The Effect of Trunk Position and Pain Location on Lumbar Extensor Muscles Recruitment Strategies.

PURPOSE: To investigate the effect of trunk positions and experimental lumbar pain location on lumbar extensor muscles recruitment strategies. METHODS: 19 healthy participants (10 men: 9 women), aged 25.3 ± 4.7 years, performed isometric back extension contractions in three positions: neutral, 45° and 90° trunk flexion and under three conditions: no pain, caudal pain and cranial pain. Lumbar muscle activation strategies were recorded using high-density surface electromyography. The effect of position and pain conditions on muscle activity amplitude and spatial redistributions was assessed. RESULTS: Muscle activity amplitude was 43% higher in 45° trunk flexion than in neutral position on both sides (p < 0.05). In the 90° trunk flexion, participants showed a more lateral spatial distribution than in the 45° trunk flexion on the left side p < 0.01, 5.4 mm difference) and the neutral position on both sides (p < 0.05, 8.2 mm difference). In the 45° trunk flexion, participants exhibited a more lateral spatial distribution compared with the neutral position on the right side (p < 0.05, 3.7 mm difference). A lateral spatial redistribution of muscle activity was observed in the caudal pain condition compared with no pain on the right side (p < 0.05, 3.0 mm difference). Individual responses to pain varied across all variables. CONCLUSIONS: Different trunk positions result in different distributions of activation within the lumbar extensor muscles, possibly based on regional mechanical advantage. No clear indication of location-specific pain adaptation, and no effect of task-dependent pain adaptation were found, whereas individual-specific adaptations were observed.

Acute Upper-Body and Lower-Body Neuromuscular Fatigue Effect on Baseball Pitchers' Velocity: A Pilot Study.

ABSTRACT: Tremblay, M, Anderson Sirois, S, Verville, W, Auger, M, Abboud, J, and Descarreaux, M. Acute upper-body and lower-body neuromuscular fatigue effect on baseball pitchers' velocity: A pilot study. J Strength Cond Res XX(X): 000-000, 2024-The purpose of this pilot study was to explore the acute effect of upper-body and lower-body neuromuscular fatigue protocols on baseball pitchers' velocity. Sixteen baseball pitchers were recruited, and a crossover design was used to meet the study purpose. Pitchers were tested twice, 7 days apart, with their upper-body and lower-body explosiveness, pitching velocity, and muscle soreness perception of their throwing arm (forearm flexors, biceps, anterior deltoid, and upper trapezius muscles) assessed before and after an upper-body and lower-body neuromuscular fatigue protocol. Two-way analysis of variances and paired t tests (p < 0.05) were used to identify and compare prescores and postscores. Following both fatigue protocols, results revealed a significant decrease in time for pitching velocity (p = 0.005, ηp2 = 0.462), and increases in muscle soreness perception of the forearm flexors (p = 0.005, ηp2 = 0.470), anterior deltoid (p = 0.045, ηp2 = 0.274), and upper trapezius (p = 0.023, ηp2 = 0.339) muscles. Paired t test results showed a significant decrease in preneuromuscular and postneuromuscular fatigue protocol in the upper-body (p < 0.01) and lower-body (p < 0.01) explosiveness scores. These pilot study results show the impact of different exercise protocols on pitchers' explosiveness, velocity, and muscle soreness perception emphasizing the need for further investigation into the acute effect of exercise targeting the upper or lower-body on pitching performance, specifically at the pitcher's position.

Effect of physical activity education on shoulder girdle pain and muscle strength in participants with fibromyalgia: a pilot experimental study.

