Effects of different modalities of afferent stimuli of the lumbo-sacral area on control of lumbar paravertebral muscles.

Somatosensory feedback to the central nervous system is essential to plan, perform and refine spine motor control. However, the influence of somatosensory afferent input from the trunk on the motor output to trunk muscles has received little attention. The objective was to compare the effects of distinct modalities of afferent stimulation on the net motoneuron and corticomotor excitability of paravertebral muscles. Fourteen individuals were recruited. Modulation of corticospinal excitability (motor-evoked potential [MEP]) of paravertebral muscles was measured when afferent stimuli (cutaneous noxious and non-noxious, muscle contraction) were delivered to the trunk at 10 intervals prior to transcranial magnetic stimulation. Each peripheral stimulation was applied alone, and subsequent electromyography (EMG) modulation was measured to control for net motoneuron excitability. MEP modulation and MEP/EMG ratio were used as measures of corticospinal excitability with and without control of net motoneuron excitability, respectively. MEP and EMG modulation were smaller after evoked muscle contraction than after cutaneous noxious and non-noxious stimuli. MEP/EMG ratio was not different between stimulation types. Both MEP and EMG amplitudes were reduced after evoked muscle contraction, but not when expressed as MEP/EMG ratio. Noxious and non-noxious stimulation had limited impact on all variables. Distinct modalities of peripheral afferent stimulation of the lumbo-sacral area differently modulated responses of paravertebral muscles, but without an influence on corticospinal excitability with control of net motoneuron excitability. Muscle stimulation reduced paravertebral activity and was best explained by spinal mechanisms. The impact of afferent stimulation on back muscles differs from the effects reported for limb muscles.