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Investigating differences in trunk muscle activity in non-specific chronic low back pain subgroups and no-low back pain controls during functional tasks: a case-control study

Background Trunk muscle dysfunction is often regarded as a key feature of non-specific chronic low back pain (NSCLBP) despite being poorly understood and variable with increases, decreases and no change in muscle activity reported. Differences in thoraco-lumbar kinematics have been observed in motor control impairment NSCLBP subgroups (Flexion Pattern, Active Extension Pattern) during static postures and dynamic activities. However, potential differences in muscle activity during functional tasks has not been established in these subgroups to date. Methods A case-control study design recruited 50 NSCLBP subjects (27 Flexion Pattern, 23 Active Extension Pattern) and 28 healthy individuals. Surface electromyography determined muscle activity during functional tasks: reaching upwards, step-down, step-up, lifting and replacing a box, stand-to-sit, sit-to-stand, bending to retrieve (and returning from retrieving) a pen from the floor. Normalised (% sub-maximal voluntary contraction) mean amplitude electromyography of bilateral musculature (transversus abdominis/internal oblique, external oblique, superficial lumbar multifidus and longissimus thoracis) was analysed using Kruskal-Wallis and post-hoc Mann-Whitney U tests. Results Transversus abdominis/internal oblique activity was significantly increased in the Flexion Pattern group compared to controls during stand-to-sit (p = 0.009) on the left side only. External oblique activity was significantly greater in the Active Extension Pattern group compared to controls during box lift (p = 0.016) on the right side only. Significantly greater activity was identified in the right Superficial lumbar multifidus during step up (p = 0.029), reach up (p = 0.013) and box replace (p = 0.007) in the Active Extension Pattern group compared to controls. However left-sided superficial lumbar multifidus activity was significantly greater in the Flexion Pattern group (compared to controls) only during stand-to-sit (p = 0.009). No significant differences were observed in longissimus thoracis activity bilaterally during any task. No significant differences between NSCLBP subgroups were observed. Conclusions Muscle activity in these NSCLBP subgroups appears to be highly variable during functional tasks with no clear pattern of activity identified. The findings reflect inconsistencies and variability in trunk muscle activity previously observed in these NSCLBP subgroups. Further work evaluating ratios of muscle activity and changes in muscle activity throughout task duration is warranted.


Sub-Maximal Voluntary Contraction Procedures
A crook-lying double leg raise was used to achieve SMVC of the abdominal muscles (knees 90°, hips 45°, feet lifted approximately 1cm off the bed, held for 3 seconds). For the LT and sLM muscles, SMVC values were obtained from a prone lying double knee lift, with the subject lying prone on the plinth (knees 90°, knees lifted approximately 5cm off the bed, held for 3 seconds).

Functional Task Protocols
Sit-to-Stand-to-Sit For the sit-to-stand task the plinth height was individually standardised to a height where the subjects' hips and knees were resting comfortably at 90 degrees, measured using a goniometer (Lafayette Instrument Co. Ltd., Lafayette, IN) with the thighs well supported on the plinth. Sit to stand was performed from a usual sitting position. The subject was instructed to sit in their usual (unsupported) sitting position on the plinth, wait for 2 seconds in standing, then return to the original position.

Box
To measure a standardised distance for moving the box during the rotational box task tape was placed at a distance equal to 70% of the total upper limb (UL) length from the midline of the plinth (NB: total upper limb length was measured in cm from the apex of the acromion process to the distal end of the middle phalanx of each hand). For this task the plinth was also set to the height of the individuals' greater trochanter. To perform the task a 2.5kg box was placed over the marked line to the left hand side of the plinth. The subject was instructed to stand with the plinth in front and move the box from left to right (to a position over the line to the right hand side) with the box starting and finishing facing the same direction. No specific directions regarding how to lift were given, however the subject was instructed to stand in a comfortable position and keep their feet stationary throughout the task. At the end of the task the subject returned to their usual standing position.

Reaching
The shelf used in the reaching task was set to the height of the ulna styloid process (right upper limb) when the shoulder was in full flexion (fully elevated). The subject stood directly in front of the custommade shelf, with the shelf base in-line with the midline of the trunk (frontal plane). The subject placed a jar onto the shelf using their right hand, allowed the jar to rest on the shelf for 2 seconds (without releasing from their hand) and returned the jar to the original position. Feet were kept stationary throughout and the subject was instructed to keep their heels on the floor at all times. The subject also kept hold of the jar at all times throughout the task.
Stepping up and down Subjects were instructed to stand in front of a 6-inch Reebok® step (Adidas International Trading, Amsterdam, Netherlands), step onto the step (with a self-selected leading-leg), wait in double-stance on top of the step for 2 seconds, and then step down (with a self-selected leading-leg). The subject was instructed that the self-selected leading-leg must remain consistent throughout trials. To ensure data could be analysed effectively in MATLAB the subject was required to wait in their usual standing position following the step down for 2 seconds to enable the end task position to be defined.
Bending to pick up a pen (and return) Subjects stood in their usual standing position with a pen (with a marker attached) placed at a point 40cm in front of them on the floor. Subjects were instructed to pick up the pen from the floor and return to their usual standing position. Subjects were encouraged to pick up the pen in whichever way they felt was most natural, however they were instructed to keep their feet stationary throughout the task. Subjects were asked to pick up the pen with their right hand to standardise the movement between subjects.

Data Processing of Functional Tasks
The five activities outlined previously (sit-to-stand-to-sit, box lift rotate and replace, bend to pick up pen, step up and down and reaching) were sub-divided into 9 separate tasks in MATLAB as outlined in the table below: *NB: only the lifting and replacing components of the task were analysed in the current study. Step up and down Step Up Step Down

Reach Up
Reach Down (NB: not included in analysis)