Background: In patients with fibromyalgia, exercise and education are recommended to decrease pain level and improve pain management. The latest scientific evidence recommends to focus interventions on the upper limb. The aim of this pilot study was to compare the immediate effect of physical activity education vs. a control group on pain and muscle capacity in fibromyalgia patients. Method: Fifty-six participants with fibromyalgia were randomized into an experimental group and a control group. The intervention consisted in watching a five-minute video that provided information about fibromyalgia, pain, kinesiophobia and physical activity. The control group watched a neutral five-minute video about beavers in Quebec. Following the video, participants performed a muscular fatigue task consisting of a repeated unilateral shoulder abduction task. At baseline and following the muscular fatigue task, maximal voluntary contraction (MVC) in shoulder abduction was assessed as well as pain level and pressure pain threshold (PPT) in the upper limb. Electromyographic activity was also assessed for upper trapezius and middle deltoid muscles. Two-way repeated measures analysis of variance was used to compare the MVC, PPT, and pain level before and after the muscular fatigue task between groups. Results: The experimental group showed a significantly lower increase in pain than the control group in the middle deltoid muscle (p = 0.002) when assessed by verbal pain rating scale. No significant interaction or main effect of Group and Time were observed for the pain level at the upper trapezius and elbow extensor muscles nor for any of the PPT measures. According to electromyographic data, the median frequency values indicate that neither group experienced muscle fatigue during the repeated contraction task. Conclusions: The preliminary results suggest that a short physical activity education video positively influenced middle deltoid pain following repeated abduction in participants with fibromyalgia. Electromyographic analysis showed no evidence of objective muscle fatigue, suggesting that there might be a partial disconnection between the perception of muscle fatigue and the physiological biomarkers associated with muscle fatigue.

Impact of lumbar delayed-onset muscle soreness on postural stability in standing postures.

BACKGROUND: Similar impact on proprioception has been observed in participants with lumbar delayed-onset muscle soreness (DOMS) and chronic low back pain (LBP), raising questions about the relevance of lumbar DOMS as a suitable pain model for LBP when assessing back pain-related postural stability changes. RESEARCH QUESTION: Does lumbar DOMS impact postural stability? METHODS: Twenty healthy adults participated in this experimental study and underwent a posturographic examination before and 24 to 36 h after a protocol designed to induce lumbar DOMS. Posturographic examination was assessed during quiet standing on both feet with eyes opened (EO), with eyes closed (EC), and on one-leg (OL) standing with eyes opened. Postural stability was assessed through center of pressure (COP) parameters (COP area, velocity, root mean square, mean power frequency) which were compared using repeated measure ANOVA. Moreover, pain, soreness and pressure pain threshold (PPT) on specific muscles were assessed. RESULTS: There was a significant main effect of the postural condition on all COP variables investigated. More specifically, each COP variable reached a significantly higher value in the OL stance condition than in both EO and EC bipedal conditions (all with p < 0.001). In addition, the COP velocity and the mean power frequency along the anteroposterior direction both reached a significantly higher value in EC than in EO (p < 0.001). In contrast, there was no significant main effect of the DOMS nor significant DOMS X postural condition interaction on any of the COP variables. There was a significant decrease in the PPT value for both the left and right erector spinae muscles, as well as the left biceps femoris. SIGNIFICANCE: Lumbar DOMS had no impact on postural stability, which contrasts findings in participants with clinical LBP. Although DOMS induces similar trunk sensorimotor adaptations to clinical LBP, it does not appear to trigger similar postural stability adaptations.

Vestibular control of deep and superficial lumbar muscles.

The active control of the lumbar musculature provides a stable platform critical for postures and goal-directed movements. Voluntary and perturbation-evoked motor commands can recruit individual lumbar muscles in a task-specific manner according to their presumed biomechanics. Here, we investigated the vestibular control of the deep and superficial lumbar musculature. Ten healthy participants were exposed to noisy electrical vestibular stimulation while balancing upright with their head facing forward, left, or right to characterize the differential modulation in the vestibular-evoked lumbar extensor responses in generating multidirectional whole body motion. We quantified the activation of the lumbar muscles on the right side using indwelling [deep multifidus, superficial multifidus, caudal longissimus (L4), and cranial longissimus (L1)] and high-density surface recordings. We characterized the vestibular-evoked responses using coherence and peak-to-peak cross-covariance amplitude between the vestibular and electromyographic signals. Participants exhibited responses in all lumbar muscles. The vestibular control of the lumbar musculature exhibited muscle-specific modulations: responses were larger in the longissimus (combined cranio-caudal) compared with the multifidus (combined deep-superficial) when participants faced forward (P < 0.001) and right (P = 0.011) but not when they faced left. The high-density surface recordings partly supported this observation: the location of the responses was more lateral when facing right compared with left (P < 0.001). The vestibular control of muscle subregions within the longissimus or the multifidus was similar. Our results demonstrate muscle-specific vestibular control of the lumbar muscles in response to perturbations of vestibular origin. The lack of differential activation of lumbar muscle subregions suggests the vestibular control of these subregions is co-regulated for standing balance.NEW & NOTEWORTHY We investigated the vestibular control of the deep and superficial lumbar extensor muscles using electrical vestibular stimuli. Vestibular stimuli elicited preferential activation of the longissimus muscle over the multifidus muscle. We did not observe clear regional activation of lumbar muscle subregions in response to the vestibular stimuli. Our findings show that the central nervous system can finely tune the vestibular control of individual lumbar muscles and suggest minimal regional variations in the activation of lumbar muscle subregions.

