The mechanics of American football cleats on natural grass and infill-type artificial playing surfaces with loads relevant to elite athletes

A mechanical comparison of the translational traction of female-specific and male soccer boots

The studded outsole of a soccer boot provides additional traction to players to minimise the risk of slipping while performing high-speed manoeuvres. As excessive traction can lead to foot fixation and injury risk, there has been significant research surrounding the influence of stud configuration on the level of traction generated. This previous research, however, has predominately focused on the stud patterns, foot morphology and lower limb loading patterns of male players. As the popularity of women's soccer increases, the aim of this investigation was to examine the differences in translational traction of female-specific soccer boots and male soccer boots currently available. A custom-built apparatus was used to determine the translational traction on both natural and artificial grass for four different movement directions. It was hypothesised that the female-specific boot in each pair would produce lower levels of translational traction as they are designed to be safer for female players who are more at risk of lower limb fixation injuries compared to males. An independent samples T-test showed that while there were some loading conditions where female boots produced lower translational traction compared to male boots, across all loading scenarios there was no significant difference between male and female boots (p = 0.818), thus the null hypothesis was rejected.

Lower Extremity Injury Rates on Artificial Turf Versus Natural Grass Surfaces in the National Football League During the 2021 and 2022 Seasons

Background

It has been argued that the use of artificial turf football fields in the National Football League (NFL) increases player injury risk compared with natural grass surfaces.

Purpose/Hypothesis

The purpose of this study was to quantify the rate of lower extremity injuries occurring in NFL players on artificial turf compared with natural grass surfaces and characterize the time missed due to injury and proportion of injuries requiring surgery. It was hypothesized that lower extremity injuries requiring surgical intervention would occur at a higher rate on artificial turf than on natural grass.

Study Design

Descriptive epidemiology study.

Methods

Lower extremity injury data for the 2021 and 2022 NFL seasons were obtained using publicly available records. Data collected included injury type, player position, player age, playing surface, weeks missed due to injury, and whether the patient underwent season-ending or minor surgery. Multivariable logistic regression was performed to determine the risk of season-ending surgery according to playing surface.

Results

When combining injuries for the 2021 and 2022 seasons (N = 718 injuries), the incidence rate of lower extremity injury was 1.22 injuries/game for natural grass and 1.42 injuries/game for artificial turf. The odds of a season-ending surgery were found to be significantly higher on artificial turf compared with natural grass (odds ratio = 1.60; 95% CI, 1.28-1.99; P < .05), while additional variables, including weather, age, position, week of injury occurrence, and history of prior injury, did not influence the odds of season-ending surgery.

Conclusion

The 2021 and 2022 NFL seasons of our analysis demonstrated a higher incidence rate of injuries on artificial turf surfaces compared with natural grass surfaces. In addition, the odds of injury requiring season-ending surgery were found to be significantly higher on artificial turf compared with natural grass.

Effect of Soccer Boot Outsole Configuration on Translational Traction Across Both Natural and Artificial Playing Surfaces

Background

Soccer boots are produced with different stud patterns and configurations to provide players with extra traction on specific surface types to minimize slipping and improve player performance. Excessive traction, however, can lead to foot fixation injuries, particularly anterior cruciate ligament tears.

Purpose/Hypothesis

The purpose of this study was to explore the translational traction properties of 5 different outsole configurations moving in 4 different directions across both natural grass and artificial grass (AG) playing surfaces. It was hypothesized that longer studs or studs with an asymmetric shape would yield a higher traction coefficient compared with the recommended stud configuration for the given playing surface.

Study Design

Descriptive laboratory study.

Methods

A custom-built testing apparatus recorded the translational traction of 5 different soccer boots moving in an anterior, posterior, medial, or lateral direction on both natural grass and AG playing surfaces. A 3-way analysis of variance was performed to determine the effect of outsole configuration on the traction, and a post hoc Tukey analysis was performed to compare different outsole configurations with a control.

Results

For the natural grass playing surface, the longer and asymmetric studs yielded a significantly higher (P < .05) traction coefficient on 75% of loading scenarios, while on AG, they yielded a significantly higher traction on 50% of loading scenarios.

Conclusion

Some soccer boots yielded higher traction values compared with the recommended configuration.

Clinical Relevance

The results highlight the importance of boot selection on different playing surfaces. Higher traction values could increase the injury risk for players due to excessive traction and foot fixation.

