Authors: Jackie Feliciano BA1, Michael P McNally PhD2,3, Andrew M Busch EdD1

1Department of Health and Human Kinetics, Ohio Wesleyan University, Delaware, OH
2School of Health and Rehabilitation Sciences, The Ohio State University, Columbus, OH
3Jameson Crane Sports Medicine Institute, The Ohio State University Wexner Medical Center, The Ohio State University, Columbus, Ohio, USA

Corresponding Author:
Andrew M. Busch, EdD
Ohio Wesleyan University
107C Edwards Gymnasium
61 S. Sandusky St
Delaware OH, 43220
ambusch@owu.edu
614-783-6917

Andrew Busch is an assistant professor at Ohio Wesleyan University and is also an alumni of the United States Sports Academy.

Performance Differences in Division III Female Field Hockey Athletes with Prior Lower Extremity Injuries Over a Competitive Season

ABSTRACT

Background: In the sport of field hockey, athletes encounter repetitive unilateral movements due to the nature of the sport, possibly leading to detectable changes in performance variables or functional movements.

Purpose: The purpose of this study was to first investigate pre-season power output, functional movement, and single leg balance differences in participants with a history of prior lower extremity injuries, and second, to examine potential changes in such measures throughout a competitive field hockey season.

Methods: Eighteen healthy collegiate female field hockey athletes (mean age = 19.3 ± 1.2 years) were assessed in different functional movement and performance measures including the Functional Movement Screen (FMSTM)deep squat, Y-balance anterior reach test (YBT), lumbar-locked thoracic rotation test (LLR), vertical jump, and a single-leg eyes-closed balance test pre- and post-competitive season.  

Results:  Fourteen participants completed the study.  Preseason testing revealed a significantly lower peak concentric rate of force development (RFD) in those reporting previous injuries of the lower extremities compared to those with no prior injuries (p = 0.017, d = 1.37).  No differences were noted post-season in previously injured participants.  Post-season testing revealed a significant decrease in LLR (Left:  p = 0.004, d = 0.35; Right: p = 0.007, d = 0.33), a decrease in multiple single-leg balance measures (center of pressure excursion: Left: p < .0005, d = -0.7; Right: p < .0005, d = -1.1; medial/lateral velocity: Left: p < .001, d = -0.24; Right: p < .0005, d = -0.74; anterior/posterior velocity: Left: p < .0005, d = -1.06; Right: p < .0005, d = -1.18) and a decrease in peak concentric rate of force development (RFD) (p = 0.03, d = .33).  There were no significant changes noted in post-season FMSTM deep squat scores, or YBT results among the participants.

Conclusion: Female field hockey athletes with a history of lower extremity injuries demonstrate significantly less concentric RFD during a vertical jump when compared to athletes with no prior injuries.  Thoracic ROM, single-leg balance performance, and concentric RFD all significantly decreased after a competitive Division III collegiate season.  FMSTM deep squat and YBT anterior reach scores did not change throughout the season. 

Applications in Sport: Field hockey athletes with a history of previous lower extremity injuries should continually focus on power development, while thoracic ROM exercises, single-leg balance training and lower body explosive exercises should be a point of focus for female field hockey athletes to maintain preseason values throughout a competitive season.

Key words: Field Hockey, Balance, Power, Performance, Functional movement

INTRODUCTION

Field hockey is one of the world’s most popular sports and is growing rapidly within the United States, with the largest growth occurring among National Collegiate Athletic Association (NCAA) Division III programs nationwide (6).  From 2002-2003 to 2015-2016, participation in NCAA women’s field hockey has increased by 11% with a total of 273 colleges now offering field hockey with 6,032 women participating in 2015-2016 (15).  However, despite its growing popularity, there is very limited research on athletes who participate in field hockey, and specifically those with a history of previous injuries.  There is also little information regarding the overall effects a competitive season has on certain performance tests and functional movement in female collegiate field hockey athletes. 

