As triathletes, our mode of travel is forward and very repetitive. Therefore, we are usually very strong in this plane. However, we are typically very weak in the sagittal plane or side-to-side plane.
During a triathlon, have you experienced or ever noticed someone leaning to one side during the run? The glute med is very weak in this example, allowing the hip to drop, therefore, leaning the body over. This weakness forces other muscles to compensate, leading to excessive fatigue and then on to potential injury as running form degrades.
Add these simple exercises to your strength routine. Do a few sets on each side, increasing time and reps as you get stronger. This winter, you could also throw in some sports that break up the training routine and exercise the sagittal plane like basketball, pickle ball, indoor soccer, etc.