Fatigue task-dependent effect on spatial distribution of lumbar muscles activity.

This study aims to identify how spatial distribution of lumbar muscle activity is modulated by different fatigue tasks. Twenty healthy adults performed two different isometric trunk extension endurance tasks (the modified Sorensen test and the inverted modified Sorensen test) until exhaustion. During these tasks, bilateral superficial lumbar muscle activity was recorded using high-density electromyography. The spatial distribution of activation within these muscles was obtained using the centroid coordinates in the medio-lateral and cranio-caudal directions. The effects of task and endurance time (left and right sides) were investigated using repeated measures ANOVA. Results revealed a significant lateral shift of the centroid throughout the fatigue tasks on both sides and no difference between tasks. Significant task × time interaction effects were found for the cranio-caudal direction on both sides showing a significantly more caudal location of the centroid in the modified Sorensen test compared to the inverted test at the beginning of the tasks. Our findings suggest that spatial distribution of lumbar muscle activity is task-dependent in a pre-fatigue stage while an alternative but similar muscle recruitment strategy is used in both tasks to maintain performance in the later stages of muscle fatigue.

Variation of corticospinal excitability during kinesthetic illusion induced by musculotendinous vibration.

Despite being studied for more than 50 years, the neurophysiological mechanisms underlying vibration (VIB)-induced kinesthetic illusions are still unclear. The aim of this study was to investigate how corticospinal excitability tested by transcranial magnetic stimulation (TMS) is modulated during VIB-induced illusions. Twenty healthy adults received vibration over wrist flexor muscles (80 Hz, 1 mm, 10 s). TMS was applied over the primary motor cortex representation of wrist extensors at 120% of resting motor threshold in four random conditions (10 trials/condition): baseline (without VIB), 1 s, 5 s, and 10 s after VIB onset. Means of motor-evoked potential (MEP) amplitudes and latencies were calculated. Statistical analysis found a significant effect of conditions (stimulation timings) on MEP amplitudes (P = 0.035). Paired-comparisons demonstrated lower corticospinal excitability during VIB at 1 s compared with 5 s (P = 0.025) and 10 s (P = 0.003), although none of them differed from baseline values. Results suggest a time-specific modulation of corticospinal excitability in muscles antagonistic to those vibrated, i.e., muscles involved in the perceived movement. An early decrease of excitability was observed at 1 s followed by a stabilization of values near baseline at subsequent time points. At 1 s, the illusion is not yet perceived or not strong enough to upregulate corticospinal networks coherent with the proprioceptive input. Spinal mechanisms, such as reciprocal inhibition, could also contribute to lower the corticospinal drive of nonvibrated muscles in short period before the illusion emerges. Our results suggest that neuromodulatory effects of VIB are likely time-dependent, and that future work is needed to further investigate underlying mechanisms.NEW & NOTEWORTHY The modulation of corticospinal excitability when perceiving a vibration (VIB)-induced kinesthetic illusion evolves dynamically over time. This modulation might be linked to the delayed occurrence and progressive increase in strength of the illusory perception in the first seconds after VIB start. Different spinal/cortical mechanisms could be at play during VIB, depending on the tested muscle, presence/absence of an illusion, and the specific timing at which corticospinal drive is tested pre/post VIB.

Lumbar muscle adaptations to external perturbations are modulated by trunk posture.

