Background. Alterations in the neural drive to trunk muscles have been implicated in low back pain (LBP). This is supported by evidence of reduced corticospinal excitability, delayed muscle activation, reduced endurance and enhanced fatigability of these muscles; whether these changes persist during pain free periods remain unclear. Neural drive (or voluntary activation-VA) can be measured using twitch interpolation and the aim of this study is to investigate if subjects with a history of LBP show reduced VA. Methods. Twenty five subjects participated (13 with a history of LBP, 12 controls). Back extensor torque was measured using a dynamometer and bilateral electromyographic (EMG) activity was recorded from erector spinae and rectus abdominis. Transcranial magnetic stimulation of the motor cortex was applied while the subject, lying prone, performed graded voluntary back extensions. VA was calculated from the size of the twitches evoked by the TMS and EMG data were analysed for evidence of altered neural drive. Results. The LBP typical VAS pain scores were 3.39±1.76(SD), with worst pain being 5.92±2.29. There were no differences in the physical activity scores between the groups. EMG data revealed no differences in the evoked responses at varying levels of voluntary torque. VA was not significantly different between the LBP and control groups (LBP: 85.30±6.45% vs C: 80.14±11.40%). Discussion. These data show that in our cohort of subjects with a history of LBP, their ability to fully activate their back muscles maximally is not reduced. Whether subjects with current LBP exhibit reduced VA remains to be established. No conflicts of interest. Funded by Imperial College London. This abstract has not been previously published in whole or substantial part nor has it been presented previously at a national meeting

Normal function of the patellofemoral joint is maintained by a complex interaction between soft tissues and articular surfaces. No quantitative data have been found on the relative contributions of these structures to patellar stability. Eight knees were studied using a materials testing machine to displace the patella 10 mm laterally and medially and measure the force required. Patellar stability was tested from 0° to 90° knee flexion with the quadriceps tensed to 175 N. Four conditions were examined: intact, vastus medialis obliquus relaxed, flat lateral condyle, and ruptured medial retinaculae. Abnormal trochlear geometry reduced the lateral stability by 70% at 30° flexion, while relaxation of vastus medialis obliquus caused a 30% reduction. Ruptured medial retinaculae had the largest effect at 0° flexion with 49% reduction. There was no effect on medial stability. There is a complex interaction between these structures, with their contributions to loss of lateral patellar stability varying with knee flexion.