Epidemiology of football-related injuries in young male football players. An additional analysis of data from a cluster-randomised controlled trial

Football carries a high risk of injury for youth players. The aim of this study was to investigate the epidemiology of football-related injuries in young male players. The data stems from a previously conducted cluster-randomised controlled trial that investigated the efficacy of 'FUNBALL', a new injury prevention programme. This study contains the data of the 503 players of the control arm. The players belonged to 22 football teams of the Under-(U)15, U17 and U19 age groups. The time-loss injuries were recorded during the season 2021-2022 according to the Football Consensus Statement. An analysis on the injury incidence (IR, calculated per 1000 hours of exposure), location, severity, category, and type was performed. Incidence rate ratios (IRRs) were used to compare the variables between the specific age groups. 187 injuries (96 in training and 91 in matches) occurred during 52 938 hours of exposure. The overall IR was 3.53 injuries/1000 h (95% confidence intervals (CI) 3.06 to 4.07). The training IR was 2.16 injuries/1000 h (95% CI 1.17 to 2.64). The match IR was 10.50 injuries/1000 h (95% CI 8.55 to 12.89). In the U19s, the overall IRR was higher compared to the U17s (IRR 1.57, CI 1.12 to 2.19; p = 0.008) and compared to the U15s (IRR 1.82, 95% CI 1.25 to 2.62; p = 0.001). The thigh was the most commonly affected body region (IR 0.92/1000 h, 95% CI 0.69 to 1.22). Muscle injuries were the most common injury type (IR 1.05/1000 h, 95% CI 0.81 to 1.37). Injury burden was 74 lost days/1000 h. The findings of this study indicate a lower injury incidence in youth players than in adult ones. We observed a higher injury incidence towards the older age groups.

Physiological and Biomechanical Monitoring in American Football Players: A Scoping Review

American football is the sport with the highest rates of concussion injuries. Biomedical engineering applications may support athletes in monitoring their injuries, evaluating the effectiveness of their equipment, and leading industrial research in this sport. This literature review aims to report on the applications of biomedical engineering research in American football, highlighting the main trends and gaps. The review followed the PRISMA guidelines and gathered a total of 1629 records from PubMed (n = 368), Web of Science (n = 665), and Scopus (n = 596). The records were analyzed, tabulated, and clustered in topics. In total, 112 studies were selected and divided by topic in the biomechanics of concussion (n = 55), biomechanics of footwear (n = 6), biomechanics of sport-related movements (n = 6), the aerodynamics of football and catch (n = 3), injury prediction (n = 8), heat monitoring of physiological parameters (n = 8), and monitoring of the training load (n = 25). The safety of players has fueled most of the research that has led to innovations in helmet and footwear design, as well as improvements in the understanding and prevention of injuries and heat monitoring. The other important motivator for research is the improvement of performance, which has led to the monitoring of training loads and catches, and studies on the aerodynamics of football. The main gaps found in the literature were regarding the monitoring of internal loads and the innovation of shoulder pads.

Force-limiting and the mechanical response of natural turfgrass used in the National Football League: A step toward the elimination of differential lower limb injury risk on synthetic turf

Lower limb injury rate in the National Football League (NFL) is greater on synthetic turf than on natural turfgrass. Foot loading in potentially injurious situations can be mitigated by damage to natural turfgrass that limits the peak load by allowing relative motion between the foot and the ground. Synthetic turf surfaces do not typically sustain such damage and thus lack such a load-limiting mechanism. To guide innovation in synthetic turf design, this paper reports 1) the peak loads of natural turfgrass when loaded by a cleated footform and 2) corridors that define the load-displacement response. Kentucky bluegrass [Poa pratensis, L.] and two cultivars of hybrid bermudagrass [Cynodon dactylon (L.) Pers × C. transvaalensis Burtt Davy] were tested with two cleat patterns in three loading modes (anterior-posterior or AP translation, medial-lateral or ML translation, and forefoot external rotation) at two power levels (full-power, which generated potentially injurious loads, and reduced-power, which generated horizontal forces similar to non-injurious ground reaction forces applied by an elite athlete during play). All tests generated peak force<4.95 kN and torque<173 Nm, which is in a loading regime that would be expected to mitigate injury risk. In full-power tests, bermudagrass withstood significantly (p < 0.05) greater peak loads than Kentucky bluegrass: (3.86 ± 0.45 kN vs. 2.66 ± 0.23 kN in AP, 3.25 ± 0.45 kN vs. 2.49 ± 0.36 kN in ML, and 144.8 ± 12.0 Nm vs. 126.3 ± 6.1 Nm in rotation). Corridors are reported that describe the load-displacement response aggregated across all surfaces tested.