Lower Extremity Injuries

Field hockey injuries have demonstrated similar overall injury rates to basketball and soccer (6).  The most frequently injured sites in field hockey players are the lower extremities, particularly ankle sprains (1,13,14).  Ankle injuries tend to occur at a rate of 0.9 injuries per 1000 days of exposure(1).  Field hockey seems to demonstrate a greater risk of subsequent injuries; including hamstring strains and anterior cruciate ligament-reconstruction (ACLR) re-injuries compared to basketball, soccer, and lacrosse (1).  An NCAA injury surveillance system reported that knee internal derangement injuries such as ACL tears were the second most common reported injury between the years of 2003 and 2004 in female field hockey athletes, with a three-fold increased risk to experience internal derangement during a game compared to practice (6).   Several studies investigating ACLR injuries have noted residual deficits in neuromuscular factors such as proprioception, peak torque, intra-muscular forces, altered gait mechanics, and functional movement patterns post-injury (3,7).  It has also been noted that ACLR may induce neurological adaptations in brain processing regarding sensory-motor control, and may alter the way an individual recruits muscles to perform tasks such as flexion and extension of both the injured and non-injured legs (8,9).

Postural Adaptations

Field hockey athletes play with short, right-handed sticks, forcing them to maintain balance in a bent-over position that may cause muscular imbalances due to repetitive motions and overuse of specific muscles.  When evaluating muscle length in professional field hockey players, Kawalek and Garsztka (2013) noted many muscles present as hypertonic (11).  Midfielders often demonstrated the highest overall number of hypertonic muscles (47%), and in the lower extremities of all players, every muscle was noted as hypertonic.  The only observed asymmetrical hypertonic muscle was the quadratus lumborum (11).  The significant difference in the quadratus lumborum muscle may likely be due to the cumulative stress a laterally-flexed position of the spine creates during competition.

When specifically looking at performance differences between left and right sides, field hockey athletes often have significantly greater trunk sway when cutting to the left side (4).  These findings may also likely result from the bent-over stick position, as it is more advantageous and common for athletes to cut right.  It has also been suggested that players can shoot with greater accuracy and power when flexing the trunk, and special training programs have been implemented for players to perform maximal flexion of the spine to teach this pattern (16).  This repetitive strain on the spine poses long-term implications for such athletes.  Computed tomography (CT) scans of the lumbar spine have demonstrated a number of functional and structural pathological changes that strongly correlate to years of training (mean = 18.5 years) in professional field hockey athletes (16).  Excessive lumbar lordosis, alterations in lumbar mobility, decreases in disc height, and changes in the shape of vertebral bodies were noted in these athletes compared to controls, suggesting the intervertebral discs were improperly absorbing vertical forces, directly transferring such forces to the vertebral bodies (16,17).     

Therefore, the stress of playing can create overuse imbalances and changes in spinal movements worsened by greater years of playing (16,17).  With such evidence demonstrating the impact residual deficits could have on motor control, and with the sport-specific stress field hockey creates within the body, a greater understanding is needed for the role previous injuries play in measures such as a vertical jump, thoracic ROM, functional movement, and single leg balance.  It is also unknown how such measures may change throughout a collegiate season.  Such information could assist strength coaches and rehab professionals in the development of safe, biomechanically based training programs to improve durability, explosiveness, and balance across a season.  The purpose of this study was to investigate whether lower body explosiveness (assessed by a countermovement vertical jump), functional movements, and single leg balance measures differ among field hockey athletes who previously sustained a lower extremity injury, and if such measures undergo deleterious effects after a collegiate season.  It was hypothesized that previous injuries would negatively affect pre-season testing measures, and greater decreases in power, functional movement, and balance, would be noticed after the season in previously injured participants, with greater differences in those who played more cumulative minutes throughout the season.

Study Design

This study used an observational design in a controlled laboratory setting to evaluate the effect previous lower extremity injuries have on power output, functional movement, and balance.  Ground reaction forces during a countermovement vertical jump and single leg balance was measured using a force plate.  The Functional Movement Screen (FMSTM) deep squat, single-leg Y-balance test (YBT) anterior reach scores, and a lumbar-locked rotation (LLR) test to measure thoracic-spine range of motion (ROM) were recorded.  Due to the low mean number of years playing field hockey in the current collegiate sample (7.2 years), compared to the 18.5 year mean in professional players suffering from structural and pathological lumbar changes (16), the authors felt any changes that might possibly occur in the lumbar spine may be too small to detect over the course of eight weeks.  Therefore, it was decided changes may be more detectable in thoracic ROM, which has much greater ROM compared to the lumbar region, and may also function as a proxy for overall stress on spinal ROM.    

