Concussion management and return to activity decision-making is a complicated process. Evidence to support management practices is scarce. Expert consensus guidelines have been developed to assist with this process.1–5 Most return-to-play guidelines are initiated with activity limitations until asymptomatic. Once asymptomatic, patients begin a gradual return to activity protocol.4,6 Because the majority of patients will become asymptomatic within a relatively short time frame,7 they respond well to activity limitations and gradual return to activity.
However, research has shown that approximately 30% of children will experience concussion symptoms beyond 4 weeks.8 In following the standard guidelines for returning to activity, these children would be limited in their physical activities during this time.9 Patients who are inactive for prolonged periods can experience secondary complications that delay recovery.10 A small randomized controlled trial on physical and cognitive rest indicated that 1 to 2 days of strict rest (no school, work, or physical activity) was superior to 5 days of rest for recovery.11 The ideal length of activity limitation after concussion is unknown.11 Removal from meaningful activities has been found to increase feelings of depression and stress, both of which are symptoms of concussion.12 Prolonged activity limitations increase concussion-like symptoms that often results in the recommendation of further activity limitations.
Recently, there has been a shift in concussion management that follows a more active approach, specifically for those who experience persisting symptoms. This approach encourages low-level, individualized activity that is believed to assist with recovery.13–17 Exercise-based strategies have been used in children and adults with promising preliminary results.
Exercise immediately after concussion has been shown to prolong recovery in animals18 and humans.19 Vigorous activity within 2 days of concussion led to longer recovery compared with those exercising at lower intensities.20 Conversely, after a period of rest in the acute phase, exercise is associated with improvement in concussion symptoms. However, most studies that have used exercise as a postconcussion intervention have started exercise at least 1 month after injury.13,21–24 Early exercise may promote recovery, assuming it is safe to do so. The purpose of this study was therefore to investigate the feasibility, safety, and potential efficacy of an exercise-based intervention [active rehabilitation (AR)] 2-weeks postconcussion.
To estimate the feasibility of implementing early AR for children and adolescents who were slower to recover from concussion. Specifically, we wished to estimate the safety and acceptability of initiating early AR at 2 weeks after injury compared with usual care (4 weeks) at the Montreal Children's Hospital Concussion clinic.
The specific aims of this project were to estimate; (1) the rates of recruitment and consent; (2) acceptability to patients/families and clinicians; (3) the utility of data collection using an online symptom and activity survey; (4) the safety of early AR; (5) the extent of patient adherence to prescribed exercise; and (6) the potential efficacy of early AR on postconcussion symptoms.
The study was conducted at The Montreal Children's Hospital of the McGill University Health Center (MCH-MUHC), a tertiary care pediatric teaching hospital affiliated with McGill University in Montreal, Canada. In this setting, AR is offered to children and adolescents who experience concussion symptoms for 3 weeks through the Mild Traumatic Brain Injury (mTBI) Program/Concussion clinic. Data were collected prospectively through the mTBI Program/Concussion clinic.
Patients were referred to the mTBI Program/Concussion clinic from various sources (emergency department and family physician). The administrative staff and clinic coordinators introduced patient/families to the study and asked their interest in being called about participating in a study. Patients were eligible if they met the following criteria: (1) physician diagnosed concussion, (2) the presence of symptoms 2 weeks postconcussion, defined as: (a) 3 individual symptoms rated above the retrospective baseline (self-report), (b) any symptom interfering with daily activities (self-report or parent report), (c) a symptom severity score greater than 7 points (parent proxy or PCSI-SR13 adolescent form), or (d) a parent proxy score of one or greater on the following question “In general, to what degree is your child acting differently than before the injury (not acting like himself or herself)? (3) aged 6 to 17 years, and (4) spoke English or French. Patients were excluded if they had a previous concussion within 6 months of the current injury, a coexisting injury preventing participation in the intervention, or if the patient engaged in moderate to vigorous physical activity/exercise/sport before enrollment.
Randomization was conducted using a sequence generated by www.randomization.com on May 26, 2016, by a research assistant (M.S.). Randomization was concealed from patients and clinicians until patients started the intervention. Participants were randomized to receiving early AR (2 weeks after injury) or to receiving usual care AR (4 weeks after injury). Participants in the usual care group did not receive clinical care between the 2- and 4-week appointments. Participants, families, clinicians, and those measuring outcomes were not blinded to group allocation. A research assistant (M.S.) and the primary author (D.M.D.) enrolled and assigned participants to groups.