Epidemiology of Sports-Specific Foot and Ankle Injuries

The epidemiology of any given topic sometimes is overlooked. This is true particularly with sports physicians and sports injuries. The identification of sports-specific injury patterns by collection and examination of data can help prevent injuries. Thus, as a physician involved in any sport, it is essential to have this knowledge because understanding it and imparting it may allow a valuable contribution to the health and safety of the athletes and success of the teams.

Incidence of Lower Extremity Injury in the National Football League: 2015 to 2018

BACKGROUND

Lower extremity injuries are the most common injuries in professional sports and carry a high burden to players and teams in the National Football League (NFL). Injury prevention strategies can be refined by a foundational understanding of the occurrence and effect of these injuries on NFL players.

PURPOSE

To determine the incidence of specific lower extremity injuries sustained by NFL players across 4 NFL seasons.

STUDY DESIGN

Descriptive epidemiology study.

METHODS

This retrospective, observational study included all time-loss lower extremity injuries that occurred during football-related activities during the 2015 through 2018 seasons. Injury data were collected prospectively through a leaguewide electronic health record (EHR) system and linked with NFL game statistics and player participation to calculate injury incidence per season and per 10,000 player-plays for lower extremity injuries overall and for specific injuries. Days lost due to injury were estimated through 2018 for injuries occurring in the 2015 to 2017 seasons.

RESULTS

An average of 2006 time-loss lower extremity injuries were reported each season over this 4-year study, representing a 1-season risk of 41% for an NFL player. Incidence was stable from 2015 to 2018, with an estimated total missed time burden each NFL season of approximately 56,700 player-days lost. Most (58.7%) of these injuries occurred during games, with an overall higher rate of injuries observed in preseason compared with regular season (11.5 vs 9.4 injuries per 10,000 player-plays in games). The knee was the most commonly injured lower extremity region (29.3% of lower body injuries), followed by the ankle (22.4%), thigh (17.2%), and foot (9.1%). Hamstring strains were the most common lower extremity injury, followed by lateral ankle sprains, adductor strains, high ankle sprains, and medial collateral ligament tears.

CONCLUSION

Lower extremity injuries affect a high number of NFL players, and the incidence did not decrease over the 4 seasons studied. Prevention and rehabilitation protocols for these injuries should continue to be prioritized.

The Influence of Soccer Playing Surface on the Loading Response to Ankle (P)Rehabilitation Exercises

CONTEXT

Contemporary synthetic playing surfaces have been associated with an increased risk of ankle injury in the various types of football. Triaxial accelerometers facilitate in vivo assessment of planar mechanical loading on the player.

OBJECTIVE

To quantify the influence of playing surface on the PlayerLoad elicited during footwork and plyometric drills focused on the mechanism of ankle injury.

DESIGN

Repeated-measures, field-based design.

SETTING

Regulation soccer pitches.

PARTICIPANTS

A total of 15 amateur soccer players (22.1 [2.4] y), injury free with ≥6 years competitive experience.

INTERVENTIONS

Each player completed a test battery comprising 3 footwork drills (anterior, lateral, and diagonal) and 4 plyometric drills (anterior hop, inversion hop, eversion hop, and diagonal hop) on natural turf (NT), third-generation artificial turf (3G), and AstroTurf. Global positioning system sensors were located at C7 and the mid-tibia of each leg to measure triaxial acceleration (100 Hz).

MAIN OUTCOME MEASURES

PlayerLoad in each axial plane was calculated for each drill on each surface and at each global positioning system location.

RESULTS

Analysis of variance revealed a significant main effect for sensor location in all drills, with PlayerLoad higher at mid-tibia than at C7 in all movement planes. AstroTurf elicited significantly higher PlayerLoad in the mediolateral and anteroposterior planes, with typically no difference between NT and 3G. In isolated inversion and eversion hopping trials, the 3G surface also elicited lower PlayerLoad than NT.

CONCLUSIONS

PlayerLoad magnitude was sensitive to unit placement, advocating measurement with greater anatomical relevance when using microelectromechanical systems technology to monitor training or rehabilitation load. AstroTurf elicited higher PlayerLoad across all planes and drills and should be avoided for rehabilitative purposes, whereas 3G elicited a similar mechanical response to NT.

