Author Archives: Darin A. Padua, PhD, ATC

Evaluating the “Threshold theory”: Can head impact indicators help?

Research Team Members: Jason Mihalik, PhD, ATC, Robert Lynall, PhD, ATC (HMSC PhD Student now at University of Georgia), Erin Wasserman, PhD (EXSS Gfeller Center Postdoc now at Datalys Center), Kevin Guskiewicz, PhD, ATC, Steve Marshall, PhD

Why did you do this study?

concussion-blog-featured-imageAs many as 1.6 – 3.8 million sport and recreation traumatic brain injuries (TBI) occur in the US on an annual basis. The direct and indirect costs for managing all forms of TBI exceed %56B annually. Proper detection and management of sport related concussion continues to challenge clinicians working with athletes. A number of options are available to clinicians, but mostly rely on subjective and clinical expertise. One example is the Sport Concussion Assessment Tool Version 3 (includes symptom inventories, mental status tests, and balance assessments). These acute injury screening tools are typically administered only after the clinician has sufficient evidence to suspect a concussion diagnosis. In the absence of obvious concussion signs (e.g., loss of consciousness, staggered gait, etc.), clinicians must rely solely on subjective symptoms reported by athletes. Research has documented a large portion of athletes either underreport concussion symptoms or fail to report them entirely. Thus, the medical field has looked to emerging technologies to fill this shortfall and provide heightened objectivity to the dilemma.

Technological advances have resulted in the emergence of commercially available head impact measurement devices. These devices typically serve two broad functions: 1) collect data for research-based inquiry, and 2) signal to clinical staff the occurrence of high-level impacts in near real-time during sports participation. Head impact indicators—the latter function—seek to identify athletes who have sustained pronounced head impacts so that they can be evaluated for symptomology. These products are usually worn directly on the head or affixed to a helmet, and are designed to indicate to medical personnel, players, coaches, and parents when a head impact magnitude has exceeded a pre-programmed threshold. There are no fewer than 20 different products that have permeated the marketplace in the last decade, and many use differing thresholds (some unknown to the user!).


Head impact indicators are believed to identify athletes who otherwise would elect not to report symptoms to the clinical staff. If an ‘alert’ is triggered, some of the manufacturers recommend the athlete be removed from activity and evaluated for a head injury, regardless of whether or not the athlete is exhibiting signs or reporting symptoms consistent with concussion. Our own work here at UNC suggests that a single impact injury threshold is not obvious, which question the clinical utility of these head impact indicators.

What did you do and what did you find in this study?

The purpose of this study was to investigate the clinical utility of head impact magnitude thresholds employed by various commercially available head impact indicators to positively predict concussion among American football players. We hypothesized these tools, by themselves, would be limited in helping clinicians make informed decisions regarding head injury during athletics due to the inherent variability of biomechanical values observed in concussed individual and the low incidence of concussion even at very high measured impact levels.

Over the last 10 years, we have collected hundreds of thousands of head impact biomechanics from hundreds of football players. A multidisciplinary clinical team independently made concussion diagnoses during this same time period (n=24). We dichotomized each impact using diagnosis (‘yes’ they were injured, ‘no’ they were not), and across a range of plausible impact indicator thresholds (10g increments beginning with a resultant linear head acceleration of 50g and ending with 120g). We then performed computations to determine the sensitivity, specificity, negative predictive value and positive predictive value, which are common measures used to assess the clinical utility of any diagnostic or screening assessment.


How do these findings impact the public?

In particular, any head impact indicator must demonstrate that it has predictive value; that is, it is an efficient use of time and resources and that it yields a practical frequency of identified concussions to be clinically useful. All thresholds we studied had low positive predictive value (<0.4%). Even when conservatively adjusting the frequency of diagnosed concussions by a factor of 5 to account for unreported/undiagnosed injuries, the positive predictive value of head impact indicators at any threshold was no greater than 1.94%. Simply put, fewer than 4 out of 1000 trigger alerts would result in a diagnosed concussion. Or, looking at it from the vantage of clinician time resources, 996 sideline evaluations would be done in vain and at the possible detriment of distracting the sideline medical personnel from observing and intervening in other emergencies and injuries during that time.

