The effects of experimental knee pain on lower limb corticospinal and motor cortex excitability.
BACKGROUND
Notable weakness of the quadriceps muscles is typically observed as a consequence of knee joint arthritis, knee surgery and knee injury. This is partly due to ongoing neural inhibition that prevents the central nervous system from fully activating the quadriceps, a process known as arthrogenic muscle inhibition (AMI). To investigate the mechanisms underlying AMI, this study explored the effects of experimental knee pain on lower limb corticospinal and motor cortex excitability.


METHODS
Twenty-four healthy volunteers participated in this study. In experiment 1, experimental knee pain was induced by the injection of hypertonic saline into the infrapatellar fat pad (n = 18). In experiment 2, isotonic saline was injected into the fat pad as a non-painful control (n = 8). Pain intensity was measured on a 10-cm electronic visual analogue scale. Transcranial magnetic stimulation and electromyography were used to measure lower limb motor-evoked potential amplitude and short-interval intracortical inhibition before and after the injection.


RESULTS
The peak VAS score following hypertonic saline (5.0 ± 0.5 cm) was higher than after isotonic saline (p <0.001). Compared with baseline, there was a significant increase in vastus lateralis (p = 0.02) and vastus medialis motor-evoked potential amplitude (p = 0.02) during experimental knee pain that was not apparent during the control condition. Biceps femoris and tibialis anterior motor-evoked potential amplitude did not change following injection (all p >0.05). There was no change in short-interval intracortical inhibition measured from vastus lateralis following injection (both p >0.05).


CONCLUSIONS
Quadriceps corticospinal excitability increases during experimental knee pain, providing no evidence for a supraspinal contribution to quadriceps AMI.
Health and Rehabilitation Research Institute, Auckland University of Technology, 55 Wellesley Street East, Auckland, 1010, New Zealand. david.rice@aut.ac.nz.|Waitemata Pain Services, Department of Anaesthesiology and Perioperative Medicine, Waitemata District Health Board, Shakespeare Road, Auckland, 0620, New Zealand. david.rice@aut.ac.nz.
David Andrew
DA
Rice
Center for Sensory-Motor Interaction (SMI), Department of Health Science and Technology, Faculty of Medicine, Aalborg University, Fredrik Bajers Vej 5, 9100, Aalborg, Denmark. tgn@hst.aau.dk.
Thomas
T
Graven-Nielsen
Health and Rehabilitation Research Institute, Auckland University of Technology, 55 Wellesley Street East, Auckland, 1010, New Zealand. gwyn.lewis@aut.ac.nz.
Gwyn Nancy
GN
Lewis
Health and Rehabilitation Research Institute, Auckland University of Technology, 55 Wellesley Street East, Auckland, 1010, New Zealand. peter.mcnair@aut.ac.nz.
Peter John
PJ
McNair
Department of Medicine, University of Auckland, 2 Park Road, Auckland, 1023, New Zealand. n.dalbeth@auckland.ac.nz.
Nicola
N
Dalbeth
D000328:Adult; D019054:Evoked Potentials, Motor / Q000502:physiology*; D005260:Female; D006801:Humans; D007719:Knee Joint / Q000473:pathology / Q000502:physiology*; D035002:Lower Extremity / Q000294:innervation / Q000502:physiology; D008297:Male; D009044:Motor Cortex / Q000502:physiology*; D010146:Pain / Q000175:diagnosis* / Q000503:physiopathology; D010147:Pain Measurement / Q000379:methods; D011712:Pyramidal Tracts / Q000502:physiology*; D052097:Quadriceps Muscle / Q000294:innervation / Q000502:physiology*; D055815:Young Adult