Incidence of Knee Injuries on Artificial Turf Versus Natural Grass in National Collegiate Athletic Association American Football: 2004-2005 Through 2013-2014 Seasons

BACKGROUND

The use of artificial turf in American football continues to grow in popularity, and the effect of these playing surfaces on athletic injuries remains controversial. Knee injuries account for a significant portion of injuries in the National Collegiate Athletic Association (NCAA) football league; however, the effect of artificial surfaces on knee injuries remains ill-defined.

HYPOTHESIS

There is no difference in the rate or mechanism of knee ligament and meniscal injuries during NCAA football events on natural grass and artificial turf playing surfaces.

STUDY DESIGN

Descriptive epidemiology study.

METHODS

The NCAA Injury Surveillance System Men's Football Injury and Exposure Data Sets for the 2004-2005 through 2013-2014 seasons were analyzed to determine the incidence of anterior cruciate ligament (ACL), posterior cruciate ligament (PCL), medial collateral ligament (MCL), medial meniscus, and lateral meniscal tear injuries. Injury rates were calculated per 10,000 athlete exposures, and rate ratios (RRs) were used to compare injury rates during practices and competitions on natural grass and artificial turf in NCAA football as a whole and by competition level (Divisions I, Divisions II and III). Mechanisms of injury were calculated for each injury on natural grass and artificial turf surfaces.

RESULTS

A total of 3,009,205 athlete exposures and 2460 knee injuries were reported from 2004 to 2014: 1389 MCL, 522 ACL, 269 lateral meniscal, 164 medial meniscal, and 116 PCL. Athletes experienced all knee injuries at a significantly higher rate when participating in competitions as compared with practices. Athletes participating in competitions on artificial turf experienced PCL injuries at 2.94 times the rate as those playing on grass (RR = 2.94; 95% CI, 1.61-5.68). When stratified by competition level, Division I athletes participating in competitions on artificial turf experienced PCL injuries at 2.99 times the rate as those playing on grass (RR = 2.99; 95% CI, 1.39-6.99), and athletes in lower NCAA divisions (II and III) experienced ACL injuries at 1.63 times the rate (RR = 1.63; 95% CI, 1.10-2.45) and PCL injuries at 3.13 times the rate (RR = 3.13; 95% CI, 1.14-10.69) on artificial turf as compared with grass. There was no statistically significant difference in the rate of MCL, medial meniscal, or lateral meniscal injuries on artificial turf versus grass when stratified by event type or level of NCAA competition. No difference was found in the mechanisms of knee injuries on natural grass and artificial turf.

CONCLUSION

Artificial turf is an important risk factor for specific knee ligament injuries in NCAA football. Injury rates for PCL tears were significantly increased during competitions played on artificial turf as compared with natural grass. Lower NCAA divisions (II and III) also showed higher rates of ACL injuries during competitions on artificial turf versus natural grass.

Higher Rates of Lower Extremity Injury on Synthetic Turf Compared With Natural Turf Among National Football League Athletes: Epidemiologic Confirmation of a Biomechanical Hypothesis

BACKGROUND

Biomechanical studies have shown that synthetic turf surfaces do not release cleats as readily as natural turf, and it has been hypothesized that concomitant increased loading on the foot contributes to the incidence of lower body injuries. This study evaluates this hypothesis from an epidemiologic perspective, examining whether the lower extremity injury rate in National Football League (NFL) games is greater on contemporary synthetic turfs as compared with natural surfaces.

HYPOTHESIS

Incidence of lower body injury is higher on synthetic turf than on natural turf among elite NFL athletes playing on modern-generation surfaces.

STUDY DESIGN

Cohort study; Level of evidence, 3.

METHODS

Lower extremity injuries reported during 2012-2016 regular season games were included, with all 32 NFL teams reporting injuries under mandated, consistent data collection guidelines. Poisson models were used to construct crude and adjusted incidence rate ratios (IRRs) to estimate the influence of surface type on lower body injury groupings (all lower extremity, knee, ankle/foot) for any injury reported as causing a player to miss football participation as well as injuries resulting in ≥8 days missed. A secondary analysis was performed on noncontact/surface contact injuries.

RESULTS

Play on synthetic turf resulted in a 16% increase in lower extremity injuries per play than that on natural turf (IRR, 1.16; 95% CI, 1.10-1.23). This association between synthetic turf and injury remained when injuries were restricted to those that resulted in ≥8 days missed, as well as when categorizations were narrowed to focus on distal injuries anatomically closer to the playing surface (knee, ankle/foot). The higher rate of injury on synthetic turf was notably stronger when injuries were restricted to noncontact/surface contact injuries (IRRs, 1.20-2.03; all statistically significant).

