Understanding the Consequences of Repetitive Subconcussive Head Impacts in Sport: Brain Changes and Dampened Motor Control Are Seen After Boxing Practice

Abstract

Objectives: The potential effects of exposure to repetitive subconcussive head impacts through routine participation in sport are not understood. To investigate the effects of repetitive subconcussive head impacts we studied boxers following customary training (sparring) using transcranial magnetic stimulation (TMS), decomposition electromyographic (EMG) and tests of memory.

Methods: Twenty amateur boxers performed three 3-min sparring bouts. Parameters of brain function and motor control were assessed prior to sparring and again immediately, 1 h and 24 h post-sparring. Twenty control participants were assessed following mock-sparring.

Results: One hour after sparring boxers showed increased corticomotor inhibition, altered motor unit recruitment strategies, and decreased memory performance relative to controls, with values returning to baseline by the 24 h follow up.

Conclusion: Repetitive subconcussive head impacts associated with sparring resulted in acute and transient brain changes similar to those previously reported in soccer heading, providing convergent evidence that sport-related head impacts produce a GABAergic response. These acute changes in brain health are reminiscent of effects seen following brain injury, and suggest a potential mechanism underlying the damaging long-term effects of routine repetitive head impacts in sport.

Keywords: TBI; boxing; motor unit behavior; subconcussive head impacts; transcranial magnetic stimulation.

FIGURE 1

(A) Difference of cSP for sparring and control groups, relative to baseline. Inhibition appeared increased following sparring, peaking at the 1 h mark with a 6% increase, and returning to pre-values by the 24 h follow up. ^∗p < 0.002 following sparring; error bars indicate 95% CI. (B) Change in cSP duration for each participant from baseline to 1 h following sparring.

FIGURE 2

Difference of corticospinal excitability for sparring and control groups, relative to baseline. Excitability appears slightly increased immediately and 1 h post sparring (though not significant). ^∗p < 0.05; error bars denote 95% CI.

FIGURE 3

Difference of the slope coefficient (A) and y-intercept (B) of the firing regression relative to baseline. There appeared to be negative shift in the regression following sparring, peaking at the 1 h time-point. ℘p = 0.011; τp = 0.008; ψp = 0.026; error bars indicate 95% CI relative to baseline.

FIGURE 4

Difference in cognitive performance following sparring or control exercise, relative to baseline. (A) PAL performance was significantly lower following sparring compared to baseline (p = 0.01), although there was no interaction effect. (B) SWM showed an interaction effects between groups (p = 0.002), though not significantly different from baseline.

FIGURE 5

Comparing the recruitment threshold (%MVC) of the individual motor units (MUs) against their average firing rate (pulses per second – p/s) using a linear regression shows a negative shift in the regression line 1 h following sparring. The finding suggests that the recruitment patterns and the firing rates of the MUs as a whole are altered; the early recruited, small motor units are activated later compared to baseline, and the frequency with which they propagate on the sarcolemma of the muscle is slowed down (showed by an increase in the average inter-pulse interval – IPI, and a decrease in how many pulses are discharged per second). The later recruited, big motor units are activated earlier compared to baseline, yet also show an increase in IPI and a decrease in the pulses discharged per second.