Influence of delayed muscle reflexes on spinal stability: model-based predictions allow alternative interpretations of experimental data.

Model-based calculations indicate that reflex delay and reflex gain are both important for spinal stability. Experimental results demonstrate that chronic low back pain is associated with delayed muscle reflex responses of trunk muscles. The aim of the present study was to analyze the influence of such time-delayed reflexes on the stability using a simple biomechanical model. Additionally, we compared the model-based predictions with experimental data from chronic low back pain patients and healthy controls using surface-electromyography. Linear stability methods were applied to the musculoskeletal model, which was extended with a time-delayed reflex model. Lateral external perturbations were simulated around equilibrium to investigate the effects of reflex delay and gain on the stability of the human lumbar spine. The model simulations predicted that increased reflex delays require a reduction of the reflex gain to avoid spinal instability. The experimental data support this dependence for the investigated abdominal muscles in chronic low back pain patients and healthy control subjects. Reflex time-delay and gain dependence showed that a delayed reflex latency could have relevant influence on spinal stability, if subjects do not adapt their reflex amplitudes. Based on the model and the experimental results, the relationship between muscle reflex response latency and the maximum of the reflex amplitude should be considered for evaluation of (patho) physiological data. We recommend that training procedures should focus on speeding up the delayed reflex response as well as on increasing the amplitude of these reflexes.