The intervention provided to both groups (early and usual care) was identical. Only the time of starting the intervention differed. The AR intervention has been published previously by Gagnon et al.13,14 The intervention included 4 components: (1) aerobic activity, (2) coordination/skill practice, (3) visualization, and (4) education. The AR intervention is delivered by trained physiotherapists who work in the mTBI Concussion Clinic. Patients are seen initially by a clinical coordinator and physiotherapist for a complete history and physical assessment. Patients are referred to the AR intervention if they continue to experience concussion signs or symptoms 3 weeks after injury. The intervention is initiated in-clinic with a physiotherapist. Patients are provided with a home program that is to be completed daily and includes the same components as those used during the physiotherapy session. Physiotherapists follow-up with patients (by phone or in-clinic) after 1 week of the intervention to modify or progress the home program.
To begin the AR intervention, aerobic exercise is started at 60% of age-predicted maximum heart rate on a stationary bicycle or treadmill for 15 minutes. Coordination and skill practice is completed for up to 10 minutes and is individualized according to the participant's sport/activity preference. Activities could include stick handling, shooting, dribbling, or other basic sport skills. Heart rate is monitored during aerobic training and coordination skill practice using a portable Polar heart rate monitor and chest strap (Polar Electro, Inc, Lake Success, NY). Visualization involves 5 to 10 minutes of sport/activity imagery based on participant preference. This visualization often mimicked the coordination and skill practice. If appropriate and able, participants were instructed to choose a specific skill and visualize it in a positive manner with realistic timing. Education is provided at each clinic visit by the coordinator or physiotherapist including information about recovery, coping with postconcussion symptoms, and the process of returning to school and sport. Patients who experience worsening of symptoms during the aerobic or coordination phases stop the activity and make note of the amount of time completed if less than 15 minutes, this duration becoming the target duration to be used as a home program of exercise. Children continue the activities at home and fill out a log to report the daily levels of symptoms as well as level of involvement in the various activities. Children are instructed to contact the physiotherapist if they could not engage in activities without an increase in symptoms for 2 consecutive days. Clinicians are in contact with all patients to monitor compliance and tolerance, review the home program, and adjust the exercise protocol as needed. Patients who are symptom-free for 1 week at rest are advised to start the stepwise, progressive return-to-play protocol.
The 2- and 4-week appointments were conducted at the Concussion Clinic. The physiotherapy appointment included a clinical assessment (standard neurological, physical examination, balance, and coordination) (Figure 1).
An online questionnaire was used in this study. The online survey contained 7 postconcussion symptoms as seen in the Post-Concussion Symptom Inventory (PCSI)25 (headache, nausea, balance problems, dizziness, fatigue, sadness, and nervous/anxious). This patient-reported measure was used to track and monitor postconcussion symptoms and the aerobic exercise portion of the AR home program. Participants were asked to provide information about their home program, specifically the mode, intensity, and duration of aerobic exercise. Participants rated postconcussion symptoms pre- and post-exercise when applicable (see Appendix A, Supplemental Digital Content 1, https://links.lww.com/JSM/A192). Online surveys were emailed to all participants everyday starting after the 2-week physiotherapy appointment until one of the following criteria were met: (1) the end of the study period (8 weeks), (2) the patient reported 5 consecutive days of being symptom-free, or (3) the patient was discharged by physical therapy.
To estimate the potential efficacy of the intervention, the total postinjury symptom severity score was obtained using the PCSI.25 The PCSI is a self-report questionnaire documenting the presence and severity of postconcussion symptoms. Two versions of the PCSI were used, the adolescent self-report form for ages 13 to 18 years (PCSI-SR13) and the older child self-report form for ages 8 to 12 years (PCSI-SR8). The highest total symptom scores for the PCSI-SR13 and PCSI-SR8 are 156 and 50, respectively.25 A higher score indicates greater severity of postconcussion symptoms. The PCSI was evaluated at 4 times: 2, 4, 6, and 8-weeks postconcussion. Evaluation at 2 and 4 weeks was conducted by a physiotherapist during the clinical visit. The 6- and 8-week evaluations were performed over the phone by a trained research assistant (M.S.) or the primary author (D.M.D.).
