Saturday, January 8, 2011

Energy Cost of Playing on Artificial Turf

A common complaint of players who play on “play on plastic” is that they feel much more tired compared to playing on natural grass. In fact, a study reviewed here (link) found that many players have a negative perception of artificial turf. They feel that turf is was more physically taxing and more technically demanding. It’s possible that the increased physical demands of artificial turf, whether real of imagined might alter how the players approach the match. It is more physically demanding to play on artificial turf? Two new Italian studies suggest that the player’s perceptions may be accurate. Playing on artificial turf may require more energy than playing on grass. However, the key may be the quality of the playing surface.

The first study (DiMichele et al., 2009) compared various physiological responses to running on artificial turf, natural grass and a treadmill. They found that when running on turf, blood lactic acid levels and exercise heart rates were slightly higher than when running on grass or a treadmill. This was the case regardless of the running speed. While energy expended was not actually measured, higher lactic acid levels and heart rates suggests increased energy cost. In the second study (Sassi et al., 2010) researchers measured energy expended. They found that running on both grass and artificial turf required more energy than running on an asphalt track. However, the energy expended on the two playing surfaces was the same.

Why the different results? Why does one study show that running on turf is more energy demanding that running on grass and the other shows no difference? The key seems to lie in the quality of the artificial turf field. Specifically, the “softness” of the turf surface may affect energy expenditure. In the first study, the mechanical characteristics of the turf were not reported. However, the comments made by the authors suggest that the synthetic surface was softer than the grass (certainly softer than the treadmill). In the second study, the shock absorption characteristics of the grass and turf were measured and were found to be nearly identical – one was as soft as the other. In fact, the mechanical characteristics of the artificial turf indicate that the field used was not high quality and would be given less that a FIFA 2-Star rating. Thus, with a softer playing surface, energy expenditure is likely to be greater. On a firmer artificial field, energy expenditure is the same a grass.

The softer surfaces probably alter the mechanical forces returned to the body with each step. Basically, softer surfaces “absorb” impact forces when the foot land. This, in turn requires that athlete to apply more muscular force during push off. Also, there is slightly less traction on artificial surfaces which requires added muscular force during push off. This concept is the same as running on sand. As the foot “sinks” into the sand and traction is less, more muscular force is needed. Thus running on sand requires much more energy that running on a track.

It should be pointed out that while artificial fields may require more energy to be expended, the “softness” and reduced traction seem to slightly lower the risk of injury. A number of studies on both soccer and American football show this to be the case.

It is not clear from either study exactly how much more energy is needed when playing on artificial turf. It may be a little or it may be a lot. Based on heart rate measurements, turf increases energy expended by 3-5%. Over the course of a 90-minute match, this could be important. Also, these studies focused on straight running only. Over the course of a match, players start, stop and change in direction many, many times. Changing movements markedly increased the energy expended compared to straight running. It is possible that artificial turf and a softer field may have a bigger influence on energy expended under these types of movements. Given that, it is possible that the playing a full match on artificial turf may require quite a bit more energy than playing on grass. However, it is also important to consider that match heart rates can easily vary by more than 5% due to various conditions such as the quality of the opponent, style of play, scoreline, and weather. And, players do seem to change their style of play opn turf compared to natural grass. Thus, any increase in energy due to artificial turf may be overshadowed by other factors that influence a player’s energy expenditure.

Despite the above and given the results of these studies, the player’s perceptions could be correct. Their post-match feelings of “tiredness” may actually have a metabolic basis rather than psychological. Playing on a good quality artificial field, one that is relatively soft may, in fact require more energy to be expended over the course of a match. However, until researchers can provide more detailed information on match conditions played on turf, it is difficult to accurately reach a firm conclusion.

References:

DiMichele R, DiRenzo AM, Ammazzalorso S, Merni F (2009) Comparison of the physiological responses to an incremental running test on treadmill, natural grass, and synthetic turf in young soccer players. Journal of Strength and Conditioning Research, 23, 939-945.

Sassi A, Stefanescu A, Menaspa P, Bosio A, Riggo M, Rampinini E (2010) The cost of running on natural grass and artificial turf surfaces. Journal of Strength and Conditioning Research, DOI:10.1519/JSC.0b013e3181c7baf9