Eliciting Quadriceps Inhibition with the Nociceptive Withdrawal Reflex: Examining Mechanisms of Arthrogenic Muscle Inhibition
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Brito Lienert, Marisa
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Abstract
Arthrogenic muscle inhibition (AMI) is the reflexive inhibition of muscles, particularly the quadriceps, due to a knee injury condition. Its mechanisms are not well understood, and there is currently no objective assessment, or treatment for AMI. The nociceptive withdrawal reflex (NWR) is a spinal reflex that triggers rapid limb withdrawal in response to nociceptive stimuli. While activating the flexor muscles of the lower leg, the NWR inhibits the extensor muscles. By examining the activity of the quadriceps during the NWR, this project aimed to examine the influence of nociceptive stimulation on ongoing quadriceps activity as a potential new biomarker of quadriceps inhibition. To that end, stimulation was applied to the medial plantar nerve on the sole of the foot to elicit an NWR response in the tibialis anterior (TA). The NWR was tested under resting and active quadriceps conditions. The active condition involved an isometric knee extension targeting approximately 10-15% of the participant’s maximum voluntary contraction. After establishing resting and active NWR threshold, 15 trials at active threshold and 1.3x active threshold were conducted to further test inhibition. Electromyography (EMG) of the TA, vastus lateralis (VL) and vastus medialis (VM) were recorded to monitor NWR response and quadriceps inhibition. Results showed that the active NWR threshold was greater than the resting NWR threshold. For inhibition, comparisons were made between muscles (VL and VM) and between stimulus intensities (threshold and 1.3x threshold). Compared to the VL, the VM had a greater magnitude of inhibition at active threshold and both a greater duration and magnitude at 1.3x active threshold. Compared to threshold stimulation, the 1.3x threshold stimulation elicited a greater magnitude of inhibition in the VL, and both a greater duration and magnitude in the VM. This suggests that the intensity of ascending nociceptive signals influences the amount of quadriceps inhibition. By investigating AMI using NWR in an active condition, this study provides a novel perspective on AMI mechanisms, offering more context to consider when improving treatment outcomes.
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Undergraduate Research Option Thesis