PURPOSE: To investigate if the recruitment of different regions within the lumbar extensor muscles in response to unexpected perturbations depends on trunk posture. METHODS: In a semi-seated posture, healthy adult participants experienced unexpected posterior-anterior trunk perturbations in three different postures: neutral, trunk flexion and left trunk rotation. High-density surface electromyography was used to identify the regional distribution of activation within the lumbar erector spinae muscles. The effect of posture and side (left vs right) on muscle activity and centroid coordinates was investigated at baseline and in response to perturbations. RESULTS: Higher muscle activity was observed in trunk flexion compared to neutral and rotation postures at baseline (multiple p < 0.001) and in response to the perturbation (multiple p < 0.01). At baseline, the centroid of the electromyographic amplitude distribution was localized more medially in trunk flexion compared to trunk neutral posture (p = 0.003), while activation was localized more laterally in response to the perturbation (multiple p < 0.05). When the trunk was rotated, the electromyographic amplitude distribution was localized more cranially on the left than the right side, both at baseline (p = 0.001) and in response to the perturbation (p = 0.001). Finally, a more lateral location of the centroid on the left side in rotation compared to neutral posture was observed in response to the perturbation (multiple p < 0.001). CONCLUSIONS: Regional differences in the distribution of electromyographic amplitude indicate that different muscle regions were recruited in different trunk postures and in response to perturbations, possibly based on regional mechanical advantage of the erector spinae muscle fibers.

Headache-related clinical features in teleworkers and their association with coping strategies during the COVID-19 pandemic.

Objectives: The objectives were (1) to describe and compare headache-related clinical features between teleworkers with migraine and those with tension-type headache (TTH) and (2) to determine the association between coping strategies and headache frequency, and intensity in the context of the COVID-19 pandemic. Methods: This cross-sectional online survey was conducted with 284 teleworkers (127 with migraine and 157 with TTH). Sociodemographic data, information related to work factors, headache clinical features, coping strategies used during the COVID-19 pandemic, and headache-related clinical features were compared between headache profiles. Bivariate logistic regression analyses were used to determine the association between coping strategies and headache frequency, and intensity. Results: Results showed that teleworkers with migraine had longer and more painful headache episodes than teleworkers with TTH (ps < 0.001). Higher migraine frequency was associated with the use of the denial coping strategy (p = 0.006) while lower migraine intensity was associated with planning (p = 0.046) and the use of positive reframing (p = 0.025). Higher TTH frequency was associated with the use of venting, self-blame, and behavioral disengagement (ps < 0.007) while higher TTH intensity was associated with substance use and behavioral disengagement (ps < 0.030). All associations remained significant after adjusting for BMI as a covariate. Discussion/conclusion: Teleworkers with migraine had more intense and longer headache episodes than teleworkers with TTH. This could be explained by the fact that a greater proportion of individuals suffering from migraine experienced headaches prior to the beginning of the pandemic compared with teleworkers suffering from TTH. Regarding coping strategies, both primary headache profiles were associated with different types of coping strategies. Most of the coping strategies associated with headache frequency or intensity were maladaptive except for planning and positive reframing that were found to be inversely associated with migraine intensity.

The effect of low back pain on neuromuscular control in cyclists.

This study was designed to identify neuromuscular adaptations of low back pain (LBP) cyclists , and the impact of a cycling effort on spinal shrinkage. Forty-eight trained cyclists rode their road bike on a smart trainer for 1-hour. Surface electromyography (EMG) recorded muscle activity of the lumbar erector spinae (LES), 3D motion analysis system recorded kinematic of the trunk, and stadiometry measured spinal height. Statistical comparisons were made using repeated measure ANOVAs. The LBP group presented increase in pain levels throughout the effort (p < 0.001). A significant group difference was only observed for the thoracic angle (p = 0.03), which was less flexed for LBP. The one-hour cycling effort (time effect) significantly increased the trunk flexion (p < 0.001) and thoracic flexion (p < 0.001) for both groups. Significant lower LES activation (35% less) was observed at the end of the effort  as well as a decrease in spinal height (p = 0.01) for both groups. Neuromuscular adaptations to cycling effort is identified by a decrease in LES EMG amplitude and an increase flexion of the trunk. Adaptation to pain is seen by an increase in thoracic flexion. Despite these adaptations, LBP cyclists could not ride their bike pain-free.

Superficial lumbar muscle recruitment strategies to control the trunk with delayed-onset muscle soreness.

