The International Classification of Functioning (ICF) defines the typical spectrum of problems in the functioning of patients with LBP and highlights the main areas and functions of interest in the study of LBP [48]. On the basis of the short version of ICF [49], the physical capacities of the patient will be assessed by assessing the flexibility of the hips in flexion and extension, the lumbar spine in flexion, the thoracolumbar segment in lateral flexion, and the trunk extensor and flexor muscle endurance. Functional abilities will be assessed from daily life activities such as gait, object lifting, forward bending, STS (and the reverse), balance, and usual posture (standing and sitting). Kinematics, kinetics, and sEMG will be assessed during the execution of these functional tasks.

The Sorensen test, which is considered as the gold standard for this measure [50], will be performed to determine trunk extensor endurance [51,52]. The participants will lie on the examining table in a prone position with the upper edge of their iliac crests' aligned along the edge of the table. The lower body will be fixed to the table by 3 straps located at the level of the pelvis, knees, and ankles. Meanwhile, the Shirado test will be performed to determine trunk flexor endurance [53]. The participants will lie on the examining table in a supine position and will raise their lower extremities until their scapulas' are off the table with a 90° flexion of the hip and knee joints. These tests are considered valid, safe, reliable, and easy to perform in participants with and without nonspecific chronic LBP [54,55]. Participants’ arms are folded across the chest for the duration of both trunk muscle endurance tests. Note that the participants will be asked to hold the original positions for as long as possible, but not exceeding a 240 s time limit. A 15 min rest is allowed between the two endurance tests.

The hip and trunk muscles flexibility in the nonspecific chronic LBP population will be evaluated using the straight leg raise test, the Thomas test, and the finger-tip-to-thigh test. These are valid, reliable, and largely used tests. The tests will assess hamstring flexibility [17,56-58], hip flexor flexibility (psoas-iliacus and rectus femoris) [59-62], and measure the lateral trunk range of motion [63,64]. All flexibility tests will be performed according to the methods set out by Norkin and White [65].

Participants’ balance in standing and sitting postures will be evaluated. For the sitting condition which limits the influence of the lower limb, the participants will be seated on an adjustable stool with the middle of the thighs on the edge of the stool and with their feet dangling. For the standing posture, the participants will stand with 10 cm between their heels and a self-selected angle between the feet [66]. For both postures, the participants must make sure their trunk is erect, fix their head in a neutral position, look ahead, keep arms along the trunk, and move as little as possible. The participants will stand on a force plate and their balance will be assessed under 4 conditions with 3 repeated trials of 30 s per condition. The conditions under which the balance of the participant will be assessed are eyes closed or opened, with stable or unstable support. To create the unstable support an Airex balance pad (50 cm length × 41 cm width × 6 cm thickness) will be used. The condition with the eyes closed are used to avoid visual compensations [67] and unstable conditions are used to challenge the participants’ balance [68].

The usual sitting and standing postures of each participant will be evaluated. For the sitting posture evaluation, an adjustable stool will be placed on a force plate and the participants will be asked to be seated in a self-selected position with their feet on another force plate. The stool height will be adjusted for each participant to fix the hip and knee flexion at 90°. For the usual standing posture evaluation, the participants will be asked to stand in a self-selected upright position with both feet on the same force plate. For both these usual static postures, the participants will look ahead, and the kinematic will be recorded for 10 s in the posture.

The participants will be asked to walk barefoot at 3 different speeds (ie, self-selected, fast, and slow) along a 10 m walkway to assess their gait. Data will be collected for at least 10 gait cycles for each participant and the speed will be monitored.

Two lifting tasks will be performed. For both tasks, the participants will start on a force plate in an upright standing position, bend down to lift a box and return to an upright standing position with 90° flexion of elbows holding the box. They will maintain this posture for 4 s, and then bend down to place the box to the ground before returning to the initial posture. This test will be performed under two conditions. The first condition is a usual lift, where the participants are asked to lift the box with a self-selected strategy, and no more instruction will be given [69]. The second condition is a standardized lift based on deadlift methods [70]. This will be used to compare the muscle strategies for the same movement between the subjects. The usual lift will be performed before the standardized lift to avoid behavioral adaptations.

Three trials will be performed per condition with 2 min rest between each condition. The weight of the box will be adjusted to 10% of the participant’s weight for each condition. The participant will be instructed to stay on the force plate for the duration of the test.

The participants will start standing in an upright position (standing phase), flex the trunk as far forward as possible with their knees extended (flexion phase), maintain this trunk full-flexion position (full flexion phase), and then return to an upright standing position (extension phase). Each phase will last for 4 s, and an audible metronome will be used to regulate the movement timing. Three trials will be performed, and only the second trial will be used for analysis [71].

The STS tasks will be performed under the following three conditions: (1) usual STS, (2) standardized STS, and (3) 5 consecutive STSs. In the usual condition, the participants will sit in a self-selected position on a stool placed on a force plate with their feet placed on another force plate. No more instructions on posture will be given for this condition. In the standardized STS, the participants will be barefoot and asked to sit upright on an adjustable stool with their trunk straight and arms crossed on the chest. The stool will be placed on a force plate, the participant’s feet will be placed on a second force plate, and the stool height will be adjusted for each participant to fix the hip and knee flexion at 90° in the starting position.

For both the usual STS and standardized STS, the participants will stand up after 4 s of sitting, maintain the upright standing position with knees fully extended for 4 s, return to the initial sitting position, and maintain it for 4 s. Three trials will be performed for both the STS and standardized STS tasks.

