Crabs move sideways. Triathletes spend most of their time in the sagittal plane (moving forwards and backwards) as they swim (freestyle), bike and run. Therefore it is very important to cross train in different planes of motion (side bending and rotating) in order to prevent injuries, increase range of motion, and provide greater stability for your body.
Sports that include side bending or rotational movements work your body in different planes and allow you a break from the repetition of forward and backward movements.
Schedule 3 plane exercises - such as gyrotonics, yoga (You can find specific yoga classes - such as yoga for runners, yoga for triathletes, yoga for swimmers - on YouTube), tennis, basketball - into your weekly routine (Pilates is saggital plane only).
Or add a few laps of butterfly or breast strokes to your routine. Balance your training.
You can't afford time off for injuries and you don't want to retire early because of the degenerative effects of wear and tear on your overloaded joints. (Did someone say arthritis?!)