Whether our youth athletes compete for fun or intend on becoming professionals, we enjoy having them participate in sport activities. Their enjoyment, as well as their safety, are our highest priorities. That being said, there are thousands of anterior cruciate ligament (ACL) injuries every year and most of them are for high school athletes (5). How can this be? A 2005 study (6) found an increased risk for ACL injuries correlated with increased knee valgus. Dynamic knee valgus is when the knee “caves in” due to hip adduction and hip internal rotation. Studies have also related increased risk of dynamic knee valgus with increased fatigue (5). Perhaps here lies the connection with our youth athletes. While still learning about life, youth athletes bear many responsibilities. Youth athletes not only participate in sports (which includes training, practice and competition), but partake in school, extracurricular activities, and maintaining a social and family life.  But, how exactly does fatigue effect their bodies? How at risk are youth athletes for injury?

Let’s take a look at the science

A 2019 study investigated the effects of fatigue on dynamic knee valgus in 85 (47 female, 38 male) youth athletes (14-18 years old). Athletes had a background in various sports, including volleyball, track and field, soccer and basketball. The experiment was carried out in 3 phases, which you can see in the image below (Figure 1). First, a baseline evaluation for each individual was assessed to determine performance and presence of dynamic knee valgus before influenced by exercise/fatigue. Second, the “fatigue protocol” or exercise session was performed. Third, individuals were re-assessed to measure any changes in performance or increases in dynamic knee valgus. (5)

What did the study find?

A minimum drop of 20% in performance was considered a “fatigued state”. It was found 26% of athletes showed a 20% drop in performance for maximal vertical jump assessment. Meaning, 22 out of 85 athletes demonstrated fatigue after performing exercise. Overall, 15.4% of youth athletes in this study had a reduction in maximal vertical jump performance after completing the exercise protocol. In regard to presence of dynamic knee valgus, 44.7% of evaluated athletes showed an increase in dynamic knee valgus after exercise. Take a look at the graph below (Figure 2). You will note the percentage of increased risk for dynamic valgus after experiencing fatigue. You can see the groups at higher risk are female youth athletes and athletes 15 years old or older. Overall, 44.7% of evaluated athletes showed an increase in dynamic knee valgus after exercise.

Youth athletes that met the “fatigue state” showed a greater increase in risk for dynamic valgus (63.6%) compared to youth athletes of lower fatigue (38.1% risk). The study found significantly more youth athletes were marked as “medium” or “high” risk after exercise compared to pre-exercise. Similarly, female youth athletes and athletes 15 years or older were found to be marked as “medium” or “high” risked significantly more after exercise compared to pre-. (5)

What does it mean?

Fatigue increases dynamic knee valgus in youth athletes. Female athletes and athletes older than 15 years are more likely to have increased levels of dynamic knee valgus. Previous studies have also suggested fatigue can change lower-limb biomechanics (4, 10, 11), leading to increased risk for ACL injury (2, 3, 7). This study found significant increases in dynamic knee valgus after exercise. This shift in lower-limb nueromuscular control was found to be associated with levels of fatigue (5). What this suggests is the greater amount of fatigue experienced by a youth athlete, the greater level of dynamic knee valgus, leading to a greater risk of ACL injury. Furthermore, this study noted a correlation between female youth athletes and higher risk for dynamic valgus. Which makes sense when you consider previous research that has demonstrated high risk for ACL injuries to be found in female youth athletes (1, 5, 8, 9).

What can we take from this?

First, in knowing the effect of fatigue on our youth athletes, it would be wise to ‘incorporate fatigue resistance training and for fatigue awareness by coaches, trainers, PE teachers’ (5) and parents. Instead of relying on “How do you feel?” or thinking “How do they look?“, Sportavida can help you know if your athletes are fatigued. Our unique reports can help target at-risk athletes and protect the kids from putting their bodies at risk.


Thank you for reading! I hope you feel more informed. If you have any questions or concerns, please feel free to leave a shout out.




  1. Agel, J., Arendt, E.A., and Bershadsky, B. Anterior cruciate ligament injury in national collegiate athletic association basketball and soccer: A 13-year review. Am J Sports Med. 2005; 33: 524–530
  2. Benjaminse, A., Habu, A., Sell, T.C. et al. Fatigue alters lower extremity kinematics during a single-leg stop-jump task. Knee Surg Sports Traumatol Arthrosc. 2008; 16: 400–407
  3. Borotikar, B.S., Newcomer, R., Koppes, R., and McLean, S.G. Combined effects of fatigue and decision making on female lower limb landing postures: Central and peripheral contributions to ACL injury risk. Clin Biomech (Bristol, Avon). 2008; 23: 81–92
  4. Cortes, N., Greska, E., Ambegaonkar, J.P., Kollock, R.O., Caswell, S.V., and Onate, J.A. Knee kinematics is altered post-fatigue while performing a crossover task. Knee Surg Sports Traumatol Arthrosc. 2014; 22: 2202–2208
  5. Fidai, M. S., Okoroha, K. R., Meldau, J., Meta, F., Lizzio, V. A., Borowsky, P., … & Makhni, E. C. (2020). Fatigue increases dynamic knee valgus in youth athletes: Results from a field-based drop-jump test. Arthroscopy: The Journal of Arthroscopic & Related Surgery36(1), 214-222.
  6. Hewett, T.E., Myer, G.D., Ford, K.R. et al. Biomechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: A prospective study. Am J Sports Med. 2005; 33: 492–501
  7. Kernozek, T.W., Torry, M.R., and Iwasaki, M. Gender differences in lower extremity landing mechanics caused by neuromuscular fatigue. Am J Sports Med. 2008; 36: 554–565
  8. Lin, C.Y., Casey, E., Herman, D.C., Katz, N., and Tenforde, A.S. Sex differences in common sports injuries. PM R. 2018; 10: 1073–1082
  9. Mihata, L.C., Beutler, A.I., and Boden, B.P. Comparing the incidence of anterior cruciate ligament injury in collegiate lacrosse, soccer, and basketball players: Implications for anterior cruciate ligament mechanism and prevention. Am J Sports Med. 2006; 34: 899–904
  10. Santamaria, L.J. and Webster, K.E. The effect of fatigue on lower-limb biomechanics during single-limb landings: A systematic review. J Orthop Sports Phys Ther. 2010; 40: 464–473
  11. Thomas, A.C., Lepley, L.K., Wojtys, E.M., McLean, S.G., and Palmieri-Smith, R.M. Effects of neuromuscular fatigue on quadriceps strength and activation and knee biomechanics in individuals post-anterior cruciate ligament reconstruction and healthy adults. J Orthop Sports Phys Ther. 2015; 45: 1042–1050