Over the recent years, interest has grown significantly relative to injury (especially of the lower leg) and sport surfaces. Are players getting injured due to changes in surfaces, are the infill synthetic turf leading to more injuries, or are there certain shoe types that are precluding injury?

Much has been written over the years regarding sports surfaces and the impending effect of surface on injury. Torg(1, 2) studied how cleat length and specific playing surface impact on injury incidence. His studies identified cleat length as a direct impact on the incidence of knee injury(1), and thus the Football Rules Committee and the Competitive Safeguards Committee of the NCAA changed rules to limit the longer cleats from being worn. The rules allow a maximum length of ½”.(3)

Rotational resistance has been found to be of significance relative to predicting injury.(4) Torg (2) identified a potential threshold for a “safe” torsional release coefficient between shoes and surfaces. In 2008, researchers at Michigan State University (5) used normal forces of 1000 N to generate theoretical safe torque of 95Nm. Most of the values in the Michigan State study exceeded the threshold level (95 Nm). Hirsh presented information (6) that the ankle can support approximately 75Nm from his cadaver studies, thus forces greater produce injury. Shoemaker(7) has reported that lower leg musculature contribute up to 70Nm of resistive torque to help protect the lower extremity from injury during controlled activities. Such control or neuromuscular influence could help stabilize the ankle during agility activity.(8)

This parameter is being studied more and more, and certainly turf’s performance is inferred to correlate with injuries. The Sports Medicine community continues to monitor this landmark measure relative to injury on turf. Discussion also abounds as to the origin of injury, whether forces are generated in the ankle-foot or between the shoe bottom and the actual sport surface. Shoe manufacturers and the turf industry need to continue to work together to solve these issues and develop surfaces and shoes which work together to minimize force production while allowing stability for effective performance.

The first of two articles conducted at Michigan State University was reported in the American Journal of Sports Medicine. (5) The authors studied the relationship between infill artificial surfaces and rotational traction in synthetic surfaces and natural grass. The makeup of the infill system with fine rubber and sand compacted into a dense structure and increased the shoe’s resistance to rotation. Synthetic infill turf of greater stitch gauge (3/4”) was reported to lead to more rotational forces, which correlates with injury. Newer generation AstroTurf systems had statistically lower mean torques than the other systems, and their nylon root zone reduced the amount of infill required and reduced the chance of compaction.

The authors used a mechanical surrogate ankle to apply rotations and measure torque at the shoesurface interface. Five football cleat patterns and four football surfaces (FieldTurf, AstroPlay, and two natural grass systems) were tested. The artificial surfaces yielded significantly higher peak torque and rotational stiffness than the natural grass surfaces. The only cleat pattern that produced a peak torque significantly different than all others was the turf-style cleat, and it yielded the lowest torque. The model of shoe had a significant effect on rotational stiffness. The findings of the research are summarized as:
• 12 Studded: Peripheral Molded Cleats – averaged .49” in length, had 14 cleats, and the mean torque generated across surfaces tested was 115.9
• Edge: Molded Cleats – averaged .47” in length, had 12 – 15 cleats, and mean torque generated across surfaces tested was 108.1 – 111.3
• Hybrid: (15+) Molded Cleats – averaged .45” in length, had 20 – 21 cleats, and mean torque generated across surfaces tested was 96.6 – 102.6
• 7 Studded (Replaceable Screw-in Cleats) – averaged .49” in length, had 7 cleats, and mean torque generated across surfaces tested was 92.1 – 92.7
• Turf: Molded Sole – averaged .26” in length, had an average of 88 cleats, and the mean torque generated across surfaces tested was 80.2.

Results of the study were very interesting to say the least – especially for those dealing with safety for athletes. The research has some very valuable information for not only coaches and medical personnel caring for athletes, but also for the athletes themselves. Based on talks I have given to athletes, they are very interested in this information. In summary, the following five categories of shoe bottoms were tested. While the research identified the specific make of the shoe, I think the information is specific to the bottom. The findings were as follows:

The second article validated some much needed science regarding shoe bottom types and performance on a variety of surfaces.(9) The study investigated the role of infill material and fiber structure on the rotational traction associated with American football shoes on infill-based artificial surfaces. The mechanical surrogate ankle was used to apply rotations and measure the torque produced at the shoe–surface interface. The mechanical surrogate was used to compare three infill materials in combination with three fiber structures, creating a total of nine unique surfaces. Infill material, fiber structure, and shoe design were all found to significantly affect rotational traction. An artificial surface with a nylon root zone yielded significantly lower peak torques than similar fiber surfaces without a nylon root zone.

The size of infill particles and the presence of a nylon root zone may influence the compactness of the infill layer. These features may act to alter the amount of cleat contact with the infill, thereby influencing rotational traction. The amount of cleat contact with the surface may also be determined by the shoe design.

In closing, these studies certainly provide some good information to the Health Care Team relative to recommendations on surfaces and shoe types. There certainly is ample room for more studies in this area. As players get bigger, faster, and stronger, we need to see how greater forces impact injury; and how this can possibly be minimized. We certainly can see that certain shoe bottoms general less rotational resistance (subsequent injury potential) and there are synthetic infill systems that perform better relative to injury incidence.

References

1. Torg JS, Quedenfeld T. Effect of shoe type and cleat length on incidence and severity of knee injuries among high school football players. Research quarterly. 1971 May;42(2):203-11.
2. Torg JS, Quedenfeld TC, Landau S. The shoe-surface interface and its relationship to football knee injuries. The Journal of sports medicine. 1974 Sep-Oct;2(5):261-9.
3. 2008 NCAA Rules and Regulation Official Guide. NCAA.
4. Livesay GA, Reda DR, Nauman EA. Peak torque and rotational stiffness developed at the shoe-surface interface: the effect of shoe type and playing surface. Am J Sports Med. 2006 Mar;34(3):415-22.
5. Villwock MR, Meyer EG, Powell JW, Fouty AJ, Haut RC. Football playing surface and shoe design affect rotational traction. Am J Sports Med. 2009 Mar;37(3):518-25.
6. Hirsch C, Lewis J. Experimental ankle-joint fractures. Acta Orthop Scand. 1965;36(4):408-17.
7. Shoemaker SC, Markolf KL, Dorey FJ, Zager S, Namba R. Tibial torque generation in a flexed weight-bearing stance. Clin Orthop Relat Res. 1988 Mar(228):164-70.
8. Meyer EG, Baumer TG, Slade JM, Smith WE, Haut RC. Tibiofemoral contact pressures and osteochondral microtrauma during anterior cruciate ligament rupture due to excessive compressive loading and internal torque of the human knee. Am J Sports Med. 2008 Oct;36(10):1966-77.
9. Villwock M, Meyer E, Powell J, Fouty A, Haut R. The effects of various infills, fibre structures, and shoe designs on generating rotational traction on an artificial surface. Proceedings of the Institution of Mechanical Engineers, Part P: Journal of Sports Engineering and Technology. 2009;223(1):11-9.

Tags: ankle sprains, sports injury, syndesmosis, turf injuries

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