Training Load and Injury Among Middle School–Aged Athletes

Sport participation rates among children and early adolescents in the United States remain high, with 69% of youths aged 6 to 12 years participating in either a team or individual organized sport.^1,2 Additionally, young athletes are being encouraged to participate in high-intensity programs intended to improve skills.³ Many youth athletes increase their training volume (TV) due to a variety of factors, such as parental pressure, coach recommendations, pursuit of a collegiate athletic scholarship, or even professional aspirations.^4–6 Youth athletes often feel that playing with an additional club team or specializing in a single sport is necessary to make their high school team.⁷ As a result of additional club participation, many youth athletes undergo year-round intensive training, often in a single sport.

Intentional year-round participation in a single sport to the exclusion of other sports has been related to overuse injuries in adolescent populations,^4,8–10 which has prompted organizations to make recommendations for safe sport participation.^11,12 Furthermore, adolescent athletes who intentionally train year-round participate in more hours per week than other athletes.^10,13 Researchers^4,9,10 suggested that athletes should not spend more hours per week participating in organized sport than their chronological age. In one study⁴ of high school athletes aged 14 to 19 years, exposure of more than 16 hours of sport participation per week was strongly associated with sport-related injury (odds ratio [OR] = 8.52; 95% confidence interval [CI] = 5.82, 12.47; P < .001).

Most authors have focused on the relationship between training load and injury while considering both duration and intensity. For example, higher daily and prior-day internal training loads (according to the session rating of perceived exertion) were related to a higher incidence of injuries among female youth soccer athletes.¹⁴ In addition, the acute:chronic workload ratio (ACWR), defined as the workload from the prior week divided by the 4-week rolling average workload, has been associated with injury.^15,16 Specifically, a nonlinear relationship has been suggested, such that large increases (very high acute loads relative to the last 4 weeks) or decreases (very low acute loads relative to the last 4 weeks) in this ratio appear to be associated with an increased risk of subsequent injury.^17–23 However, some²⁴ have questioned the interpretation of this relationship and raised the possibility of spurious correlations between ACWR and injury, as the acute training load is part of both the numerator and denominator. Additionally, variations in training schedule may confound the relationship between ACWR and injury.²⁵ Yet research in preadolescent athletes to evaluate the association between acute on chronic TV and injury risk is limited.

In an attempt to reduce the risk of injuries among youth athletes, the American Medical Society for Sports Medicine¹¹ has provided recommendations regarding weekly organized sport participation by this population. Unfortunately, earlier studies^26,27 of youth sport participation often did not incorporate or account for TV as it is related to injury and were focused primarily on older adolescent athletes. Compared with measures of internal and external training load, guidelines about hours of weekly participation may be more easily understood and implemented by parents of younger children. For example, a relationship between TV and shoulder injury rates among elite youth handball athletes (adolescents aged 14–18) was identified.²⁸ Still, to our knowledge, the relationships between TV and injury risk have not been prospectively examined in middle school athletes (adolescents aged 10–14 years) across a variety of sports and injury types. Therefore, the purpose of our study was to (1) prospectively evaluate the association between acute and chronic TV (hours per week) and overall, acute, and overuse injuries; and (2) evaluate the relationship between various ratios of acute:chronic TV and injury while controlling for sex, age, and prior injury in middle school-aged athletes.

METHODS

This study was approved by the Institutional Review Board of the University of Wisconsin–Madison. We followed a prospective cohort design that involved weekly anonymous surveys using Qualtrics (Provo, UT). Parents of youth athletes were initially contacted by email to participate in the weekly anonymous survey. The weekly survey lasted for 39 weeks of the academic year, starting in September of 2017. Data were included in the final analysis if the participant completed 80% or more of the weekly surveys. Children and their parents were instructed to complete the surveys together, and the data were recorded in a deidentified format.

RESULTS

Of an initial 278 participants who completed the baseline survey, 244 (88%) completed 80% or more of the weekly surveys, and 125 (51%) completed 100% of the surveys. Participants reported their primary sports as basketball (n = 85, 35%), soccer (29, 12%), volleyball (26, 11%), tennis (21, 9%), baseball (20, 8%), football (16, 7%), cross-country (13, 5%), softball (11, 5%), swimming or diving (11, 5%), ice hockey (3, 1%), cheer or dance (2, 1%), gymnastics (1, <1%), wrestling (1, <1%), or other (5, 2%). Among the 244 participants included in the analysis (Table 1), 117 (48%) reported 219 injuries, with 149 being classified as acute and 70 as chronic or overuse (Figure). This represented an overall injury rate of 2.44 injuries per 1000 hours of physical activity. Baseline measures including age, sex, and prior injury were not different between injured and uninjured participants (Table 1). Weekly reported hours per week in organized sport were not different among 10-, 11-, 12-, 13-, or 14-year-old participants (Table 2). An increase in the average TV over the prior 4 weeks decreased the risk of reporting an acute injury (OR = 0.97; 95% CI = 0.93, 1.00; P = .05; Table 3). Additionally, higher average TV over the prior 2 (OR = 0.96; 95% CI = 0.93, 0.99; P = .04), 3 (OR = 0.95; 95% CI = 0.91, 0.99; P = .02), and 4 (OR = 0.95; 95% CI = 0.91, 0.99; P = .05) weeks was associated with fewer reported acute injuries the following week (Table 3). However, no duration of prior TV (prior week or prior 2, 3, or 4 weeks) was significantly related to overuse injury (Table 3). Similarly, no significant relationship was present between acute:chronic TV and overall, acute, or overuse injury the following week (Table 3).