CONCLUSION

These results support the biomechanical mechanism hypothesized and add confidence to the conclusion that synthetic turf surfaces have a causal impact on lower extremity injury.

Development of a Footwear Sizing System in the National Football League

Context:

Footwear performance and injury mitigation may be compromised if the footwear is not properly sized for an athlete. Additionally, poor fit may result in discomfort and foot injury such as fifth metatarsal stress fracture, foot deformities, turf toe, and blisters. Current footwear fitting methods consist of foot length and width measurements, which may not properly describe the shape of the individual foot, correlated with shoe size descriptors that are not standardized. Footwear manufacturers employ a range of sizing rubrics, which introduces shoe size and shape variability between and even within footwear companies. This article describes the synthesis of literature to inform the development and deployment of an objective footwear fitting system in the National Football League (NFL). The process may inform athletic footwear fitting at other levels of play and in other sports.

Evidence Acquisition:

Literature related to footwear fitting, sizing, and foot scanning from 1980 through 2017 was compiled using electronic databases. Reference lists of articles were examined for additional relevant studies. Sixty-five sources are included in this descriptive review.

Study Type:

Descriptive review.

Level of Evidence:

Level 5.

Results:

Current methods of footwear fitting and variability in the size and shape of athletic footwear complicate proper fitting of footwear to athletes. An objective measurement and recommendation system that can match the 3-dimensional shape of an athlete’s foot to the internal shape of available shoe models can provide important guidance for footwear selection. One such system has been deployed in the NFL.

Conclusion:

An objective footwear fitting system based on 3-dimensional shape matching of feet and shoes can facilitate the selection of footwear that properly fits an athlete’s foot.

Does variability within natural turfgrass sports fields influence ground-derived injuries?

Natural turfgrass sports fields exhibit within-field variations due to climatic conditions, field construction, field management, and foot traffic patterns from field usage. Variations within a field could influence the playing surface predictability and require athletes to make abrupt or frequent adjustments that lead to increased ground-derived injury occurrence. This study introduces a new methodology aimed at evaluating the potential relationship between within-field variations of turfgrass sports field properties and ground-derived athlete injuries. Collegiate Club Sport athletes self-reported ground-derived injuries over two years. Soil moisture, turfgrass quality, surface hardness, and turfgrass shear strength were quantified from their two home fields. Hot spot analysis identified significantly high (hot spots) and low (cold spots) values within the fields. Injury locations were compared to hot spot maps each month. Binomial proportion tests determined if there were differences between observed injury proportions and expected proportions. Twenty-three ground-derived injuries were reported overall. The observed injury proportions occurring in turfgrass quality cold spots [0.52 (95% CI 0.29-0.76)] and soil moisture hot spots [0.43 (95% CI 0.22-0.66)] was significantly higher than expected [0.20 (p < .001) and 0.21 (p < .05), respectively]. Most injuries in significant areas of turfgrass quality, soil moisture, and surface hardness were along edges of hot and cold spots. These results suggest a potential relationship between within-field variations and ground-derived injuries, particularly in transition areas between non-significant and significant high and low values. Future larger-scale studies can incorporate the reported methodology to validate this relationship and implement strategies that reduce ground-derived injuries.

The Athletic Shoe in Football

BACKGROUND

Foot and ankle injuries are common in sports, particularly in cleated athletes. Traditionally, the athletic shoe has not been regarded as a piece of protective equipment but rather as a part of the uniform, with a primary focus on performance and subjective feedback measures of comfort. Changes in turf and shoe design have poorly understood implications on the health and safety of players.

EVIDENCE ACQUISITION

A literature search of the MEDLINE and PubMed databases was conducted. Keywords included athletic shoewear, cleated shoe, football shoes, and shoewear, and search parameters were between the years 2000 and 2016.

STUDY DESIGN

Clinical review.

LEVEL OF EVIDENCE

Level 5.

RESULTS

The athletic shoe is an important piece of protective sports equipment. There are several important structural considerations of shoe design, including biomechanical compliance, cleat and turf interaction, and shoe sizing/fit, that affect the way an athlete engages with the playing surface and carry important potential implications regarding player safety if not understood and addressed.

CONCLUSION

Athletic footwear should be considered an integral piece of protective equipment rather than simply an extension of the uniform apparel. More research is needed to define optimal shoe sizing, the effect that design has on mechanical load, and how cleat properties, including pattern and structure, interact with the variety of playing surfaces.