PURPOSE: The lumbar region offers various muscle recruitment strategies to achieve a task goal under varying conditions. For instance, trunk movement control can be reorganized under the influence of low back pain. How such task-modulation is obtained is not fully understood. The objective of this study was to characterize superficial lumbar muscle recruitment strategies under the influence of delayed-onset muscle soreness (DOMS) during unexpected trunk perturbations. METHODS: Healthy participants experienced a series of 15 sudden external perturbations with and without the influence of low back DOMS. During these perturbations, high-density surface electromyography was used to characterize recruitment strategies of superficial lumbar muscles, while kinematics sensors were used to characterize movements of the trunk. Lumbar muscle recruitment strategies, characterized by the amplitude of muscle activity amplitude, the latencies of the reflex activity and the spatial distribution of muscle activity, were compared across perturbations trials and with and without DOMS. RESULTS: An attenuation of lumbar muscle activity amplitude was observed across perturbation trials without DOMS, but not with DOMS. The spatial distribution of muscle activity was similar with and without DOMS. No significant changes in reflex activity latency and trunk flexion movement were observed. CONCLUSIONS: Following an unexpected trunk perturbation under DOMS effects, trunk movement are controlled using two different superficial lumbar muscles control strategies: keeping a constant level of their overall muscle activity and using a variable muscle recruitment pattern.

Factors associated with headache and neck pain among telecommuters - a five days follow-up.

BACKGROUND: The current sanitary crisis brought on by the COVID-19 recently forced a large proportion of workers to adopt telecommuting with limited time to plan transition. Given that several work-related risk factors are associated with headache and neck pain, it seems important to determine those associated with headache and neck pain in telecommuters. The main objective of this study was to identify which telecommuting and individual associated factors are related with headache and neck pain occurrence in telecommuters over a five days follow-up. The second objective was to evaluate the impact of wearing a headset on headache and neck pain intensity in telecommuters. METHODS: One hundred and sixty-two participants in telecommuting situation were recruited. Baseline assessment included sociodemographic data, headache and neck pain-related disability (6-item Headache Impact Test (HIT-6) and Neck Bournemouth Questionnaire (NBQ)), headache and neck pain frequency and intensity as well as questions about the wearing of a headset (headset wearing, headset type and headset wearing hours). A prospective data collection of headache, neck pain and headset wearing was conducted using daily e-mail over a 5-day follow-up. A stepwise multivariate regression model was performed to determine associated factors of headache or neck pain occurrence during the follow-up. A t-test was conducted to assess the impact of headset wearing on headache and neck pain intensity during the follow-up. RESULTS: Regarding headache, the stepwise multivariate regression model showed that the HIT-6 score was associated with future headache occurrence in telecommuters (OR (95% CI) = 1.094 (1.042-1.148); R2 = 0.094; p <  0.001). For neck pain, the stepwise multivariate regression showed that the NBQ score was related to future neck pain occurrence in telecommuters (OR (95% CI) = 1.182 (1.102-1.269); R2 = 0.182; p <  0.001). T-test showed no difference between participants that wore a headset and participant that did not wore a headset on mean headache (p = 0.94) and neck pain (p = 0.56) intensity during the five days follow-up. CONCLUSION: Although several work-related risk factors are associated with headache and neck pain in workers, telecommuting did not present the same risks. Working set-up did not have a significant impact on headache and neck pain as headache-related disability was the only associated factor of future headache episodes and neck-pain related disability was the only associated factor of future neck pain episodes. Also, wearing a headset had no impact on headache and neck pain in telecommuters.

A task-relevant experimental pain model to target motor adaptation.

