Inflection points in longitudinal models: Tracking recovery and return to play following concussion

Abstract

The return to play (RTP) process may occur during longitudinal studies tracking recovery after concussion. This factor, which is often omitted within statistical designs, could affect the fit and overall interpretation of the statistical model. This article demonstrates the difference in results and interpretation between 2 linear mixed-model designs: (1) a between-group longitudinal (GROUP) analysis and (2) a between-group longitudinal model that used an inflection point to account for changes around the time of RTP (RTP analysis). These analyses were conducted on instrumented balance data collected on 23 concussed athletes and 25 controls over 8 weeks following concussion. Total sway area and the range of mediolateral acceleration were used as outcome measures. No significant findings were found in the GROUP design for either outcome measure. In contrast, the RTP analysis revealed significant effects of time (P = .007) and RTP change (P = .007), and group*time (P = .028) and group*RTP change (P = .022) interactions for total sway area, and effects of group (P = .011), time (P = .010), and RTP change (P = .014), and group*time (P = .013) and group*RTP change interactions (P = .013) for range of mediolateral acceleration. For both outcomes, the RTP model fit the data significantly better on comparison of likelihood ratios (P ≤ .027). These results suggest that allowing for an inflection point in the statistical design may assist understanding of what happens around clinically meaningful time points. The choice of statistical model had a considerable effect on the interpretation of findings, and provokes discussion around the best method for analyzing longitudinal datasets when important clinical time points like RTP exist.

Longitudinal Assessment of Balance and Gait After Concussion and Return to Play in Collegiate Athletes

“…Our approach and statistical design have been previously reported. 32 Linear mixed models were fit to each outcome measure to assess whether groups differed over the recovery period. Group was entered as a fixed factor to assess group differences.…”

“…Healthy participants were tested using the same time intervals for comparison. 32 Athletes completed testing sessions at their universities of enrollment. For consistency across universities, testing was conducted in a well-lit, straight hallway with a firm surface.…”

“…Our balance-assessment procedures have been previously described. 32 Briefly, balance assessment was conducted using (1) the Balance Error Scoring System (BESS) protocol 12,35 to obtain the subjective clinical error count and (2) instrumented sway metrics. The BESS test was modified to 30 seconds of stance to obtain instrumented measures, 36,37 a requirement for collecting more reliable sway data.…”

Inertial Sensors Reveal Subtle Motor Deficits When Walking With Horizontal Head Turns After Concussion

“…Gait and balance was longitudinally tested in people with SRC over 9 times over 8 weeks at a mean (SD) timeline of 2 (0.6), 5 (1), 10 (0.7), 17 (3), 23 (3), 30 (3), 37 (2), 44 (2), and 51 (2) days post-concussion, as described by Parrington et al 24 . Control subjects were tested on a similar schedule of 2 (0), 5 (0.7), 10 (1), 17 (1), 25 (1), 31 (1), 38 (2), 45 (1), and 52 (1).…”

“…Participant recruitment, inclusion, and exclusion criteria was previously reported by Parrington et al 24 . Briefly, twenty-four collegiate athletes who suffered a sports-related concussion (SRC) and 25 healthy control athletes were recruited from colleges and universities in the greater Portland, Oregon area.…”