The Influence of Movement Profile on The Female Athlete’s Biomechanical Resilience & Training Load Response to Exercise

Research Team Members: Barnett Frank, PhD, ATC, Claudio Battaglini, PhD, Troy Blackburn, PhD, ATC, Anthony Hackney, PhD, DSc, Darin Padua, PhD, ATC

Why did you do this study?

Lack of physical activity is directly responsible for 9% of global premature mortality. Remarkably, exercise is consistently identified as a fundamental health behavior to effectively reduce one’s risk of disease. However, exercise participation carries a concerning high risk of musculoskeletal injury. Musculoskeletal injury amounts to a socioeconomic burden >6% of the U.S. gross domestic product. Paradoxically, injury is the primary barrier to exercise participation. Thus there is a need to prevent exercise-related musculoskeletal injury to promote the health and quality of life enhancing benefits of exercise.

Faulty movement patterns (i.e. knee collapsing and stiff hips and knees when landing – Figure 1) and elevated biochemical markers of musculoskeletal tissue stress are predictive of future injury during physical activity participation. Exercise interventions aimed at correcting faults in motion during physical activity reduce risk of injury. However, the underlying physiological mechanisms by which movement patterns modify risk for injury are unknown.


Abnormal movement patterns are theorized to impart a greater cumulative physical stress and systemic demand on the body during exercise, resulting in musculoskeletal system tissue failure and ultimately injury. Currently, it is unknown if there is a combined effect of an individual’s movement profile and exercise exposure on tissue and systemic stress measures associated with injury.

The purpose of this research was to investigate the influence of an individual’s movement profile on their physiological and biomechanical response to high training loads experienced during exercise and sport participation. Specifically, we investigated if a high injury risk / “stiff” or a low injury risk / “soft” movement profile affects the body’s systemic stress (cortisol), muscle loading (creatine kinase), cartilage degradation, and biomechanical response to high training load exposure.

What did you do and what did you find in this study?


43 college-aged female athletes were enrolled in this study and were assigned to a low-risk / “soft” (n=22) or a high-risk / “stiff” (n=21) movement profile group using a clinical movement injury risk assessment – The Landing Error Scoring System (Figure 1.) Jump-landing 3D biomechanics and blood samples were collected prior to and following a high training load exercise bout (Figure 2 & 3). Changes in biomechanics, circulating biomarkers of joint cartilage (cartilage oligomeric matrix protein) and skeletal muscle loading (creatine kinase), and of systemic stress (cortisol), were compared between movement profiles to better understand the influence of movement profile on the body’s response to the demands of exercise exposure (Figures 2 & 3).


We observed the high-risk / “stiff” landing group to experience greater degradation of movement strategies that effectively and efficiently dissipate landing forces experienced during high-intensity exercise. Specifically, we observed the high-risk / “stiff” group to land with a high-load landing posture and greater landing forces compared to the low-risk / low-load group when exposed to exercise. Furthermore, we observed movement profile to influence systemic stress hormone levels. Individuals with a high-risk / “stiff” movement profile exhibited an elevated stress level in contrast to their low-risk / “soft” landing profile counterparts. Additionally, it seems the low-risk / “soft” movement profile is linked to greater utilization of dynamic muscle tissue to efficiently dissipate the high loading stresses experienced during exercise and physical activity.

Interestingly, we did not observe a direct influence of movement profile on cartilage loading during exercise. However, we observed greater variability of cartilage loading responses in the high-risk / “stiff” landing group (standard deviation = ±43.9%) with over 1.5 times the range of responses compared to the low-risk / low-load group (standard deviation = ±29.4%). Implicating individuals with a low-risk / “soft” movement profile have a more uniform cartilage loading response compared to their high-risk / “stiff” landing counterparts.

How do these findings impact the public?