METHODS

Participants

An institutional review board approval and informed consent were obtained prior to any data collection.  Eighteen healthy female NCAA field hockey athletes from a single university were recruited to participate (age = 19.3 ± 1.2 years, height = 1.66 ± 0.075 m, mass = 66.84 ± 8.99 kg, playing experience = 7.8 ± 2.3 years).  Each participant received an email describing the study, and completed an athletic injury history questionnaire.  Previous injuries in the lower extremity were defined as: any prior surgery or injury in the lower extremity requiring a minimum time-loss of two weeks within the past four years.  The primary positions of the 18 participants originally enrolled in this study were: goalkeepers (n = 2), defenders (n = 5), midfielders (n = 7), and forwards (n = 4).  Participants were included if they were listed on the active roster at the time of testing pre- and post-season, and were free of current injuries at the time of testing.  They were excluded if they did not complete the season with the team, or were injured at the time of testing, therefore altering their ability to perform the testing protocol.  A total of four athletes were excluded from the post-season testing because three encountered season-ending injuries, and one quit the team during the season.  Fourteen participants completed both pre- and post-season testing.

Data Collection.

Pre-season testing began one week prior to any official competition.  The season consisted of 18 games spanning eight weeks in duration, and post-season testing was performed the week immediately after the last game.  Each testing period was divided into two days.  On day one, participants performed a five-minute warm-up jog at a self-selected pace, followed by a series of moderate-intensity dynamic stretches used in previous research that are commonplace in athletic settings (18).  Participants performed three maximal effort countermovement vertical jumps on a portable force plate measuring takeoff ground reaction forces (Bertec- 4060-05, Columbus, OH).  A VerTec jump-measuring device was used to record jump height, and was placed 35 centimeters in front of the force plate, resulting in athletes jumping slightly forward to avoid landing on the restricted surface size of the force plate (Figure 1).  The takeoff ground reaction forces (GRF) were used to calculate peak vertical GRF, eccentric and concentric rate of force development (RFD), peak vertical power, and energy generation as the integral of vertical power during takeoff.  Each participant then performed a single leg balance test on the same force platform, randomizing the starting leg between participants.  They were instructed to stand on one leg in the center of the force plate, closing their eyes once balance was sustained, and remain in this position for 30 seconds (Figure 2).  The first and last five seconds of each trial were eliminated to ensure steady-state balance, and from this, average velocity in the medial-lateral (ML) and anterior-posterior (AP) directions were calculated, along with total center of pressure (COP) excursion (Figures 3 & 4).

On the second day of testing, all participants were individually screened in random order in the Functional Movement Screen (FMSTM) deep squat, the Y-balance anterior reach test (YBT), and a lumbar-locked thoracic rotation (LLR) test, which have all been shown to have excellent to good reliability in appropriately trained individuals (10,19,20).  The examiner for all participants was a certified FMSTM, and YBT practitioner, with over 6 years of experience screening individuals.  Pilot testing data demonstrated high intra-rater reliability for each test: FMSTM :100%, YBT: ICC = .847 (95% CI =.569-.945, p < .0001), LLR: ICC = .820 (95% CI = .518-.933, p < .001). 

The LLR scores were obtained with the athletes kneeling on an examination table, with hips sitting back on their heels. One elbow was placed between the knees, with the forearm outstretched flat on the table. The opposite hand was placed on the lower back.  A handheld digital inclinometer (Acumar- ACU001, Lafayette, IN) was placed perpendicular to the spine between the inferior angles of the scapula.  Participants were instructed to rotate their torso as far as possible toward the sky while maintaining contact of the forearm with the table.  Once maximum rotation was achieved, their angle of rotation was recorded.  This was performed two times for each side, recording the best angle (Figure 5).

Statistical Analysis

Statistical analyses were conducted with theStatistical Package for the Social Sciences version 23.0 (SPSS, Inc., Chicago, IL).  Statistical significance was determined using a two-tailed priori at p < 0.05.  Descriptive statistics were utilized.  Left and right side performance were compared for bilateral assessments.  Chi-square analyses were performed to assess FMSTM deep squat differences between participants with or without previously reported injuries.  Independent t-tests were used to compare preseason right and left sides, ground reaction forces, YBT, LLR, and all single leg balance measures between participants with or without previously reported injuries.  Paired t-tests were used to compare pre- and post FMSTM deep squat, YBT, LLR, ground reaction forces and single leg balance measures.  To determine the effect playing time had on performance changes, the total number of minutes played during the season was tabulated and compared to the performance measures.  Effect sizes were interpreted as very small (<0.2), small, (0.2-0.5), medium (0.5-0.8), or large (>0.8). 

