THE EFFECTS OF CONCUSSION ON MOTOR NEURON REFLEX EXCITABILITY

Objective: Individuals who participate in high impact sports are prone to concussive injuries. Assessments used to measure concussion severity mainly examine cognitive domains and only minimally assess the motor system. Testing the motor system is important for athletes whose performance and safety rely heavily on motor control. To our knowledge, there are limited studies assessing motor dysfunction at the spinal level. The goals of this study were to assess the effect that a head injury has on 1) resting spinal motor neuron reflex excitability (MNRE) and 2) MNRE levels over time when preparing for a volitional movement (Motor Preparation). Participants and Methodology: Sixty-four participants were recruited from high risk club sports teams at The University of Rhode Island and received baseline reflex excitability testing. MNRE was assessed using the standardized soleus H-reflex technique (Riego and Ruegg, 1987). Motor preparation (MP) was evaluated by randomly eliciting a series of H-reflexes before and after a "GO" signal was given to the participant. H-reflexes were elicited at 11 sampling times from 300ms before to 200ms after the “GO” signal at 50ms intervals. Three evoked H-reflex waves were measured and averaged for each sampling time. H-reflexes were elicited at 15-25% of Mmax, amplified 1000X and digitized at a sampling frequency of 4000Hz. Follow up experiments were conducted on four participants who reported a head or neck injury that may have resulted in concussion. The returning participants completed three postinjury evaluations at three separate times, 2-3 (P1), 6-12 (P2) and 13+ (P3) days post injury. Sixty-four participants were baseline tested. Data analyses were conducted on these four data sets. The change in H-reflex amplitude was used to assess spinal MNRE at rest and during MP. Descriptive statistics, paired t-tests and Cohen D effect size (ES) were used to detect changes between preand postconcussion values. Results: Resting MNRE showed a moderate to large reduction in post-concussion values when compared with baseline for two of the three post-injury testing sessions (P1, ES = 0.448; P2 ES=0.842). By the third follow up study, resting MNRE was approaching values similar to those obtained at baseline (P3, ES= 0.025) MP showed a moderate inhibition (~25% decline) of the H-reflex across all sampling times for the P2 testing session (ES= 0.45-0.76). This flat depressive response was not characteristic of the normal response observed at baseline testing. Conclusion: These results show that head injury may affect normal MNRE at rest and modulation of MNRE in preparation for volitional movement. The inhibition of reflex activity appears to continue until 6-12 days post injury. If soleus muscle findings in this study are extrapolated to other muscles, our results suggest that mTBI/concussion may contribute to decreased motor function that can last days to weeks after injury. Therefore, we suggest athletes who sustain a concussion have a full motor evaluation before being allowed to return to play or wait approximately 12 days after injury. Further study is needed to determine what the functional outcome our results may impose on sports performance.