Persistent motor system abnormalities in formerly concussed athletes

Abstract

Context: The known detrimental effects of sport concussions on motor system function include balance problems, slowed motor execution, and abnormal motor cortex excitability.

Objective: To assess whether these concussion-related alterations of motor system function are still evident in collegiate football players who sustained concussions but returned to competition more than 9 months before testing.

Design: Case-control study.

Setting: University laboratory.

Patients or other participants: A group of 21 active, university-level football players who had experienced concussions was compared with 15 university football players who had not sustained concussions.

Intervention(s): A force platform was used to assess center-of-pressure (COP) displacement and COP oscillation regularity (approximate entropy) as measures of postural stability in the upright position. A rapid alternating-movement task was also used to assess motor execution speed. Transcranial magnetic stimulation over the motor cortex was used to measure long-interval intracortical inhibition and the cortical silent period, presumably reflecting y-aminobutyric acid subtype B receptor-mediated intracortical inhibition.

Main outcome measure(s): COP displacement and oscillation regularity, motor execution speed, long-interval intracortical inhibition, cortical silent period.

Results: Relative to controls, previously concussed athletes showed persistently lower COP oscillation randomness, normal performance on a rapid alternating-movement task, and more M1 intracortical inhibition that was related to the number of previous concussions.

Conclusions: Sport concussions were associated with pervasive changes in postural control and more M1 intracortical inhibition, providing neurophysiologic and behavioral evidence of lasting, subclinical changes in motor system integrity in concussed athletes.

Figure 1.

A, Approximate entropy values expressed as the center-of-pressure (COP) oscillation regularity in the mediolateral and anteroposterior directions (range, 0–2). Greater approximate entropy values reflect more COP oscillation randomness. B, Root mean square amplitude of COP displacement for the anteroposterior and mediolateral directions. Smaller COP displacement amplitude reflects better postural stability. ^aP <. 05.

Figure 2.

A, Long-interval intracortical inhibition, expressed as the ratio of the conditioning stimulus-test stimulus/test stimulus. Intensity of the conditioning stimulus was set at 120% of the resting motor threshold, and test stimulus intensity was adjusted to induce motor evoked potentials of approximately 1 mV peak-to-peak amplitude. B, Cortical silent period duration when transcranial magnetic stimulation at 3 intensities (110%, 120%, and 130%), expressed as a percentage of the resting motor threshold, was applied to the vertex while participants in each group performed a voluntary isometric muscle contraction of the right hand first dorsal interosseous muscle at approximately 10% of maximum strength. ^aP < .05.