KEY POINTS: Motor adaptation is thought to be a strategy to avoid pain. Current experimental pain models do not allow for consistent modulation of pain perception depending on movement. We showed that low-frequency sinusoidal stimuli delivered at painful intensity result in minimal habituation of pain perception (over 60 s) and minimal stimulation artefacts on electromyographic signals. When the amplitude of the low-frequency sinusoidal stimuli was modulated based on the vertical force participants applied to the ground with their right leg while standing upright, we demonstrated a strong association between perceived pain and motor adaptation. By enabling task-relevant modulation of perceived pain intensity and the recording electromyographic signals during electrical painful stimulation, our novel pain model will permit direct experimental testing of the relationship between pain and motor adaptation. ABSTRACT: Contemporary pain adaptation theories predict that motor adaptation occurs to limit pain. Current experimental pain models, however, do not allow for pain intensity modulation according to one's posture or movements. We developed a task-relevant experimental pain model using low-frequency sinusoidal electrical stimuli applied over the infrapatellar fat pad. In fourteen participants, we compared perceived pain habituation and stimulation-induced artefacts in vastus medialis electromyographic recordings elicited by sinusoidal (4, 10, 20 and 50 Hz) and square electrical waveforms delivered at constant peak stimulation amplitude. Next, we simulated a clinical condition where perceived knee pain intensity is proportional to the load applied on the leg by controlling sinusoidal current amplitude (4 Hz) according to the vertical force the participants applied with their right leg to the ground while standing upright. Pain ratings habituated over a 60 s period for 50 Hz sinusoidal and square waveforms but not for low-frequency sinusoidal stimuli (P < 0.001). EMG filters removed most stimulation artefacts for low-frequency sinusoidal stimuli (4 Hz). While balancing upright, participants' pain ratings were correlated with the force applied by the right leg (R2  = 0.65), demonstrating task-relevant changes in perceived pain intensity. Low-frequency sinusoidal stimuli can induce knee pain of constant intensity for 60 s with minimal EMG artefacts while enabling task-relevant pain modulation when controlling current amplitude. By enabling task-dependent modulation of perceived pain intensity, our novel experimental model replicates key temporal aspects of clinical musculoskeletal pain while allowing quantification of neuromuscular activation during painful electrical stimulation. This approach will enable researchers to test the predicted relationship between movement strategies and pain.

Mechanisms Underlying Lumbopelvic Pain During Pregnancy: A Proposed Model.

Up to 86% of pregnant women will have lumbopelvic pain during the 3rd trimester of pregnancy and women with lumbopelvic pain experience lower health-related quality of life during pregnancy than women without lumbopelvic pain. Several risk factors for pregnancy-related lumbopelvic pain have been identified and include history of low back pain, previous trauma to the back or pelvis and previous pregnancy-related pelvic girdle pain. During pregnancy, women go through several hormonal and biomechanical changes as well as neuromuscular adaptations which could explain the development of lumbopelvic pain, but this remains unclear. The aim of this article is to review the potential pregnancy-related changes and adaptations (hormonal, biomechanical and neuromuscular) that may play a role in the development of lumbopelvic pain during pregnancy. This narrative review presents different mechanisms that may explain the development of lumbopelvic pain in pregnant women. A hypotheses-driven model on how these various physiological changes potentially interact in the development of lumbopelvic pain in pregnant women is also presented. Pregnancy-related hormonal changes, characterized by an increase in relaxin, estrogen and progesterone levels, are potentially linked to ligament hyperlaxity and joint instability, thus contributing to lumbopelvic pain. In addition, biomechanical changes induced by the growing fetus, can modify posture, load sharing and mechanical stress in the lumbar and pelvic structures. Finally, neuromuscular adaptations during pregnancy include an increase in the activation of lumbopelvic muscles and a decrease in endurance of the pelvic floor muscles. Whether or not a causal link between these changes and lumbopelvic pain exists remains to be determined. This model provides a better understanding of the mechanisms behind the development of lumbopelvic pain during pregnancy to guide future research. It should allow clinicians and researchers to consider the multifactorial nature of lumbopelvic pain while taking into account the various changes and adaptations during pregnancy.

Regional activation in the human longissimus thoracis pars lumborum muscle.

KEY POINTS: •Longissimus activity in the lumbar region was measured using indwelling electromyography to characterize the territory of its motor units. •The distribution of motor units in the longissimus pars lumborum muscle was mainly grouped into two distinct regions. •Regional activation of the longissimus pars lumborum was also observed during functional tasks involving trunk movements. •The regional activation of the longissimus pars lumborum muscle may play a role in segmental stabilization of the lumbar spine. ABSTRACT: The longissimus pars lumborum contributes to lumbar postural control and movement. While animal studies suggest a segmental control of this muscle, the territory of motor units constituting the human longissimus pars lumborum remains unknown. The aims of this study were to identify the localization of motor unit territories in the longissimus and assess the activation of this muscle during functional tasks. Eight healthy participants were recruited. During isometric back extension contractions, single motor-unit (at L1, L2, L3 and L4) and multi-unit indwelling recordings (at L1, L1-L2, L2, L2-L3, L3, L3-L4 and L4) were used to estimate motor unit territories in the longissimus pars lumborum based on the motor-unit spike-triggered averages from fine-wire electrodes. A series of functional tasks involving trunk and arm movements were also performed. A total of 73 distinct motor units were identified along the length of the longissimus: only two motor units spanned all recording sites. The majority of the recorded motor units had muscle fibres located in two main rostro-caudal territories (32 motor units spanned L1 to L3 and 30 spanned ∼L3 to L4) and 11 had muscle fibres outside these two main territories. We also observed distinct muscle activation between the rostral and caudal regions of the longissimus pars lumborum during a trunk rotation task. Our results show clear rostral and caudal motor unit territories in the longissimus pars lumborum muscle and suggest that the central nervous system can selectively activate regions of the superficial lumbar muscles to provide local stabilization of the spine.