This study is the first to identify movement profile as a moderator of systemic responses to exercise. Collectively our findings suggest that an individual with a movement profile associated with a lower risk of injury may be more mechanically and systemically resilient to exercise exposure. Decreased system resilience in individuals with high-risk / “stiff” movement profiles may explain their elevated risk of sustaining a debilitating musculoskeletal injury during physical activity. Correcting a physically active individual’s faulty movement patterns may enhance their response to exercise while also decreasing their risk of musculoskeletal injury.

East Africian Distance Runners: Female Athlete Triad and Relative Energy Deficiency in Sport (RED-S) Conditions

Research Team Members: Martin Mooses, Anthony C. Hackney, Diresibachew H Wondimu, Robert Ojiambo and Amy R. Lane


International research team (R to L): Amy Lane, Marin Mooses, Silva Suvi, Robert Ojiambo, Diresibachew Wondimu, and Anthony Hackney

Why did you do this study?

The Female Athlete Triad (TRIAD) and more recently, Relative Energy Deficiency in Sport (RED-S) health conditions in men (male hypogonadism) have been linked a state called “low energy availability” (LEA). LEA occurs when an individual’s energy intake (food) minus their exercise energy expenditure is below a level that will ensure adequate energy for exercise as well as all physiological processes within the body (<30 kcal/kg body weight/day).


Maximal oxygen uptake test of elite athlete

LEA results in hormonal, metabolic and bone disorders which compromise overall health, but are specially related to reproductive system dysfunctions. These disorders also can have detrimental effects on exercise performance, injury rate, as well as impact aspects of the athletes’ health later in life (e.g., increased risk of osteoporosis).

Athletes who participate in endurance sports are at increased risk for LEA due to the extremely high volumes of exercise training they perform. The research conducted in this area so far has been done predominantly in Caucasian (US and European) populations. Very little is known, however, about prevalence in East African endurance athletes, who are some of the best runners in the world based upon the numerous world records and Olympic medals.

Earlier research by our group suggests that the low body mass and BMI of these African runners have benefitted their performance; i.e., through a more advantageous running economy, but those same anthropometric factors (body mass, BMI) could also put them at risk for the TRIAD or RED-S conditions. Therefore, the focus of our study is to identify the prevalence and risk factors for the TRIAD and RED-S in East African elite distance runners (both females and males). The intent is to collect data in field settings where the athletes live and train.

What did you do and what did you find in this study?


Field tests for blood lactate responses (lactate threshold test)

The study has been operating for the last 9 months. This past fall and winter, members of our international team from Ethiopia, Kenya, Estonia as well as UNC-CH have been on-site in Kenya working collecting data on male and female athletes at a running training camp at Eldoret, Kenya (2200 meters elevation, southwestern Kenya). UNC-CH EXSS professor Anthony Hackney and his doctoral student Amy Lane have been on sight collecting body composition, training, blood samples, nutritional and psychological data from the research subjects (pictures 1,2,3). The logistics of how to collect some of this data in rural areas of Kenya have been challenging at times.

The work is moving towards a commencement of the final data collection in fall 2017. Participants will include 30 female and 30 male elite East African endurance runners, with 30 female and 30 male adult non-athletes matched for age and ethnicity as controls. This study is funded on a 2 year grant through the International Athletics Foundation (project # 417) and is  truly collaborative international project. The main member of the research team are shown in picture 4.

How do these findings impact the public?


UNC doctoral student interviewing one of the research subjects relative to there training and dietary history

The findings from this study will help build knowledge about the impact that LEA has on elite East African endurance athletes. Through the identification of prevalence rates, this work can contribute to development of future interventions to minimize the prevalence, prevent occurrence and improve recovery from TRIAD and/or RED-S. Additionally, bringing this research to Kenya may increase education about energy availability to a broader audience.

Enhancing Public Dissemination and Understanding of Injury Risk in Sport

This week’s EXSS Impact Post is by Dr. Zack Kerr, Assistant Professor in Exercise and Sport Science.  Dr. Kerr is an epidemiologist and explores different reporting methods of injury epidemiology data to improve public dissemination and understanding.

