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I interpreted this to mean that the walking group suffered a greater number of leg and foot problems than did the non-walking group.

Adverse events related to the lower extremities includes any reported event -- like blisters, sore feet, twisted ankles -- and other things that you would expect to see increase in a group of people who were encouraged to walk versus a sedentary group.

same overall probability of adverse events in both groups, but the nature of adverse events reported differed between the two.