Short-term effect of delayed-onset muscle soreness on trunk proprioception during force reproduction tasks in a healthy adult population: a crossover study.

PURPOSE: The aim of this study was to evaluate the effects of lumbar muscle delayed-onset muscle soreness (DOMS) on the ability of the trunk muscles to reproduce different levels of force. METHODS: Twenty healthy adults (10 males and 10 females) were recruited for this study. Force reproduction in trunk extension and flexion was assessed at 50 and 75% of participants' maximal isometric voluntary contraction in flexion and extension before and after a lumbar muscle DOMS protocol. Trunk proprioception was evaluated and compared between these conditions using different variables such as constant errors (CE), absolute errors (AE), variable errors (VE) and time to peak force (TPF). For each variable, repeated measure ANOVAs were conducted. RESULTS: AE were higher when participants had to reach the target post-DOMS protocol in extension compared to flexion and in the presence of higher demand of force (p = 0.02). For VE, results showed that participants were more variable in extension than in flexion when the required force was higher (p = 0.04). CE variable was higher when participants had to reach the force target in extension compared to flexion under the effect of DOMS (p = 0.02). Results also showed that participants took less time to reach the force target post-DOMS protocol in extension (0.62 ± 0.20 s) and in flexion (0.53 ± 0.19 s) than pre-DOMS protocol in extension (0.55 ± 0.15) and in flexion (0.50 ± 0.20) (p < 0.001). CONCLUSION: Lumbar muscle DOMS affects trunk proprioception during force reproduction tasks especially in trunk extension and at higher force.

Paraspinal muscle function and pain sensitivity following exercise-induced delayed-onset muscle soreness.

PURPOSE: The aim of this study was to evaluate the effectiveness of an exercise protocol designed to induce delayed-onset muscle soreness (DOMS) in paraspinal muscles and its effects on low back functional capacities. METHODS: Twenty-four healthy participants were asked to perform four series of 25 trunk flexion-extension in a prone position (45° inclined Roman chair). The protocol was performed using loads corresponding to participant's trunk weight plus 10% of their trunk extension maximal voluntary contraction. Perceived soreness and pain were assessed using an 11-point numerical analogue scale three times a day during 5 day post-DOMS protocol. Pressure-pain thresholds (PPT) in paraspinal muscles (L2 and L4 bilaterally) and the vastus medialis (control site), and trunk extension maximal voluntary contraction were assessed 24-36 h post-protocol and compared to baseline (t tests). RESULTS: Muscle soreness (3.8/10) and pain (2.1/10) peak scores were observed 24-36 h post-protocol (mean of 28 h). A significant reduction in trunk extension maximal voluntary contraction was observed post-protocol (p = 0.005). Significant reductions in PPT were observed post-protocol for all trunk extensor sites (ps < 0.01), but not for the control site (p = 0.40). CONCLUSIONS: The exercise protocol efficiently led to low back muscle DOMS, reduced functional capacities, and increased pain sensitivity locally. Such protocol could be used as an efficient and safe experimental low back pain model.

Motor adaptations to trunk perturbation: effects of experimental back pain and spinal tissue creep.