Table 1 Baseline Data for Injured and Uninjured Athletes^a

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Table 2 Weekly Training Volume (h) by Age Group

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Table 3 Mixed-Effects Logistic Regression Model Results for All, Acute, and Chronic Injuries

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DISCUSSION

Our primary finding was that higher chronic TV was associated with a reduced risk of injury the following week after adjusting for age, sex, and prior injury. To the best of our knowledge, we are the first to prospectively evaluate the relationship between various durations of prior TV and injury risk in a population of middle school-aged athletes from various sports²⁸ and suggest that athletes with higher chronic TVs may gain some protective benefit against acute injury. Although this finding has been confirmed in studies of adult athletes,^15,29–31 we were the first to demonstrate this in a middle school-aged population while controlling for previous injury.

We explored ratios of acute:chronic TV to determine whether changes in participation hours relative to the habitual level of activity were associated with subsequent injury, but they were not. This result is different from that in previous research²⁸ on youth handball players in which a 60% increase in handball participation compared with the previous 4 weeks increased the risk of shoulder injuries. These variations may reflect differences in methods or populations. Our measures were calculated in a similar way to ACWR; therefore, the objections regarding the ACWR calculation may apply to our calculations as well.²⁴ Traditional measures of ACWR incorporate both training duration and intensity and have been associated with injury in other studies.^15,17,29,30 However, important methodologic differences in our work could account for these differences. Our middle school athletes did not report changes in weekly TV that were as significant as those seen in college-aged or professional athletes. We observed relatively small variances in TV and ratio variables. This may have simply resulted in fewer times when the ratio reached a level that has been suggested to increase the injury risk in older athletes. However, we found that relative fluctuations in TV in younger athletes were not related to injury risk, whereas a chronic and consistent TV may help reduce the injury risk in younger athletes. More research is needed to confirm this theory.

Although higher chronic TV in the present study was associated with a reduced injury risk the following week, this does not necessarily mean that higher and higher TVs will offer more and more protection. The average TV among our participants was typically 10 to 11 hours per week and rarely reached a level that would be considered dangerous based on recent recommendations.³² We cannot directly address this factor using our data, yet a limit beyond which the benefit of higher chronic TV plateaus in this population seems possible. An unexpected finding was that higher chronic TV was related to acute injuries and not chronic injuries. Previous investigators²⁷ identified that high-volume, year-round participation in a single sport may be related to an increased risk of overuse injuries. Increases in acute training load negatively affected a number of measures of subjective wellbeing, including fatigue, mood, stress, and soreness, which were risk factors for injury.³³ The specific mechanisms for this relationship are not clear; psychological states, such as increased stress, may increase the risk for injury through physiological (eg, increased muscle tension) or psychological (eg, distractibility and poor decision-making) mechanisms.^34–36 Within our group of middle school athletes, higher levels of chronic TV may lead to fitness increases that not only improve the tolerance of physical constructs to increased loads but also attenuate the impairments in subjective wellbeing after acute increases in TV, thereby reducing the risk of subsequent acute injury.

Finally, our results may differ from those of other studies^15,29–31 because earlier authors failed to control for prior injury in the risk prediction model. Previous injury has been consistently identified as being associated with future injury, and adjusting for this factor may help clarify the true role of risk factors, such as TV, by eliminating the potential confounding role of prior injury. This may be particularly true among youth athletes who are involved in high volumes of year-round sport participation, which is associated with both increased TV and higher rates of prior injury.

CONCLUSIONS

Higher chronic TV was associated with a reduced risk of injury the following week among middle school-aged athletes, even after adjusting for age, sex, and prior injury. Specifically, higher 2-, 3-, and 4-week TV was associated with a decreased risk of acute injuries, and acute or chronic TV was not related to overuse injury the following week. No TV ratio was associated with overall, acute, or overuse injury in this population. These results suggest that consistent, higher levels of physical activity may offer protection against acute injury in young athletes.