Pain and muscle fatigue are two factors that are linked to musculoskeletal injuries. Not only do both factors interact with each other, but their interactions depend on the specific tasks used to induce pain or fatigue. The goal of this thesis was to explore the interaction between low back pain developed from prolonged standing and fatigue of the hip abductors muscle group. Specifically, how does perturbing those who do and do not develop pain in standing postures using muscle fatigue affect the muscular responses and postures adopted while standing? The hip abductors were chosen as they represent the muscle group with the greatest potential to influence other muscular or postural responses during standing with respect to musculoskeletal function and pain generation.

Forty young healthy participants (20 male, 20 female) were recruited to perform two sessions of standing work for two hours each. Participants performed a side-lying leg raising (hip abduction) exercise to fatigue prior to one of the standing sessions, the other session acted as a control. Surface electromyography (EMG) of six muscles bilaterally, motion capture of the trunk and lower limbs, and force plate data under each foot were measured continuously during each standing session. EMG data were also collected during fatiguing exercise trials. Self-reported pain and isometric hip abductor strength were assessed at discrete time points using 100 mm visual analog scales and uni-axial force transducers tethered to participant’s legs respectively. Pain measures were taken at baseline and every 15 minutes during standing in each session. Strength measures were taken at time points 0 minutes, 1 minute, 2 minutes, 3 minutes, 5 minutes, 10 minutes, 15 minutes 30 minutes and every 15 minutes thereafter during standing and were normalized to a baseline strength measure occurring immediately after instrumentation. EMG data from the standing exposures were expressed as a percentage of maximal voluntary isometric contraction then used to compute cocontraction indices of 10 muscle pairs and resting gap measures on all 12 muscles. EMG data from the exercise protocol were used to assess fatigue through decreases in mean power frequencies. Motion capture data were used to compute low frequency postural responses through averaging lumbar spine, pelvic tilt and bilateral hip angles in 15 minute blocks. Force plate data were used to quantify transient movements through centre of pressure motion and medial-lateral body weight transfers. Pain scores were used to classify participants into pain developers (PDs) and non-pain developers (NPDs) based on a 10 mm threshold in low back pain scores during the control standing session.

PDs (8 male, 8 female) reported decreases in low back pain while standing during the fatigue session compared to the control session (10.9 ± 11.7 mm reduction) while NPDs reported minimal changes in pain with fatigue (1.5 ± 3.9 mm increase). While there were decreases, PDs were still experiencing low back pain of intensities above the 10 mm threshold in the fatigue session. Also, male and female PDs reported different pain patterns during the fatigue session. A decrease in trunk (1033.9 ± 528.6 %MVIC reduction across R-LES/R-EXO, L-LES/L-EXO and R-LES/L-LES CCIs in first hour) and gluteal cocontraction indices (398.8 ± 792.4 %MVIC decrease in the 15 minute block) and an increase in the number of anterior-posterior centre of pressure fidgets (13.5 ± 25.4 increase in the 30 minute block) within the first hour of the fatigue session were associated with fatigue-related pain reductions in PDs of both genders. Female PDs had more posterior pelvic tilt with fatigue (3.9 ± 9.2° more posterior with fatigue from 30 to 90 minutes) that distinguished them from male PDs, whose pelvises were more anteriorly tilted with fatigue (7.0 ± 11.3° more anterior with fatigue from 15 to 120 minutes). Additional changes seen exclusively in PDs during the fatigue session, such as larger force residuals with time, lateral migration of centre of pressures and increases in tensor fascia latae activity, indicate that hip abductor fatigue did not recover while standing for PDs.

The fatigue protocol resulted in reductions in mean power frequency in four to nine of the 12 muscles measured, and affected both pain groups similarly with respect to EMG frequency shifts and strength lost. Females had longer times to fatigue (F:​ 21.7 ± 12.5 minutes;​ M:​ 17.2 ± 7.1 minutes;​ p = 0.0031) and smaller force losses with fatigue than males (F:​ 7.2 ± 10.4 %Baseline;​ M:​ 12.8 ± 11.1 %Baseline;​ p = 0.0500), and NPDs had longer times to fatigue than PDs (NPDs:​ 20.1 ± 11.2 minutes;​ PDs:​ 18.7 ± 9.3 minutes;​ p = 0.0106).

This study provides evidence that the hip abductor musculature is likely one causative factor in the low back pain developed from standing, although it appears as though both static and dynamic postural responses are important in the low back pain pathway of prolonged standing. Also, this study provides further evidence that a two hour standing exposure can identify those likely to develop chronic low back pain in the future in that PDs had fatiguing characteristics of persons suffering from chronic low back pain, only without any pain present. Exercise protocols aimed at fatiguing a single hip abductor do not appear to be muscle-specific, and the muscles fatigued by gross motor exercise are different between participants and cannot be predicted based on low back pain development during standing.