Purpose:​ Biomechanical differences between people with low back pain (LBP) and healthy controls have been shown in multiple research studies. Because of the typically used cohort research design, it is impossible to determine whether these are predisposing factors or adaptive responses to the LBP condition. LBP development has been associated with static standing postures in occupational settings, and previous experimental work has shown that a percentage of individuals will develop considerable LBP with a 2-hour functional standing exposure. This transient pain-generating model allows for comparisons between pain developers (PD) and non-pain developers (NPD) to determine whether pre-disposing factors exist. There were two major objectives for this thesis. The first was to utilize a multifactorial approach including physiotherapy clinical assessment tools, psychosocial assessments, and biomechanical measures to characterize differences between PD and NPD individuals. Acute responses to the 2-hour standing exposure were also investigated. The second objective was to investigate the impact of an exercise intervention on LBP development during standing, and to determine whether the intervention resulted in changes in any of the previously identified factors.

Methods:​ Forty-three participants without any prior history of LBP volunteered for this study. A clinical assessment was conducted on each participant by a licensed physiotherapist. Participants completed a compilation of psychosocial questionnaires. Participants performed a trunk extensor endurance test, pre- and post-standing functional movements (forward flexion in standing, squatting, single leg stance), and 2-hours of standing. Continuous electromyography (EMG) data were collected from 16 trunk and hip muscles, kinematic and kinetic data were used to construct an 8-segment rigid link model to calculate time-varying 3-dimensional trunk and pelvis angles and the reaction moment at the L₅S₁ joint. Average and peak vertebral joint rotation stiffness (VJRS) measures during the functional tasks were calculated with an EMG-assisted anatomical model. Participants completed 0-100 mm visual analog scales (VAS) rating their LBP every 15 minutes during the 2-hr standing exposure. Participants were classified as PD or NPD based on the threshold criteria of a greater than 10 mm increase in VAS score during standing. Participants were randomly assigned to exercise (EX) intervention or control (CON) groups. The EX group completed a standardized home exercise program (HEP) focused on hip and trunk control during dynamic movement for 4-weeks. There were weekly meetings with the investigator to monitor and progress the HEP. The CON group was instructed to maintain their usual activity level during the 4-week period. All participants returned for a second data collection following the 4-week period.

Results:​ Forty percent of participants developed LBP during the 2-hours of standing. The PD group had hypoactivity of the gluteus maximus muscles during standing forward flexion compared to NPD (p < 0.05). The PD group had elevated gluteus medius and trunk flexor/extensor muscle co-activation prior to reports of pain development (p < 0.05). PD and NPD demonstrated markedly different patterns of muscle activation during the 2-hr standing exposure, with PD decreasing trunk flexor/extensor co-activation as standing duration progressed. PD demonstrated higher average hip muscle activation levels during standing, with shorter muscle rest periods than NPD. There were no PD/NPD group differences in questionnaire responses, total VJRS, or extensor muscle fatigability. The only clinical assessment tool that predicted LBP development was the active hip abduction test.

Following 2-hours of standing, there was a decrease for all participants in VJRS about the lateral bend axis during unilateral stance. PD had increases in peak gluteus medius muscle activation during single leg stance (SLS). Males showed a decrease in stability during unilateral stance as shown by increased centre-of-pressure excursion.

PD_EX had decreased VAS scores during the second data collection (p = 0.007) compared with PD_CON. There was a trend (p = 0.08) for PD_EX to show improvement on the active hip abduction test. Male PD_EX had a decrease in gluteus medius co-activation during standing (p < 0.05). PD_EX had increased trunk flexor/extensor co-activation during the middle stages of standing. PD_EX showed a variable response in gluteus medius muscle rest periods during standing compared with the other groups. Between-day repeatability for the CON groups was excellent with intraclass correlation coefficients > 0.80 (p < 0.05) for the majority of the outcome measures.

Conclusions:​ There were clear differences between PD/NPD groups in muscle activation patterns, prior to subjective reports of LBP, supporting the hypothesis that some of the differences observed between these groups may be predisposing rather than adaptive. A prolonged standing exposure does not appear to have detrimental effects on functional movement performance. An exercise intervention resulted in positive changes in the PD group, both in subjective pain scores as well as muscle activation profiles. Control groups had excellent between-day repeatability, showing that in the absence of intervention these outcome measures remain stable over time. Elevated gluteus medius and trunk co-activation in the first 15-30 minutes of standing may indicate that an individual is at increased risk for LBP during standing. Trunk co-activation during a sustained postural task may be beneficial, and can be facilitated through appropriate exercise intervention.