RESULTS

Fourteen of the eighteen participants completed the study.  Three were injured during the season and did not regain medical clearance, and one quit the team.  Upon initial demographic and injury questionnaire data, nine participants reported experiencing an injury in the last four years in the lower extremities, yet all were medically cleared and listed on the active roster at the time of testing.  The most reported previous injury was an ACL tear (4), followed by ankle sprains (2), turf toe (2), and metatarsal fracture (1).  For the preseason data analysis, all eighteen initial participants were included.  Independent samples t tests of preseason data revealed significantly lower peak concentric rate of force development (RFD) in participants with prior reported lower extremity injuries (10830.8 ± 8250.9 N/s) than those with no prior injuries (40978 ± 29921.4 N/s) a statistically significant difference of 30147.2 N/s (95% CI, 6823.7 to 53470.7), (t(16) = 2.914, p = 0.017, d = 1.37).  No differences were noted in FMSTM deep squat, YBT, LLR, or balance measures between participants with or without previous injuries. 

Post-season data analysis only included fourteen participants who completed the study, with significant findings presented in Table 1.  There were no differences to report in any performance measure between previously injured and non-injured participants.  Paired samples t tests revealed significant decreases in the LLR assessment, as ROM decreased significantly on both the left (p = 0.004) and right (p = 0.007) sides, with no differences to note between sides.  Significant increases were noted in all single-leg balance measures recorded for both legs: Left COP excursion (p < 0.005), ML velocity (p = 0.001), AP velocity (p < 0.005), and Right COP excursion (p < 0.005), ML velocity (p < 0.005), AP velocity (p < 0.005).  Additionally, there was a significant decrease in peak concentric RFD (p = 0.03).  There were no significant changes to report in FMSTM deep squat, or YBT scores, or in players with greater playing time throughout the competitive season.

Table 1. Comparison of Preseason and Postseason Test Performance
Measure Direction/Limb Preseason mean (SD) Postseason mean (SD) Mean Difference (SD) 95% CI of Difference Effect Size P Value
Lumbar-Locked Rotation (LLR)a Left Rotation 69.92 (14.27) 57.67 (10.11) 12.25 (11.77) (4.77, 19.73) 0.35 0.004
Right Rotation 68.25 (11.54) 55.83 (14.89) 12.42 (13.11) (4.09, 20.75) 0.33 0.007
Balance (Center of Pressure Excursion)b Left 2.31 (1.01) 4.07 (0.49) -1.76 (.916) (-2.29, -1.23) -0.7 <.0005
Right 2.25 (0.65) 4.06 (0.5) -182 (.865) (-2.32, -1.32) -1.1 <.0005
Balance (Medial/Lateral Velocity)c Left .071 (.041) .115 (.014) -0.044 (.037) (-.065, -.023) -0.24 .001
Right .074 (.023) .116 (.016) -.0421 (.025) (-.056, -.028) -0.74 <.0005
Balance (Anterior/Posterior Velocity)c Left .076 (.027) .146 (.018) -.070 (.027) (-.085, -.055) -1.06 <.0005
Right .073 (.018) .143 (.017) -.0693 (.024) (-.083, -.056) -1.18 <.0005
Peak Concentric Rate of Force Development (RFD)d Bilateral Vertical Jump 21430.3 (23631.9) 5833.8 (2746.5) 15596.5 (23976.8) (1752.7, 29440.3) 0.33 .030

Significance observed (p < .05)
aValues are degrees of passive ROM
bValues are meters (m)
cValues are m/s
dValues are N/s
Note: Effect size = Cohen’s d

DISCUSSION

There were several primary findings of this research.  Initial data collection revealed a relationship between previous lower extremity injuries and RFD.  Participants who experienced lower extremity injuries in the previous four years had significantly lower RFD compared to those with no history of injury.  This may be due in part from residual deficits existing after those athletes were cleared to play. 

