Comparison of the reticulospinal drive to lumbar erector spinae muscles in postural and voluntary tasks using the StartReact paradigm

Abstract

Introduction: While lesion and neurophysiological animal studies point toward a notable involvement of subcortical pathways in the control of low back muscles, little attention has been dedicated to the subject in humans. The StartReact paradigm may allow to indirectly test the potential contribution of the reticulospinal system during motor control, thus addressing this gap of knowledge. In this study, we aimed to compare the potential contribution of the reticulospinal system in the control of low back muscles during voluntary (lumbar spine extension) and postural (upper limb movement eliciting anticipatory postural adjustment) tasks using the StartReact paradigm.

Methods: The reaction time (RT) of the lumbar erector spinae was measured within a simple precued RT task while conditioned by startling (SAS-116 dB) or non-startling (NSAS-80 dB) acoustic stimuli.

Results: The reduction in RT was similar during the postural and voluntary tasks. However, RT was more shortened with the SAS condition compared to the NSAS condition in both tasks. This finding was replicated using a cumulative distribution functions analysis.

Discussion: For the first time, a StartReact effect of back muscles was demonstrated during a voluntary task and was shown to be similar to that observed in a postural task. Therefore, these results suggest a contribution of the reticulospinal tract in the postural and voluntary control of back muscles in humans.

Keywords: StartReact; anticipatory postural adjustment; erector spinae; paravertebral muscle; reticulospinal; volitional control.

Figure 1

Schematic representation of the starting position and motor tasks used for the reaction time paradigm. This position was also used when startle and non-startle auditory stimulation were used outside the reaction time paradigm. (A) Participants sat on a chair without backrest, arms along the body and feet on the floor or on a step to maintain ≈80° of hip flexion, with an audio speaker placed 50 cm behind their head. (B) The postural task consists of a bilateral shoulder flexion up to ≈90°, which elicits an APA of back muscles. (C) The voluntary task consists of an anterior pelvic tilt, where LES act as agonists of the lumbar spine extension. APA, Anticipatory Postural Adjustments; LES, Lumbar erector spinae.

Figure 2

Schematic representation of the experimental design. (A) For the reaction time tasks, a visual warning cue (orange light) informed participants that an imperative cue (blue light) would turn on (1,500 ms later). The delay period corresponds to the time between the two visual cues. The motor execution period corresponds to the time between the imperative cue and LES EMG onset. (B) Description of the StartReact paradigm. The presentation of an auditory stimulus (SAS or NSAS) is presented simultaneously with the imperative cue during a simple RT task. A greater reduction of the motor execution period with SAS compared to NSAS suggest potential reticulospinal drive. LES, Lumbar erector spinae; SAS, Startling acoustic stimuli; NSAS, Non startling acoustic stimuli; RT alone, Reaction time at baseline.

Figure 3

Average (black line) raw EMG of LES, AD and SCM of one participant for the SAS and NSAS conditions without motor task. The blue line corresponds to the first time that (A) SAS or (B) NSAS was presented. Note the early inhibition (present in the first trial—blue trace) followed by excitation of LES elicited by the first SAS. SAS, Startling acoustic stimuli; NSAS, Non startling acoustic stimuli; LES, Lumbar erector spinae; AD, Anterior deltoid; SCM, Sternocleidomastoid.

Figure 4

Average (black line) and individual (gray lines) EMG traces of a participant for RT alone, NSAS (80 dB) and SAS (116 dB) in (A) SCM, (B) AD and (C) LES for the postural task. The arrows represent the EMG onset elicited by the motor task. Non-rectified EMG is displayed to appreciate the motor pattern elicited by the postural task. RT alone, Reaction time at baseline; LES, Lumbar erector spinae; AD, Anterior deltoid; SCM, Sternocleidomastoid.

Figure 5

Average (black line) and individual (gray lines) EMG traces of a participant for RT alone, NSAS (80 dB) and SAS (116 dB) in (A) SCM and (B) LES for the voluntary task. The arrows represent the EMG onset elicited by the motor task. Non-rectified EMG is displayed to appreciate the motor pattern elicited by the voluntary task. RT alone, Reaction time at baseline; LES, Lumbar erector spinae; SCM, Sternocleidomastoid.

Figure 6

(A) Individual raw values (n = 15), means and standard deviation for lumbar erector spinae RT in the different conditions: alone, combined with 80 dB and 116 dB for both tasks [postural (red dots) and voluntary (blue dots)]. (B) Estimated means and 95% confidence interval of the log-transformed RT extracted from the linear mixed model. Note that the RT alone was used as a covariate due to the substantial RT differences between tasks. *p < 0.001. RT alone, Reaction time when the task was realized only with visual cues; dB: decibels.