Why did you do this study?

Over the past decade, I have immersed myself in the world of sports injury surveillance. The findings are of great importance because they are the basis for many data-driven decisions regarding the rules and safety in sports organizations such as the National Collegiate Athletic Association (NCAA) and the National Federation of State High School Associations (NFHS).

However, at times, I found it difficult to discuss findings to stakeholders of sports organizations, such as parents, coaches, and administrators, because we in academia like to rely on the injury rate to measure injury incidence. The injury rate is defined as:


Injury rates are preferred because they account for all injury events in the numerator, regardless of whether or not these were sustained by the same athletes. They also account for variation in the amount of exposure time via the denominator; thus, an athlete that plays more across a season provides more exposure time. But for many parents, what matters most to them is knowing the risk; in other words, what is the probability that their child will get injured within a specific timeframe (e.g. one season).

As a result, using data from the NCAA Injury Surveillance Program, I worked with a team of epidemiologists and athletic trainers to examine a variety of methods of measuring injury incidence. This team included:

  • Karen G. Roos, California State University – Long Beach
  • Aristarque Djoko, Datalys Center for Sports Injury Research and Prevention
  • Sara L. Dalton, Datalys Center for Sports Injury Research and Prevention
  • Steve P. Broglio, University of Michigan
  • Stephen W. Marshall, University of North Carolina at Chapel Hill
  • Thomas P. Dompier, Datalys Center for Sports Injury Research and Prevention

Given the interest, we opted to examine these measures with concussion.

What did you do and what did you find in this study?

We used concussion data from the NCAA Injury Surveillance Program during the 2011/12-2014/15 academic years. The NCAA Injury Surveillance Program has been in existence since the early 1980s and have been assisting the NCAA in assessing sport-related safety in their sponsored programs. Participation in the NCAA Surveillance Program varied by sport and academic year. Data were collected by team athletic trainers that worked with these sport programs during the season.

We computed four measures of concussion incidence in a 13 different sports:

Men’s sports Women’s sports
–   Baseball

–   Basketball

–   Football

–   Ice Hockey

–   Lacrosse

–   Soccer

–   Wrestling

–   Basketball

–   Field hockey

–   Ice hockey

–   Lacrosse

–   Softball

–   Volleyball

The four measures are described in the table below.

Concussion rate

At what rate are concussions sustained during at-risk exposures?

Example: Across 10,000 NCAA football athlete-exposures, we expect to see 6-7 concussions

One-season risk of concussion

What is the probability of an athlete obtaining a concussion in one season?

Example: In one season, we expect 1 in 20 NCAA football players to have a concussion?

Average # concussion per team per season

How many concussions does a team sustain in one season?

Example: In one season, we expect 5-6 concussions within a NCAA football team

% teams with a concussion

How many teams have a concussion occur within a season?

Example: In one season, we expect 80.6% of all NCAA football teams to have at least one concussion

The computations for the measures included alongside rates are included below:




What we found is that despite some variation in the rank-order of included sports, full contact sports such as wrestling, football, and ice hockey consistently generated the highest incidence of concussion.





Despite the similar rankings across sports, we believe that squad size may serve as a confounder, particularly in football. Furthermore, such measures can be biased when comparing incidence across teams (or sports) that vary greatly by the number of athletic sessions per season. Thus, it is important for readers to understand the strengths and limitations of measures of injury incidence utilized by various researchers.

How do these findings impact the public?

Although injury rates are the most preferred method of gauging injury incidence in academia, they may not be intuitive to non-scientists, including members of sports organizations concerned about the incidence of injury among their players. To help parents, coaches, and athletes, and to drive the development of data-driven, evidence-based policy and rule changes, we need to ensure that we are providing our findings in an easily understood manner.