Safety was defined as the ability to perform AR with minimal symptom exacerbation. We measured safety with an adverse event definition. An adverse event was defined as an increase in the 7-symptom online scale reported during or after aerobic exercise in which there was: (1) an individual symptom greater than one point (6-12 years) or 2 points (13-18 years) and (2) a total severity score increase of 2 points (6-12 years) or 3 points (13-18 years). A stopping rule was implemented if an adverse event occurred on 3 subsequent aerobic exercise sessions.
Acceptability was defined as: (1) a willingness of clinicians to prescribe comparable home programs to both groups and (2) a willingness of patients to follow clinician recommendations. Acceptability was evaluated by comparing the online activity survey responses and patient charts.
Adherence was defined as the extent to which participants reported exercise characteristics that were consistent with those prescribed by clinicians.
Feasibility was composed of safety and acceptability. Feasibility was also inferred based on recruitment, utility of data collection, and the potential efficacy of the intervention.
A sample size of 20 participants was determined sufficient for the objectives of this pilot study.26,27
The trial was registered with the US National Institutes of Health NCT03103529 (www.clinicaltrials.gov). Reporting results of this trial were guided by the CONSORT guidelines for randomized pilot and feasibility trials.28 Ethics approval was obtained from the Research Ethics Board of the Montreal Children's Hospital, McGill University Health Center.
Analysis of data was conducted using R version 3.1.2 (2014-10-31). Descriptive statistics were compiled for feasibility objectives. To estimate patient adherence, descriptive statistics were obtained for exercise frequency, intensity, and duration. The total postconcussion scores were calculated by summing the scores of each individual symptom. Symptom scores on the PCSI-SR8 were transformed to a 7-point scale by multiplying each individual item by 3. This transformation was conducted so that all participants' symptoms could be reported on the same scale. Variables not meeting normality assumptions are presented as the median and range.
Recruitment and Consent (Objective 1)
Fifty patients were contacted for participation in the study. Participants were between the ages of 9 to 17 years old. At the initial screening, 50 patients met the eligibility criteria and 50% (25/50) enrolled in the study. The most common reason for not participating was scheduling. Of the 25 patients who consented to participate in the study, 5 became symptom-free before the 2-week appointment making them ineligible. After allocation, no participants dropped out of the study.
Recruitment occurred between May 21 and November 17, 2016. The final follow-up was completed on December 29, 2017. Figure 2 shows the flow of participants in the trial. Groups were similar with respect to sex, age, and history of: concussion, migraines, anxiety, learning disabilities, or sleep disorders. Most participants (17/20) reported academic accommodations such as reduced school days, breaks during the day, limited workload, or examination exemptions over the study period. The 2- and 4-week visits occurred an average of 16- and 30-day postconcussion, respectively (Table 1).
TABLE 1. -
||Early AR (n = 10)
||Usual Care (n = 10)
|Age (mean, SD)
|History of concussion (n)
|History of anxiety (n)
|History of migraines (n)
|Learning disability (n)
|History of sleep disorder (n)
|Initial symptoms (n) (at time of injury)
Acceptability (Objective 2)
Clinicians demonstrated a willingness to prescribe AR to patients at 2-week postconcussion. Exercise characteristics prescribed to patients were similar in both groups indicating acceptability of clinicians (Table 2).
TABLE 2. -
Aerobic Exercise Characteristics
||Early AR (n = 10)
||Usual Care (n = 7*)
|Frequency (mean), d/wk
|Intensity (mean (SD), P-CERT)
|Duration (mean (SD), min)
*Three participants were discharged before starting AR.
Utility of Data Collection (Objective 3)
During the study period, the online questionnaire was emailed to participants 478 times. We received 326 completed surveys for a response rate of 68%. We were able to measure the PCSI at all 4 time points. One participant was unreachable for the 8-week phone call and we carried forward their PCSI value from the 6-week visit. The 6- and 8-week follow-up phone calls were conducted an average of 6.1 ± 0.69 and 7.8 ± 1.06 weeks postconcussion, respectively.
Safety (Objective 4)
Two adverse events occurred during the study. Events occurred in one female participant (early AR group) and one male participant (usual care group) at 31- and 37-days postconcussion, respectively. Participants reported increased symptom severity on the online survey after aerobic exercise. In both cases, symptoms resolved within 1 hour of terminating exercise. Both participants continued with the intervention.
Patient Adherence (Objective 5)
Exercise characteristics obtained from the online survey for frequency, intensity, and duration of aerobic exercise are displayed in Table 2. Patients reported being adherent to the recommendations prescribed by clinicians for exercise intensity and duration. Patients reported being less compliant with prescribed frequency. The most common modes of exercise were walking/jogging and stationary cycling.