Significant increases in single leg balance measures were observed among all participants in each measure of balance after the season, suggesting overall balance decreased, as greater excursion distances and ML/AP velocities are interpreted as more ‘sway’ in the foot.  The Cohen’s effect size values suggest a high practical significance for all three balance measures.  This suggests the decrease in postural stability could be from the demands of the season.  Previous research has demonstrated that decreases in postural stability are due to weakness or fatigue in the surrounding muscle groups of the lower extremities, along with fatigue of the neuromuscular system (2,5)  

Another important finding in this study was the overall decrease in the LLR test that measures thoracic ROM and the Cohen’s effect size value suggests a high practical significance.  Field hockey requires athletes to repeatedly run in a bent-over position where the spine is flexed and/or laterally flexed while often forcefully rotating their spine in to the left when passing or shooting.  This data suggests the strain of this repetitive motion throughout the season can lead to significant decreases in overall thoracic ROM.  Previous research has shown ROM at specific joints may or may not cause changes in movement due to compensation for that deficit (12), however, compensation can lead to asymmetries which can eventually lead to disruptions in the kinetic chain during athletic performance.  The vertebral joints of the spine are very important when performing full-range symmetrical movements (12).  Even though every stick is right-handed in field hockey, thoracic ROM decreased in both directions.  While this finding opposes the logic of the imbalanced sport movement, the decreased ROM may have simply resulted from the overall demands of the sport.  It is unknown whether such decreases influence other areas of muscle imbalance or asymmetry because the YBT and balance measures did not result in significant between-limb asymmetries.  

The knowledge gained from this study is helpful for coaches and athletes to understand the effects prior lower extremity injuries have on the RFD, and the effects a competitive season has on thoracic ROM, single-leg balance, and RFD.  These findings provide support for regularly incorporating power development in previously injured athletes, along with focusing on balance, stability, and thoracic ROM exercises within strength and conditioning programs for field hockey athletes to continually target proprioceptive awareness and mobility of the spine.  Although this study did not track lower back injuries or pain symptoms, it seems plausible that continual decreases in balance performance and thoracic spine ROM could increase the risk of lower extremity or lower back injuries.

This study does have some limitations when interpreting the data.  The sample was a convenience sample from a single university, which could impact results due to their specific training regimen.  Only 14 participants were on the roster at the end of the season to therefore complete the study.  This may have underpowered the prospective analysis of comparing performance measures post-season.  Practice time, warm-up routines, and strength and conditioning practices were not investigated, which could also play an important role in the outcomes measured. 

Clinical Relevance

Based on these results, coaches and trainers who work with field hockey athletes should continually emphasize safe power development in athletes with a history of previous lower extremity injuries.  Additionally, balance training with a focus on single-leg exercises that challenge athletes’ proprioceptive awareness should be regularly implemented.  Finally, incorporating a variety of thoracic ROM exercises into a warm-up routine are recommended to maintain baseline levels of spinal rotation through a season in which much of the stress is unilateral due to posture when playing field hockey.

CONCLUSIONS

This study determined that Division III female field hockey athletes with a history of lower extremity injuries demonstrate significantly less concentric RFD during a vertical jump when compared to athletes with no prior injuries.  Balance performance and thoracic ROM significantly decreased in all athletes after the season.  Overall power performance, FMSTM deep squat and YBT anterior reach scores did not change throughout the season.  This data extends the knowledge of the effects prior injuries have on lower extremity performance measures, and how a competitive season effects functional movement, balance and power performance in collegiate female field hockey athletes.

APPLICATIONS IN SPORT

Coaches and trainers who work with field hockey athletes should continually emphasize safe power development in athletes with a history of previous lower extremity injuries.  Additionally, balance training with a focus on single-leg exercises that challenge athletes’ proprioceptive awareness should be regularly implemented.  Finally, incorporating a variety of thoracic ROM exercises into a warm-up routine are recommended to maintain baseline levels of spinal rotation through a season in which much of the stress is unilateral due to posture when playing field hockey.

ACKNOWLEDGMENTS

The authors would like to thank The Ohio State University for help in equipment usage during the data collection process.  None of the authors of this article have any conflicts of interest or financial conflicts to report.  There are no conflicts of interest to report and no funding was received for this study.

