actuator. Closed-loop system identification was used to determine the trunk translational admittance and reflexes as frequency response functions. The admittance describes the actuator displacement as a function of contact force and to assess reflexes muscle activation was related to actuator displacement. Trunk admittance gain decreased after unsupported flexion, while reflex gain and lumbar range of motion increased after both conditions. Significant interaction effects confirmed a larger increase in lumbar range of motion and reflex gains at most frequencies analysed following unsupported flexion in comparison to supported flexion, probably compensating for decreased passive tissue stiffness. In contrast with some previous studies we found that prolonged intermittent flexion decreased trunk admittance, which implies an increase of the lumped intrinsic and reflexive stiffness. This would compensate for decreased stiffness at the cost of an increase in cumulative low back load. Taking into account the differences between conditions it would be preferable to offer upper body support during activities that require prolonged trunk flexion.

Effects of Muscle Fatigue, Creep, and Musculoskeletal Pain on Neuromuscular Responses to Unexpected Perturbation of the Trunk: A Systematic Review

“…Such result is surprising since it is expected that creep deformation will lead to an increase muscle activity amplitude (Olson et al, 2009; Abboud et al, 2016), which is believed to act as a spinal stabilization mechanism. Interesting new findings have shown that following a prolonged intermittent trunk flexion of 1 h, an increase of trunk stiffness is observed (Voglar et al, 2016). This observation confirms previous findings suggesting that in the first 30 min of cyclic trunk flexion, a decrease in intrinsic stiffness occurs, whereas, the following 30 min, spinal stiffness increases (Parkinson et al, 2004).…”

The Effect of Bed Rest and Hypoxic Environment on Postural Balance and Trunk Automatic (Re)Actions in Young Healthy Males

“…After the load release, the participants' task was to return the bar to the initial position, as quickly as possible. For both measurements, all trials were triggered in random manner every 5–12 s. Reliability of the applied test procedures for CoP sway during quiet stance tasks and trunk muscles' stability (re)actions has been tested in previous studies (Markovic et al, 2014 ; Voglar and Sarabon, 2014a , b ; Sarabon and Rosker, 2015 ; Voglar et al, 2016 ) and we adopted the suggested reliability optimization guidelines in this study.…”

Standing Up from a Chair with an Asymmetrical Initial Foot Position Decreases Trunk and Masticatory Muscle Activities in Healthy Young Men

“…These findings indicate that the asymmetrical strategy with the dominant leg behind decreases the movement of the trunk and reduces ES muscle activation by increasing the activity of the lower extremity muscles during STS. In addition, prolonged trunk flexion increases the lumbar load [ 31 ]; when lumbar load is increased, back extensor muscle activation is increased, rather than abdominal muscle activation [ 29 ]. Therefore, as our asymmetrical foot strategy decreases ES activation, the lumbar load may also be reduced.…”