over the 9-year period; in fact, the boxers improved on some of the tests in comparison with the controls.
Studies of soccer players have been conducted to evaluate the association of TBI with long-term health outcomes, particularly neurocognitive outcomes. Soccer is a popular sport that is considered relatively safe for the general population; however, it is designated a contact sport because rates of concussion in soccer players are high and have been found to be equivalent to those in football players (Matser et al., 1998).
Matser et al. (1998), Guskiewicz et al. (2002), Rutherford et al. (2005), and Straume-Naesheim et al. (2005) conducted studies of neurocognitive outcomes related to soccer-related head injuries. Two studies found neuropsychologic impairment in head-injured soccer players. Matser and colleagues (1998) assessed neurocognitive impairment in soccer players with chronic TBI and found that soccer players performed worse than controls on neurocognitive tests of planning, memory, and visuoperceptual tasks. The number of concussions was inversely related to scores on neurocognitive tests. Rutherford and colleagues (2005) studied neuropsychologic impairment in amateur soccer, rugby, and non-contact-sports players and found that the number of head injuries was a significant predictor of scores on the Trails B response test (p = 0.014) and the Test of Attention Performance Divided Attention (p = 0.020). The latter study was designed to be exploratory.
Two other studies, however, did not find a relationship between soccer-related TBI and neurocognitive outcomes. Guskiewicz and colleagues (2002) evaluated neurocognitive outcomes in collegiate athletes (including participants in soccer and other sports) and found no significant relationship between a history of soccer-related concussions and scholastic aptitude or neurocognitive performance. Straume-Naesheim and colleagues (2005) studied neuropsychologic impairment in head-injured Norwegian elite soccer players; lifetime heading exposure was not associated with neuropsychologic test performance.
As with much of the sports literature discussed above, diagnosis of TBI in studies of soccer players was generally based on self-reports of exposure (for example, questionnaires that asked about number of concussions in the past or number of headings in previous matches) or on surrogates of exposure (such as number of games played). Reliance on self-reports of exposure may introduce recall bias, and this should be considered in evaluating the results of the studies.