REFERENCES

  1. Beynnon, B. D., Vacek, P. M., Murphy, D., Alosa, D., & Paller, D. (2005). First-time inversion ankle ligament trauma: the effects of sex, level of competition, and sport on the incidence of injury. Am. J. Sports Med., 33, 1485-91.
  2. Bisson, E.J., Chopra, S., Azzi, E., Morgan, A., & Bilodeau, M. (2010). Acute Effects of Fatigue of the Plantarflexor Muscles on Difference Postural Tasks. Gait & Posture., 32(4), 482.
  3. Boyle, M. J., Butler, R. J., & Queen, R. M. (2016). Functional movement competency and dynamic balance after anterior cruciate ligament reconstruction in adolescent patients. J Ped Ortho., 36(1), 36-41.
  4. Braun, H. J. (2015). Differences in ACL biomechanical risk factors between field hockey and lacrosse female athletes. Knee Surgery, Sports Traumatology, Arthroscopy: Official J of the ESSKA., 23(4), 1065-1070.
  5. Corbeil, P., Blouin, J.S., Begin, F., Nougier, V., & Teasdale, N. (2003). Perturbation of the postural control system induced by muscular fatigue. Gait & Posture.,18(2), 92-100.
  6. Dick, R., Hootman, J. M., Agel, J., Vela, L., Marshall, S. W., & Messina, R. (2007). Descriptive epidemiology of collegiate women’s field hockey injuries: National Collegiate Athletic Association injury surveillance system, 1988-1989 through 2002-2003. J Ath Train., 42(2), 211-220.
  7. Fulton, J., Wright, K., Kelly, M., Zebrosky, B., Zanis, M., Drvol, C., & Butler, R. (2014). Injury risk is altered by previous injury: A systematic review of the literature and presentation of causative neuromuscular factors. Int J Sports Phys Ther., 9(5), 583-595.
  8. Grooms, D., Page, S., Nichols-Larsen, D., Chaudhari, A., White, S., & Onate, J. (2017). Neuroplasticity associated with anterior cruciate ligament reconstruction. J Orthop Sports Phys Ther., 47(3), 180-189.
  9. Grooms, D., Page, S., & Onate, J. (2015). Brain activation for knee movement measured days before second anterior cruciate ligament injury: Neuroimaging in musculoskeletal medicine. J Ath Train., 50(10), 1005-1010.
  10. Johnson, K. D., Kyung-Min Kim, Byung-Kyu Yu, Saliba, S. A., & Grindstaff, T. L. (2012). Reliability of Thoracic Spine Rotation Range- of-Motion Measurements in Healthy Adults. Journal of Athletic Training (National Athletic Trainers’ Association), 47(1), 52–60.
  11. Kawalek, K., & Garsztka, T. (2013). An analysis of muscle balance in professional field hockey players. TRENDS in Sport Sciences., 20(4), 181-187.
  12. Kiesel, K., Burton, L., Cook, G., & Mattacola, C. G. (2004). Mobility Screening for the Core. Athletic Therapy Today, 9(5), 38-41.
  13. Murtaugh, K. (2009). Field Hockey Injuries. Current Sports Medicine Reports American College Of Sports Medicine., 8(5), 267-272.
  14. Murtaugh, K. (2001). Injury patterns among female field hockey players. Med & Sci  Sports & Exercise., 33(2), 201-207.
  15. NCAA Sports Sponsorship & Participation Rates Report (n.d.). Retrieved from https://www.ncaapublications.com/p-4445-2015-16-ncaa-sports-sponsorship-and-participation-rates-report.aspx
  16. Ogurkowska, M. B., & Kawałek, K. (2017). Evaluation of functional and structural changes affecting the lumbar spine in professional field hockey players. Acta of Bioeng & Biomech., 19(2), 51-58.
  17. Ogurkowska, M. B., & Kawałek, K. (2016). Pathological changes in the lumbar intervertebral discs among professional field hockey players. J of Sports Med & Phys Fit., 56(1/2), 85-91.
  18. Ryan, E., Everett, K., Smith, D. B., Pollner, C., Thompson, B. J., Sobolewski, E. J., & Fiddler, R. (2014). Acute effects of different volumes of dynamic stretching on vertical jump performance, flexibility and muscular endurance. Clin Phys, and Funct. Imaging., 34(6), 485-492.
  19. Shaffer, S. W., Teyhen, D. S., Lorenson, C. L., Warren, R. L., Koreerat, C. M., Straseske, C. A., & Childs, J. D. (2013). Y-balance test: a reliability study involving multiple raters. Military Medicine, 178(11), 1264–1270.
  20. Teyhen, D. S., Shaffer, S. W., Lorenson, C. L., Halfpap, J. P., Donofry, D. F., Walker, M. J., & Childs, J. D. (2012). The Functional Movement Screen: A Reliability Study. Journal of Orthopaedic & Sports Physical Therapy, 42(6), 530–540.
Print Friendly, PDF & Email