The 5 consecutive STS task provides information on the global capacity of the participant to perform the STS task. The participants will have the same start position as the standardized STS task. The participants will then be asked to perform 5 consecutive STS movements as fast as possible. As a precautionary measure, an investigator will stand near the participant to prevent possible falls. To evaluate the total task duration, the start and end points of the task will be defined by the mean value of the anterior-posterior center of the pressure displacement during the usual sitting phase before and after the task was completed [72].

The psychosocial profile will be explored using patient-reported outcomes to evaluatef anxiety, depression, functional disability, fear of pain, avoidance belief, and physical activity (PA). All the questionnaires will be self-completed before the experiments, except for the PA questionnaire which will be completed by the investigator with the participant during the course of the experiments.

Anxiety and depression are parameters which play an important role in the sustainability of pain; hence, they are factors of pain chronicity [42]. The Hospital Anxiety and Depression Scale (HADS) [73] is widely used to evaluate mental disorders [74] in the LBP population [75]. This study will use the French version of the HADS introduced by Lépine et al [76], which has been used in other studies conducted on French-speaking populations [77-79].

The functional capacity evaluation is recommended when studying LBP [80]. Functional capacity is indeed an interesting parameter to evaluate the interference of pain on daily life [81]. One of the most used and recognized assessment tools is the Oswestry Disability Questionnaire (ODI) [82], which is specific for LBP [83]. The French validated version of the ODI [84] will be used for this study.

A systematic review shows that pain catastrophizing can predict the degree of pain, disability, and mediated treatment efficacy in the nonspecific chronic LBP population [85]. The pain catastrophizing scale (PCS) was introduced by Sullivan et al [86], and his validated French version [87] will be used for this study.

Fear avoidance beliefs are reported to be factors for the delayed recovery and chronicity of pain in the nonspecific chronic LBP population [88]. One of the questionnaires used to identify fear avoidance beliefs is the Fear Avoidance Beliefs Questionnaire (FABQ) [89]. This study will use the French version of the FABQ validated by Chaory et al [90].

PA plays an important role in the prevention of nonspecific chronic LBP. The participant’s weekly PA will be assessed using the Global Physical Activity Questionnaire (GPAQ) developed by the World Health Organization [91]. This questionnaire has already been used in studies on the nonspecific chronic LBP population [92].

Pain is a key symptom in nonspecific chronic LBP, therefore, its evaluation during the course of the study is recommended [49,93]. The intensities of current pain, pain in the last 24 h, pain in the last week, pain in the last month, and pain in the last 3 months will be quantified with a visual analogue scale largely used in the nonspecific chronic LBP population [29,94-96].

The sEMG will be bilaterally collected from 3 back muscles (ie, lumbar multifidus, iliocostalis lumborum, and lumbar erector spinae), 2 abdominal muscles (ie, transverse fibers of the abdominal external oblique and rectus abdominus), gluteus medius, semitendinosus, and the rectus femoris muscle. Moreover, 16 active surface electrodes (model: Trigno, Delsys Inc, Boston, MA, USA) will be used to collect the sEMG signals at a sampling frequency of 1000 Hz. The skin at the electrode sites will be shaved, abraded, and cleaned with alcohol prior to measurement. The electrodes will then be positioned relative to the muscle fiber direction, following the surface EMG for noninvasive assessment of muscles project recommendations [97].

The sEMG activation pattern, time of cocontraction (TCC), and cocontraction index (CCI) [98] will be calculated for gait, STS, and lift tasks. The TCC is the time for simultaneous activation of a pair of muscle groups over a specified number of data points (activation thresholdbaseline+3 SD; duration activity threshold: 5 ms) [99]. The CCI is defined as the degree of coactivation for a pair of muscle groups over a specified number of data points. The TTC and CCI will characterize muscle coordination. The flexion-relaxation ratio will be calculated and used to detect and quantify exacerbated back muscle activity for trunk forward bending [71]. The muscle fatigability of the back and abdomen muscles will be evaluated using the sEMG median frequency evolution during endurance tasks [100].

The kinematic parameters will be recorded using a 12-camera motion analysis system (Oqus7+, Qualisys, Göteborg, Sweden) set at a sampling frequency of 100 Hz. The participants will have 35 reflective markers (14 mm diameter) placed on the skin at defined anatomical and technical landmarks on the head, trunk, and pelvis and bilaterally on the arms, thighs, shanks, and feet according to the full-body Plug-in-Gait model [101]. Additional markers will be placed on the spinous process of T2, T4, T6, T8, L1, L3, L5, and S1 to assess the sagittal plane curve of the spine [102,103].

The thorax, lumbar, pelvis, hip, knee, and ankle kinematics (maximum angle, range of motion, and speed) will be calculated in 3 planes for all tasks. The lumbar/hip ratio will be calculated for the trunk forward bending, STS, and lift tasks [104,105]. The relative phase between the pelvis and the thorax segment and the spatiotemporal parameters (ie, walking speed, cadence, stance phase, and step length) will be calculated during gait [106]. The thorax movement during the balance tasks will characterize the trunk sway [107].

Two force plates (AMTI Accugait, Watertown, NY, USA) at a sampling frequency of 1000 Hz will be used to measure the ground reaction forces. The center of pressure displacement (range and speed) will be calculated for the balance tasks to assess the balance capacity [68,108,109]. All kinetic, kinematic, and sEMG data will be synchronized together.