This research presents a collection of “alternative facts” that still utilize the data collected by athletic trainers in a valid manner, but may be easier to interpret and disseminate to stakeholders. Better yet, these measures are applicable to other injuries and settings. This more diverse “toolbox” of measures, in combination with traditional athlete-based rates, may help sports organizations better identify specific athletes at risk for injury.

Epidemiology of Hip Flexor and Hip Adductor Strains in National Collegiate Athletic Association Athletes, 2009/10-2014/15

Research Team Members: Timothy G. Eckard, PT, DPT, OCS, Darin A. Padua, PhD, ATC, Thomas P. Dompier, PhD, ATC, Sara Dalton M.Ed, LAT, ATC, Kristian Thorborg, PT, PhD, Zachary Y. Kerr, PhD, MPH

hip-adductorsWhy did you do this study?

Lower extremity muscle strains are common in collegiate sports. Strains result in significant participation restriction time for athletes, as they often require extensive rehabilitation and frequently recur. Two muscle groups vulnerable to strain injury in sports are the hip flexor and hip adductor groups. The hip flexor muscles, including the psoas major and the iliacus (i.e., the iliopsoas group), are injured in athletic activities such as sprinting, kicking, and cutting. The adductor group shares the same mechanisms of injury and consists of muscles in the medial compartment of the thigh including the pectineus, adductor magnus, adductor longus, adductor brevis, obturator externus, and gracilis. Elucidating the rates and patterns of these injuries in collegiate athletes will help determine the need for primary and secondary prevention programs in various NCAA sports. However, research on hip flexor and hip adductor strains has been mostly limited to professional athletes and frequently does not distinguish between strains of the disparate muscle groups around the hip. Therefore, the purpose of our study was to describe the epidemiology of hip flexor and hip adductor strains in 25 NCAA sports.

hip-flexorsWhat did you do and what did you find in this study?

Rates and patterns of hip flexor and hip adductor strains in collegiate sports were examined in a sample of NCAA varsity teams from 25 sports. Rates and distributions of strains by mechanism, recurrence, and time loss were examined. Rates were then compared within and between sports by event type (practice versus competition), sex, mechanism, recurrence, and time loss. A total of 770 hip flexor and 621 hip adductor strains occurred over the six academic years examined. The highest rates were in men’s soccer and men’s ice hockey. Most strains occurred in practice, but the rate of both types of strains was higher in competition. In sex-comparable sports, the hip flexor strain rate did not differ between the sexes but the hip adductor strain rate was higher in men than women. Non-contact was the most common mechanism for both types of strains, and most resulted in less than one week of time loss.

soccer-playerHow do these findings impact the public?

Our findings suggest that prevention programs for hip flexor strains should be developed and implemented across male and female sports teams, particularly in soccer and ice hockey. Male sports teams, especially soccer and ice hockey teams, should place an emphasis on prevention programs for hip adductor strains. Secondary prevention programs involving thorough rehabilitation and strict return to play criteria should be developed and implemented to curb the high recurrence rate of these injuries, particularly in ice hockey.


Sponsorship Revenue Forecasting for Sport Organizations: A Survival Analysis Approach

Research Team Members: Jonathan A. Jensen*, Brian A. Turner†, Natalie Caneja*, David Head*, Akash Mishra*, and Tyler Wisniewski*

* University of North Carolina, Chapel Hill, NC; † The Ohio State University, Columbus, OH

Why did you do this study?

university-athletic-departments-are-increasngly-reliant-on-apparel-sponsorships-and-multimedia-rights-agreementsOne of the more important evolutions in the sport industry over the past decade has been the marked increase in the application of advanced methodologies to ascertain patterns in data, or analytics. Numerous new methodological approaches are now being applied to assist sport organizations in decision-making relative to scouting, player development, and resource allocation. However, analytics are just now beginning to be applied off of the field to the business side of sport organizations, in areas such as ticket pricing and sales.

One area that has yet to be impacted by this trend in the application of analytics is revenue projections and forecasting. Despite monumental gains in other areas, revenue forecasts for many sport organizations still largely depend on the renewal rate, simply the annual percentage of sponsors or ticket holders who renew their relationship with the organization.

