Tuesday, November 30, 2010

Fatigue and Skill Performance in Professional Matches

Few sports are more energy costly than soccer. Given that over the course of a 90-minute match, some players may run more than 10km (6.2 mi), it is not surprising that physical performance declines from the initial minutes to the final whistle. This decline in physical performance is usually interpreted as fatigue. Studies show that as soccer players experience fatigue, technical or skill performance also begins to suffer. In a fatigued state, he/she probably has more difficulty completing passes, receiving and controlling the ball. Thus, one would assume that the match progresses, development of fatigue leads to deterioration in skill performance. However, a new study from LOSC Lille Métropole Football Club suggests that the relationship between physical and skill performance may be more complex that first thought.

Christopher Carling, one of the leading experts in match analysis, headed the study. He examined both physical and skill performance of midfield players during and across 35 French League 1 games. Physical performance was defined as the total amount of distance covered (running and walking), the distance covered during high speed sprints and the distance running with the ball. Skill performance was defined by a number of variables such as the number of passes attempted, passes completed, possessions gained and lost, and duels won. To assess the effects of fatigue, these variables were compared across 15-minute intervals within each match.

The researchers were also interested in performance in matches played with limited recovery time. To answer this question, they compared physical and skill performance across consecutive matches played within a 7-day period. Three matches were played, one on Sunday, one on either Tuesday or Wednesday and one on the following Sunday.

During an individual match, midfield players covered slightly more than 11km (7 mi). High-speed running accounted for about 24% of that distance. As expected, physical performance declined over the course of the match. Total distance covered during the final 15 minutes was 7.5% less than the first 15 minutes. High speed running was also about 12% less. These are relatively small changes but definitely a reduction in performance.

Despite the changes in physical performance, skill performance did not suffer as the match progressed. All variables remained unchanged across each 15-minute analysis period. The number of passes attempted (~7 per 15 minutes), percentage of passes completed (~70%), possessions gained and lost (2-2 ½) and the number of touches per possession (~2.2) all remained constant throughout the match. The only decline in skill performance was found when comparing the initial 5-minutes of the match to the final 5-minutes. The number of passes attempted and number of possessions were reduced during the final minutes.

As for successive matches played with limited rest, none of the physical or performance variables decreased from match one to match three. Distance covered, shots taken, passes attempted and completed, touches per possession and possessions won or lost per match were all constant across the matches.

The authors conclude that despite a reduction in physical performance over the course of a 90-minute match, players are still able to maintain their level of skill performance. Also, the effects of multiple matches played with limited recovery time are minimal. If anything, most performance variable slightly increased from match one to match three. This indicates that these athletes can fully recover from match play within a few days. Clearly, they are either incredibly fit and/or using very effective post-match recovery strategies.

Other studies show that when players experience fatigue, their technical abilities decline. This is at odds with the present study. Why the difference? One explanation is how fatigue and technical skill is determined. Previous studies used a set amount of exercise or simulated match play to induce fatigue. They also measured technical performance using tightly controlled field tests such as passing at a target. What is unique about Chris Carling’s study is that it is first to examine physical and skill performance within a match. Players exercised at a level appropriate for the match conditions and skill was assessed in an ever changing environment. Thus, the different findings between studies probably reflect different “experimental” conditions.

In the context of a match, skill performance depends on the player executing the skill as well as the influence of an opponent who is trying to disrupt that skill. It is possible that over the course of the match, both players and their opponents experience similar fatigue. As the match progresses, any loss in skill by the offensive player may be met with less aggressive defending. Thus, in contrast to a tightly controlled field test, match performance has a more variable “target”. For example, less aggressive defending late in a match may make it easier to complete passes.

It is also assumed that the decline in physical performance indicates fatigue. As fatigue sets in, players run less and sprint less. However, the authors point out that the decline running may not be an accurate indicator of fatigue. As the match progresses, players may use a pacing strategy to avoid fatigue. Slowing down and running less may reserve energy stores needed to respond to the opponent and to maintain technical abilities. This would help them keep up their skill performance as the match winds down.

Finally, as the match progresses, strategies change deepening on the score line. For example, midfielders may need to run and sprint may change depending on whether the team is trying to equalize of protect a lead. These variables also depend on the interaction between other players involved in the match - teammates and opponents. This is particularly true when comparing the initial and final 5-minutes of the match.

Thus, the relationship between physical and skill performance during a match is more complex than first thought. Over the course of a professional match, physical performance declines. However, skill performance is maintained. These players are either able to maintain their technical abilities in the face of physical fatigue or they adapt a pacing strategy that prevent their skill performance from suffering.

Reference:

Carling C, Dupont G (2010) Are declines in physical performance associated with a reduction in skill-related performance during professional soccer match play? Journal of Sports Sciences, DOI: 10.1080/02640414.2010.521945.