Performance of the exercise task for the fastest surface during the exercise period. The figure illustrates the distribution of median trial times over all subjects separately for each block of trials. Due to variation in progression between subjects the number of blocks of trials performed in this condition ranged from 11 to 20. Therefore, for block 1–11 n = 14 and for block 12–20 n = 13, 11, 11, 10, 9, 8, 6, 3 and 1 respectively. For each box plot, the whiskers represent maximum and minimum values, the top and bottom of the box represent the 75th and 25th quartiles and the horizontal line in the box represent the median.