Adductors are a funny muscle.
They sit quietly and do their job, we never work on them and we forget they are there.
Till the end of a triathlon/ironman and BOOM we suddenly discover that they exist and they hurt!
Adductors act to, bring the thigh in, they also internal rotate at the hip, then can also act as hip extensors in acute hip angles (top of the pedal stroke).
They are also a key stabiliser in running, as they act, with the abductors on the outside, to keep the leg straight as we hold in stance phase.
If they didn’t work we would be all over the place.
Problem is, we often don’t use them effectively, particularly in swim and bike, therefore by the end of the run they are giving up.
In swimming, poor kickers can use them to try and hold the legs in to stop them splaying. Many people do this conciously in a wetsuit, OR if you have spent too much time with the pull buoy when you actively use the adductors to keep the buoy in place.
Then, on the bike they can be used as hip extensors if activation of the glutes is weak, in the context of a more extreme aero position.
They are also over-used on the bike if you have excessive movement around the knee, caused by bad cleat setup, or no arch support (collapse inwards causing the need for more control by the adductors).
The fitter Steve Hogg often sighted pain in the adductors as a result of not being balanced on the bike, meaning you were using them to keep yourself stable.
Then, we start to run, this requires a lot of adductor activation and strength, towards the end of the race this goes.
And you see what happens.
So, you not only need to increase their strength, but you need to improve your swimming kick, to not over-use them and improve your glute activation/bike position to not overload them there.
Then they will be fine for the run!