Potential Efficacy (Objective 6)
Visual inspection of postconcussion symptoms improved over time for both groups (Table 3). Two participants in the early AR group were excluded from symptom analysis. One participant admitted to falsifying symptom reports at 4 weeks. The other participant was enrolled based on parent-reported symptoms and self-reported no symptoms on the PCSI. We excluded their data as it artificially influenced the descriptive statistics.
TABLE 3. -
Median (Range) Postconcussion Symptom Severity Scores
||Early AR (n = 8*)
||Usual Care (n = 10)
|Youth (n = 8) (PCSI-SR13)
||Youth (n = 8) (PCSI-SR13)
||Child† (n = 2) (PCSI-SR8)
*Two patients excluded from the symptom analysis.
†Child scores on the PCSI-SR8 have been transformed to a 7-point scale (all scores multiplied by 3).
The primary objective of this study was to determine the feasibility of a future clinical trial implementing early AR. Due to the feasibility nature of this study, we did not conduct inferential statistics. Literature regarding feasibility trials indicates that analysis should be descriptive in nature rather than hypothesis-driven.29,30
Recruitment and Consent
The recruitment rate was 50%, which is considered high. Recruitment is a common problem in clinical trials with rates as low as 3% to 20%.31 Our study had only one additional clinical appointment compared with usual care provided at the concussion clinic. Our recruitment strategy allowed patients and families to get information about the study before being asked to participate.
Our data collection strategy to monitor safety and adherence was a strength of the study. Monitoring and adherence are 2 methodological components that have not been well controlled in previous studies. Although previous studies reported the use of log books to measure adherence, none have presented data on these variables.17,32,33 Exacerbation of symptoms during recovery from concussion is one of the most commonly cited reasons to limit activity.5,34,35 Given this concern, monitoring what patients are doing and how it influences their symptoms is critical to a management strategy involving physical activity. Half of our families elected to have the online survey delivered to both the participant and parent/guardian email addresses. In other cases, the email was sent only to the parent/guardian email address (n = 6) or the participants' email address (n = 4). The rate of response was highest for delivery to the parent's email address (78%) compared with both parent/participant (67%) and the participant only (53%). To the best of our knowledge, this method of monitoring has not been used previously during recovery from concussion. Importantly, we demonstrated it was a feasible method to identify adverse events. Similar to a recent randomized trial,17 we found serial repetition of the PCSI to be feasible in our study.
Regarding the prescribed exercise, we identified differences between groups. The usual care group reported higher frequency, intensity, and durations of aerobic exercise compared with the early AR group. Patients in both groups displayed a high level of adherence to the prescribed intensity and duration. Patient adherence to prescribed interventions varies considerably.36 If future studies aim to estimate the efficacy of this concussion management approach, exercise characteristics and adherence must be measured.
At the study conclusion, most patients (15/20) demonstrated symptom scores that were at or below the level of their preinjury, retrospective baseline. Although expert consensus advocates patients to be asymptomatic before return to full activity,5 there is growing evidence that youth experience concussion-like symptoms in the absence of concussive injury.37 Healthy teen athletes (no concussion) aged 13 to 17 years displayed symptom severity scores ranging from 4.06 to 9.17 on the same symptom scale we used in this study (PCSI).38 Both groups achieved comparable levels of postconcussion symptoms at the end of the study period. Of note, patients in the early group reached a level of symptom severity that could be considered “normal” (equal to baseline in healthy youth) at 4-weeks postconcussion, compared with 6 weeks in the usual care group. Starting exercise 2-weeks postconcussion did not delay recovery.
Considering our findings, we acknowledge a few limitations. We do not have a true control group for comparison, and the evaluation of outcomes was not blinded. The study sample consisted mostly of older adolescents with only 3 of our participants younger than 12 years. The low number of participants younger than 12 years may be a result of differing rates of recovery in these age groups. Research has shown that patients 13 years and older are more likely to experience prolonged symptoms compared with younger patients.8 We cannot say with certainty that our findings can be generalized to a younger sample of patients. It is also unclear if our data collection methods (online surveys) would be suitable for younger children. We acknowledge that the online activity surveys were patient-reported and coupling this with an objective measure would provide better accuracy. Physical activity reporting in children has low to moderate correlation with direct measures such as accelerometers. Furthermore, children have been found to both overestimate and underestimate their level of physical activity when compared with direct measures.39 Finally, related to the intervention, we did not measure all components (coordination drills and visualization). Although we did not exclude patients who were receiving additional care outside of the study protocol, 3 patients (early care n = 1 and usual care n = 2) reported receiving treatment (osteopathy and physiotherapy) for concurrent neck injuries during the study period.