In complex anatomical systems, such as the trunk, motor control theories suggest that many motor solutions can be implemented to achieve a similar goal. Although reflex mechanisms act as a stabilizer of the spine, how the central nervous system uses trunk redundancy to adapt neuromuscular responses under the influence of external perturbations, such as experimental pain or spinal tissue creep, is still unclear. The aim of this study was to identify and characterize trunk neuromuscular adaptations in response to unexpected trunk perturbations under the influence of spinal tissue creep and experimental back pain. Healthy participants experienced a repetition of sudden external trunk perturbations in two protocols: 1) 15 perturbations before and after a spinal tissue creep protocol and 2) 15 perturbations with and without experimental back pain. Trunk neuromuscular adaptations were measured by using high-density electromyography to record erector spinae muscle activity recruitment patterns and a motion analysis system. Muscle activity reflex attenuation was found across unexpected trunk perturbation trials under the influence of creep and pain. A similar area of muscle activity distribution was observed with or without back pain as well as before and after creep. No change of trunk kinematics was observed. We conclude that although under normal circumstances muscle activity adaptation occurs throughout the same perturbations, a reset of the adaptation process is present when experiencing a new perturbation such as experimental pain or creep. However, participants are still able to attenuate reflex responses under these conditions by using variable recruitment patterns of back muscles. NEW & NOTEWORTHY The present study characterizes, for the first time, trunk motor adaptations with high-density surface electromyography when the spinal system is challenged by a series of unexpected perturbations. We propose that the central nervous system is able to adapt neuromuscular responses by using a variable recruitment pattern of back muscles to maximize the motor performance, even under the influence of pain or when the passive structures of the spine are altered.

Trunk proprioception adaptations to creep deformation.

PURPOSE: This study aimed at identifying the short-term effect of creep deformation on the trunk repositioning sense. METHODS: Twenty healthy participants performed two different trunk-repositioning tasks (20° and 30° trunk extension) before and after a prolonged static full trunk flexion of 20 min in order to induce spinal tissue creep. Trunk repositioning error variables, trunk movement time and erector spinae muscle activity were computed and compared between the pre- and post-creep conditions. RESULTS: During the pre-creep condition, significant increases in trunk repositioning errors, as well as trunk movement time, were observed in 30° trunk extension in comparison to 20°. During the post-creep condition, trunk repositioning errors variables were significantly increased only when performing a 20° trunk extension. Erector spinae muscle activity increased in the post-creep condition, while it remained unchanged between trunk repositioning tasks. CONCLUSIONS: Trunk repositioning sense seems to be altered in the presence of creep deformation, especially in a small range of motion. Reduction of proprioception acuity may increase the risk of spinal instability, which is closely related to the risk of low back pain or injury.

Influence of Lumbar Muscle Fatigue on Trunk Adaptations during Sudden External Perturbations.

Introduction: When the spine is subjected to perturbations, neuromuscular responses such as reflex muscle contractions contribute to the overall balance control and spinal stabilization mechanisms. These responses are influenced by muscle fatigue, which has been shown to trigger changes in muscle recruitment patterns. Neuromuscular adaptations, e.g., attenuation of reflex activation and/or postural oscillations following repeated unexpected external perturbations, have also been described. However, the characterization of these adaptations still remains unclear. Using high-density electromyography (EMG) may help understand how the nervous system chooses to deal with an unknown perturbation in different physiological and/or mechanical perturbation environments. Aim: To characterize trunk neuromuscular adaptations following repeated sudden external perturbations after a back muscle fatigue task using high-density EMG. Methods: Twenty-five healthy participants experienced a series of 15 sudden external perturbations before and after back muscle fatigue. Erector spinae muscle activity was recorded using high-density EMG. Trunk kinematics during perturbation trials were collected using a 3-D motion analysis system. A two-way repeated measure ANOVA was conducted to assess: (1) the adaptation effect across trials; (2) the fatigue effect; and (3) the interaction effect (fatigue × adaptation) for the baseline activity, the reflex latency, the reflex peak and trunk kinematic variables (flexion angle, velocity and time to peak velocity). Muscle activity spatial distribution before and following the fatigue task was also compared using t-tests for dependent samples. Results: An attenuation of muscle reflex peak was observed across perturbation trials before the fatigue task, but not after. The spatial distribution of muscle activity was significantly higher before the fatigue task compared to post-fatigue trials. Baseline activity showed a trend to higher values after muscle fatigue, as well as reduction through perturbation trials. Main effects of fatigue and adaptation were found for time to peak velocity. No adaptation nor fatigue effect were identified for reflex latency, flexion angle or trunk velocity. Conclusion: The results show that muscle fatigue leads to reduced spatial distribution of back muscle activity and suggest a limited ability to use across-trial redundancy to adapt EMG reflex peak and optimize spinal stabilization using retroactive control.