Sports Medicine Corner

Dr. Don Kirkendall

A neighbor with kids in the local soccer leagues is concerned about injury and frequently knocks on my door. This time – artificial turf vs. natural grass.

Two recent comparisons were published about college sports. One looked at ACL knee injuries in American football and the other was on overall injuries in soccer. On the surface, they reported different results so would I please explain the differences. Bottom line – more ACL tears on artificial turf in American football, but no differences in overall injury rates in soccer.

The source for both projects was the National Collegiate Athletic Association’s (NCAA) Injury Surveillance System (ISS).

A few NCAA facts: 3 divisions of sport (D1-D3) based (mostly) on financial commitment to sport – 345 D1 members, 300 in D2, and 444 in D3 – 650 compete in American Football across the 3 divisions – 819 sponsor men’s soccer and 1017 sponsor women’s soccer.

The ISS is designed to be representative of the NCAA members, thus those that participate (this changes each year) are stratified to achieve proportional reporting across the three divisions. While Division I gets most of the media attention and is a fruitful source of future professional athletes, the ISS does not focus on just the very best collegiate athletes. The ISS collects data on competitive 18-22 year old college athletes of all levels, not just ‘the elite’.

ISS participation in voluntary. To make up for the lack of randomization, the ISS randomly selects from a stratified sample of the volunteers. Be impressed with the logistics required to collect the data for the ISS.

Injury incidence (the number of new injuries) is determined, but the key figure is a rate. The ISS reports injury rate per 1,000 Athlete Exposures (AE). That’s an overall rate, which can be broken down into training and game rates.

An AE is one athlete participating in one exposure where there was the possibility of injury. In Europe, rates are reported per 1,000 hours and the two methods are not directly comparable. For comparisons, people report ratios (no units like hours or AE) such as sport to sport, males vs. females, grass to artificial turf, etc.

With that intro, it was time to look at each paper, one by Colin Fuller (of the UK) on soccer and the other by Jason Dragoo on American football. To avoid confusion, ‘turf’ = artificial turf’ and ‘grass’ = natural grass).

First, the reports differ on two major aspects. Fuller reports on all injuries in soccer (in men and women). Dragoo reports ONLY on ACL injuries in football (men only).

Fuller presents rates (per 1,000 hrs) for men and women, by training and match, by turf and grass. I’ll just focus on match rates.

Overall injury rate for men on turf was 25.43 and for grass was 23.92. The ratio (turf/grass) was 1.06, which means the turf injury rate for matches was 6% higher than the grass injury rate. For women, the ratio was 0.88 or 12% lower on turf. Nothing statistical or practical there.

Dragoo reported the overall rate of ACL tear on turf to be 0.17/1,000AE vs. 0.12/1,000AE on grass (ratio=1.42 or 42% higher rate on turf vs. grass). That ratio is both statistical and practical. Dragoo also included other rates that Fuller didn’t like pre vs. in season, by division, by type of turf, by position, by type of play (offense, defense, special teams).

Over 50% of ACL injuries were due to contact. In soccer, most reports show that ACL tears happen with no direct contact, especially in women.

What did Fuller get just for ACL injuries? Their match ACL injury rate for males turf was 0.42 and the grass rate was 0.47 (ratio = 0.89 or 11% lower rate on turf vs. grass). Remember, you can’t compare the actual rate because the units are different.

For women, the turf rate was 1.3 and for grass was 1.64 (ratio=0.79 or 21% lower rate on turf). Neither rate was of statistical importance.

Now let’s look at the pattern of injury as reported by Fuller. The top three match injuries for men on either turf or grass were ankle sprains, hamstring strains, and concussions. Rates on turf were a bit higher on turf for each.

For women, the top three turf injuries were concussion, ankle sprains, and ACL tear. On grass, the order was slightly different – ankle sprain, concussion, ACL tear. In all three, the rate on grass was a bit higher than the rate on turf, a minor shift in the ranking for women, but not for men. Each rate for men was higher on turf while each rate for women was highest on grass.

By now, my neighbor is thoroughly flummoxed. Is one surface ‘better’ or ‘worse’? The lesson is in the differences between the two sports. Soccer and football are remarkably different in the type of running
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