When you exercise with intensity, lactate builds up in your bloodstream. This occurs at different rates depending on the individual and their level of fitness, and a new study published in the Journal of Sports Science & Medicine suggests that rate of lactate accumulation can vary between sets depending on whether you’re working upper body or lower body muscle groups.

On separate occasions, 10 healthy male subjects performed 3 sets of curls using one or both arms, and 3 sets of leg extensions using one or both feet. Levels of blood lactate decreased during the third set of curls, and also during the second and third set when leg training, before increasing again during recovery.

True Strength Moment: To explain why the rise in blood lactate wasn’t steady, scientists theorized that in addition to lactate being metabolized by muscle fibers, the short duration decrease might be explained by shifts in blood volume or other factors. Whatever the cause, the effect may help determined athletes push through those last few reps.