a-b. Figure 6a is an example of an alternative causal diagram to figure 2a. The only difference between the two is an additional causal relationship where previous injury causes a decrease in pre-game proprioception (we have also included the additional conditional associations that occur as a result of this change with dotted lines). We are still interested in the causal effects of warm-up on injury risk. Because previous injury is an ancestor of warm up exercises (previous injury causes a decrease in pre-game proprioception which causes an increase in warm up exercises), it is not deleted in Step 2. This leads to two effects. First, contact sport is now a common cause of exposure and outcome. Second, there are additional conditional associations in Step 4 (dotted lines) even if "Previous Injury" is not conditioned on in the statistical model because one is already conditioning on a descendant of previous injury (i.e. the main exposure of interest, warm-up); the effect estimate of warm-up on injury is biased if the statistical model includes only warm-up, neuromuscular fatigue and tissue weakness. Figure 6b shows the same causal diagram as 6a (without the conditional associations), but now a causal link is added from pre-game proprioception to intra-game proprioception.