Robert A. Panariello MS, PT, ATC, CSCS
Professional Physical Therapy
Professional Athletic Performance Center
New York, New York
With the fall sports season upon us, the expectation of ACL injuries is a reality. The reported incidence of ACL ruptures is approximately 200,000 in the United States annually. Most of these (58%-70%) are non-contact injuries occurring in young athletes. Many coaches, parents, and athletes often inquire how such a high ACL injury rate transpires without the incidence of physical contact by an opposing player. One main reason, considered by many, is the athlete’s shoe – playing surface interface.
There is epidemiologic evidence that increased traction at the shoe surface interface may lead to enhanced sports performance at the expense of increased risk of ACL injury. There is documentation as far back as the 1980’s that state ACL sprains are at higher risk on artificial turf, but at that time was thought by some to occur only in certain game situations as kickoffs and punts. There is also documentation noted in the 1990’s demonstrating that during NFL game exposure there was almost a five (5) times greater of ACL injury on grass vs. turf. However, for practice sessions the reverse was true. When reviewing the total exposures (practice and games) an incidence density ratio was calculated that revealed a 90% increase in ACL injuries on artificial turf per 1000 athlete exposures. In a similar but more recent study it was reported that more ACL injuries (per 1000 athletic exposures) ensue on turf vs. grass surfaces in professional football players.
In 2010 my good friend Dr. Mark Drakos published a study on this very topic. Dr. Drakos and his associates compared four (4) experimental groups evaluating the strain that occurred to the ACL during the application of an axially applied load with a standardized rotational moment (simulated cutting movement). These four (4) groups were as follows:
1. AstroTurf – turf shoes
2. Modern playing turf – turf shoes
3. Modern playing turf – cleats
4. Natural grass – cleats
The results of the study exhibited the natural grass and cleat combination (group 4) produced less strain upon the ACL than the modern playing turf – cleat (group 3), modern playing turf – turf shoe (group 2) and AstroTurf –turf shoe (group 1) combinations. It is also important to note that although the depth of the cleats was slightly larger than the depth of the turf shoe studs, the displacement into modern playing turf was similar for a given axial load.
The coefficient of friction and the coefficient of restitution
The resultant complex interaction between the bottom of the shoe and top of the surface is explained via the coefficient of friction and the coefficient of restitution. The coefficient of friction (grip strength) is closely related to torque and studies have demonstrated a higher incidence of ACL injuries with surfaces that have a higher coefficient of friction. The coefficient of restitution is the ability of the surface to absorb shock. It is measured by using the G-Max value where one “G” represents one unit of gravity. The United States Consumer Products Safety Commission determined that fields with a G-Max of greater than 200 are unsafe for athletic play.
The explanations for non-contact ACL injury are not entirely clear. Additional research is needed to assist in the clarity of this subject matter. Artificial turf surfaces allow for higher velocity (a faster game) due to the increased traction provided by the shoe – surface interface. However, with these higher velocity stadium surfaces there appears to be a related increased risk of non-contact ACL injuries. This coupled with many other identified risk factors makes the non-contact ACL injury a significant concern and health care matter in otherwise young healthy athletes.
Robert A. Panariello MS, PT, ATC, CSCS is a founding partner and Chief Clinical Officer at Professional Physical Therapy and the Professional Athletic Performance Center with 99 facilities in the NY, NJ, and CT tristate area. He has more than 35 years of experience in the related professional fields of sports physical therapy and the strength and conditioning of athletes, is well published in scientific and S&C journals, and lectures nationally on these related topics. He is a former Division I Collegiate and Professional Head Strength and Conditioning Coach and has studied the science of strength and conditioning and weightlifting in Bulgaria, the former Soviet Union and former East Germany. Rob was the recipient of the 2016 National Strength and Conditioning Association (NSCA) Sports Medicine/Rehabilitation Specialist of the Year Award, the 2015 American Physical Therapy Association (APTA) Sports Physical Therapy Section Lynn Wallace Clinical Educator Award, and was elected to the USA Strength and Conditioning Coaches Hall of Fame in 2003.