The results from this study indicate that a clinical trial estimating the efficacy of AR 2-week postconcussion is feasible. Little evidence exists to determine when patients with prolonged symptoms should resume physical activity. The introduction of physical activity (at 2 weeks after injury) that we implemented is important given that extended physical inactivity is associated with prolonged recovery. This study provides a foundation on which a future efficacy study could be conducted. Future work building on the current study would have a large impact on concussion rehabilitation and the use of exercise-based approaches. Further study is needed to determine the superiority of this strategy over current treatment approaches.
The authors thank the children and families for participating in this research study. The authors thank Fatemeh Bahrpeyma for assistance in recruiting participants. The authors thank Jérôme Gauvin-Lepage for translating study documents into French.
1. McCrory P, Meeuwisse W, Dvorak J, et al. Consensus statement on concussion in sport—the 5th International Conference on Concussion in Sport held in Berlin, October 2016. Br J Sports Med. 2017;51:838–847.
2. Aubry M, Cantu R, Dvorak J, et al. Summary and agreement statement of the 1st International Symposium on Concussion in Sport, Vienna 2001. Clin J Sport Med. 2002;12:6–11.
3. McCrory P, Johnston K, Meeuwisse W, et al. Summary and agreement statement of the 2nd International Conference on Concussion in Sport, Prague 2004. Br J Sports Med. 2005;39:196–204.
4. McCrory P, Meeuwisse W, Johnston K, et al. Consensus statement on concussion in sport: the 3rd International Conference on Concussion in Sport held in Zurich, November 2008. Br J Sports Med. 2009;43(suppl 1):i76–i84.
5. McCrory P, Meeuwisse W, Aubry M, et al. Consensus statement on concussion in sport—the 4th International Conference on Concussion in Sport held in Zurich, November 2012. Phys Ther Sport. 2013;14:e1–e13.
6. McCrory P, Meeuwisse WH, Aubry M, et al. Consensus statement on concussion in sport: the 4th International Conference on Concussion in Sport held in Zurich, November 2012. Br J Sports Med. 2013;47:250–258.
7. Lovell MR, Iverson GL, Collins MW, et al. Measurement of symptoms following sports-related concussion: reliability and normative data for the post-concussion scale. Appl Neuropsychol. 2006;13:166–174.
8. Zemek R, Barrowman N, Freedman SB, et al. Clinical risk score for persistent postconcussion symptoms among children with acute concussion in the ED. JAMA. 2016;315:1014–1025.
9. Wells EM, Goodkin HP, Griesbach GS. Challenges in determining the role of rest and exercise in the management of mild traumatic brain injury. J Child Neurol. 2016;31:86–92.
10. Buckley TA, Munkasy BA, Clouse BP. Acute cognitive and physical rest may not improve concussion recovery time. J Head Trauma Rehabil. 2015;31:233–241.
11. Thomas DG, Apps JN, Hoffmann RG, et al. Benefits of strict rest after acute concussion: a randomized controlled trial. Pediatrics. 2015;135:213–223.
12. Silver JM. Effort, exaggeration and malingering after concussion. J Neurol Neurosurg Psychiatry. 2012;83:836–841.
13. Gagnon I, Galli C, Friedman D, et al. Active rehabilitation
for children who are slow to recover following sport-related concussion. Brain Inj. 2009;23:956–964.
14. Gagnon I, Grilli L, Friedman D, et al. A pilot study of active rehabilitation
for adolescents who are slow to recover from sport-related concussion. Scand J Med Sci Sports. 2016;26:299–306.
15. Leddy JJ, Kozlowski K, Donnelly JP, et al. A preliminary study of subsymptom threshold exercise training for refractory post-concussion syndrome. Clin J Sport Med. 2010;20:21–27.
16. Hugentobler JA, Vegh M, Janiszewski B, et al. Physical therapy intervention strategies for patients with prolonged mild traumatic brain injury symptoms: a case series. Int J Sports Phys Ther. 2015;10:676.