This research offers a new approach to the analysis and forecasting of revenue from an increasingly important revenue source for sport organizations, commercial sponsorship. Many non-profit sport organizations, such as the International Olympic Committee (IOC), the United States Olympic Committee (USOC) and their many National Governing Bodies (NGBs), as well as the National Collegiate Athletic Association (NCAA) and intercollegiate athletic departments at its member institutions, depend on commercial sponsorship for an increasingly larger portion of their annual revenue.

What did you do and what did you find in this study?

the-non-profit-international-olympic-committee-ioc-relies-on-sponsorship-for-44-of-its-annual-revenueGiven these challenges, the purpose of this research is to apply advanced methodological approaches to assist these various types of organizations

in sponsorship revenue forecasting. Specifically, this research represents the first application of survival analysis modeling approaches in an empirical investigation of the duration of sponsorships. Typically utilized in the biostatistics and medical fields, survival analysis approaches can provide a wealth of additional information about the duration of sponsorships. For example, rather than simply providing information on how many sponsors typically renew, this approach unearths a variety of additional information, including when the probability of a sponsor ending its relationship is the highest and the median lifetime of the sponsorships.

Initial results of the research, which was named a finalist in the research paper competition at the 2016 MIT Sloan Sports Analytics Conference and is slated to be published in Sport Marketing Quarterly, has utilized the context of global sport organizations, such as the International Olympic Committee (IOC) and the Fédération Internationale de Football Association (FIFA). Current research being performed in conjunction with students in the UNC Sport Administration program extends the research to the context of intercollegiate athletics, including athletic apparel sponsorships, multimedia rights agreements, title sponsorships of postseason bowl games, and naming rights agreements of facilities. Future research involves the insertion of covariates into the models, to ascertain whether there are certain conditions or factors that can actually predict the end of these partnerships.

How do these findings impact the public?

from-2011-14-the-fifa-world-cup-generated-more-than-1-6-billion-in-revenue-from-sponsorshipThe impact of the research involves assisting non-profit sport organizations in particular in improving their understanding of when these marketing partnerships are most susceptible to dissolution and their ultimate duration, as well as what factors may be predictive of the end of such partnerships. Given that many non-profit sport organizations depend on sponsorship revenue for their survival, these efforts help them to better forecast revenue they receive from this increasingly important source. In addition, the identification of covariates that may be statistically significant predictors of the dissolution of such partnerships may help these organizations isolate certain factors that should be closely monitored throughout the relationship, and identify the types of sponsors more likely to enter into longer-term relationships that can help guarantee the survival of the organization for many years to come.

Testing Strength in ACL Reconstructed Patients: Is Symmetry the Answer?

This is a summary of a recently published article in Medicine and Science of Sport and Exercise entitled “Quadriceps Strength Predicts Self-Reported Function Post ACL Reconstruction”.

Dr. Pietrosimone’s co-authors on this paper include Adam Lepley, Matthew Harkey, Brittney Luc- Harkey, Troy Blackburn, Phillip Gribble, Jeffrey Spang and David Sohn.

Why Did We Perform this Study

quadriceps-musclesQuadriceps muscle weakness is common following anterior cruciate ligament (ACL) injury and ACL reconstruction. This muscle weakness often persists many years after someone has undergone knee surgery and returned to participation in physical activity. Quadriceps weakness often leads to greater disability in those with ACL reconstruction, and there is evidence that quadriceps dysfunction may negatively impact gait mechanics that can increase the risk of developing knee osteoarthritis. Therefore, it is critical for patients who have undergone ACL reconstruction to maximize quadriceps strength following surgery and maintain optimal quadriceps strength throughout their lifetime. Unfortunately, it remains unclear how much quadriceps strength is needed to function at a high level following ACL reconstruction. Furthermore, there are no established best practice guidelines for quantifying quadriceps strength in patients with an ACL reconstructions. Traditionally, many clinicians and researchers have compared quadriceps strength of the injured limb to that of the uninjured limb; thereby trying to maximize the strength symmetry between limbs after ACL injury. Conversely, we have previously reported that greater overall quadriceps strength, normalized to the body mass of the individual, is strongly associated with self-reported disability in those with an ACL reconstruction. Therefore, in this study, we wanted to determine the best method for using quadriceps strength to predict self-reported function following ACL reconstruction. We felt that this study would help us develop valuable clinical cuff-off scores that could be used to guide strengthening regimens for patients with an ACL reconstruction.