17. Kurowski BG, Hugentobler J, Quatman-Yates C, et al. Aerobic exercise for adolescents with prolonged symptoms after mild traumatic brain injury: an exploratory randomized clinical trial. J Head Trauma Rehabil. 2016;32:79–89.
18. Griesbach GS, Hovda D, Molteni R, et al. Voluntary exercise following traumatic brain injury: brain-derived neurotrophic factor upregulation and recovery of function. Neuroscience. 2004;125:129–139.
19. Asken BM, McCrea MA, Clugston JR, et al. “Playing through it”: delayed reporting and removal from athletic activity after concussion predicts prolonged recovery. J Athl Train. 2016;51:329–335.
20. Maerlender A, Rieman W, Lichtenstein J, et al. Programmed physical exertion in recovery from sports-related concussion: a randomized pilot study. Dev Neuropsychol. 2015;40:273–278.
21. Gagnon I, Grilli L, Friedman D, et al. A pilot study of active rehabilitation
for adolescents who are slow to recover from sport-related concussion. Scand J Med Sci Sports. 2016;26:299–306.
22. Dobney DM, Grilli L, Kocilowicz H, et al. Evaluation of an active rehabilitation
program for concussion management in children and adolescents. Brain Inj. 2017;31:1–7.
23. Chan C, Iverson GL, Purtzki J, et al. Safety of active rehabilitation
for persistent symptoms after pediatric sport-related concussion: a randomized controlled trial. Arch Phys Med Rehabil. 2018;99:242–249.
24. Chrisman SPD, Whitlock KB, Somers E, et al. Pilot study of the Sub-Symptom Threshold Exercise Program (SSTEP) for persistent concussion symptoms in youth. Neurorehabilitation. 2017;40:493–499.
25. Sady MD, Vaughan CG, Gioia GA. Psychometric characteristics of the Post-Concussion Symptom Inventory in children and adolescents. Arch Clin Neuropsychol. 2014;29:348–363.
26. Isaac S. Handbook in Research and Evaluation; a Collection of Principles, Methods, and Strategies Useful in the Planning, Design, and Evaluation of Studies in Education and the Behavioral Sciences.
San Diego, CA: R.R. Knapp; 1971.
27. Hertzog MA. Considerations in determining sample size for pilot studies. Res Nurs Health. 2008;31:180–191.
28. Eldridge SM, Chan CL, Campbell MJ, et al. CONSORT 2010 statement: extension to randomised pilot and feasibility trials. Pilot Feasibility Stud. 2016;2:64.
29. Arain M, Campbell MJ, Cooper CL, et al. What is a pilot or feasibility study? A review of current practice and editorial policy. BMC Med Res Methodol. 2010;10:67.
30. Lancaster GA, Dodd S, Williamson PR. Design and analysis of pilot studies: recommendations for good practice. J Eval Clin Pract. 2004;10:307–312.
31. Swanson GMG. Recruiting minorities into clinical trials: toward a participant-friendly system. J Natl Cancer Inst. 1995;87:1747–1759.
32. Grabowski P, Wilson J, Walker A, et al. Multimodal impairment-based physical therapy for the treatment of patients with post-concussion syndrome: a retrospective analysis on safety and feasibility. Phys Ther Sport. 2017;23:22–30.
33. Cordingley D, Girardin R, Reimer K, et al. Graded aerobic treadmill testing in pediatric sports-related concussion: safety, clinical use, and patient outcomes. J Neurosurg Pediatr. 2016;25:693–702.
34. Moser RS, Schatz P. A case for mental and physical rest in youth sports concussion: it's never too late. Front Neurol. 2012;3:171.
35. Majerske CW, Mihalik JP, Ren D, et al. Concussion in sports: postconcussive activity levels, symptoms, and neurocognitive performance. J Athl Train. 2008;43:265.
36. DiMatteo MR. Variations in patients' adherence to medical recommendations: a quantitative review of 50 years of research. Med Care. 2004;42:200–209.
37. Alla S, Sullivan SJ, McCrory P. Defining asymptomatic status following sports concussion: fact or fallacy? Br J Sports Med. 2012;46:562–569.
38. Hunt AW, Paniccia M, Reed N, et al. Concussion-like symptoms in child and youth athletes at baseline: what is “typical”? J Athl Train. 2016;51:749–757.
39. Adamo KB, Prince SA, Tricco AC, et al. A comparison of indirect versus direct measures for assessing physical activity in the pediatric population: a systematic review. Int J Pediatr Obes. 2009;4:2–27.