What we did in this study


We tested isometric quadriceps strength at 90-deg of knee flexion. Most individuals following ACL injury and reconstruction can be tested in this position at multiple time points. Also, testing in this manner may be more easily conducted with a range of strength testing instrumentation in clinical and research settings.

We recruited 96 individuals with a primary ACL reconstruction on only one limb (62 females and 34 males; 21.7±3.85 years old; 37.04 ± 36.7 [range 6-161] months post ACLR). We tested the maximal strength quadriceps strength on each limb in a random order using a dynamometer. Each participant performed maximal voluntary isometric contractions in a seated position with their knees flexed to 90° (See Figure 1). On the same day participants completed the subjective section of the International Knee Documentation Committee Index questionnaire in order to determine the magnitude of self-reported disability for each participant. We considered anyone scoring over 90% on the International Knee Documentation Committee Index to be at a high level of function (we termed these individuals “High Functioners”). Next we created Receiver Operating Characteristic Curves to determine how well quadriceps strength predicted who would be a High Functioner. After determining the quadriceps strength cut-off scores that maximized the sensitivity and specificity for predicting the High Functioner status, we calculated odds ratios to determine how well our cut-off scores were at determining a High Functioner status.

What we found and how it Impacts Healthcare

Overall, regardless of quantification method, quadriceps strength was able to significantly predict which individuals would be High Functioners. Interestingly we found that overall quadriceps strength, normalized to body mass ,demonstrated a higher accuracy for predicting who would be a High Functioner, compared to quadriceps strength symmetry. Specifically, those who were able to produce overall quadriceps strength that exceeded 3.10 Newton-meters per kilogram of body mass demonstrated 8.15 times higher odds of being a High Functioner. Therefore, a 150lb individual (68.18 kg) would need to generate 211.36 Newton-meters of torque with their quadriceps in order meet the prescribed cut-off score. We found that individuals who achieved a strength symmetry value of 96.5% (injured limb strength/ uninjured limb strength) would have a 2.78 times higher odds of being a High Functioner. Therefore, an individual who can produce 250 Newton-meters of torque with the uninjured limb would need to produce 241.25 Newton-meters of torque on the ACL reconstructed limb to achieve this cut-off value.

Overall Strength vs Strength Symmetry

Strength symmetry has traditionally been used to determine which participants are able to generate enough strength to return to participation in physical activity. While improving strength symmetry is still important for ACL reconstructed patients, achieving symmetry alone may not be sufficient for optimizing therapeutic outcomes. It is possible that some people may demonstrate symmetrical quadriceps strength, yet they may not exhibit enough strength relative to the size of their body. Our data suggests that maximizing overall quadriceps strength to support the size of the person is critical in determining the functional status for that individual. Bilateral quadriceps weakness may be of particular concern in individuals who have a history of bilateral ACL injury. Rehabilitation goals for ACL reconstructed individuals may need to be amended to include the development of strength relative to a patient’s size rather than the contralateral limb. Bilateral strengthening may be critical for many patients in order to achieve the desired ratio on the ACL reconstructed limb (3.10 Newton-meters/ kilograms of body mass), while still marinating strength symmetry between limbs. It may also be necessary to address the maintenance of a healthy body weight, or when prudent, to lose excess non-lean body weight which will also improve the ratio of quadriceps strength to body mass. Future ACL rehabilitation guidelines should consider testing patients’ overall strength relative to their body mass in addition to strength symmetry outcomes.