During the off season, what matters is a focus on addressing muscle imbalances, and areas of chronic tightness. Strong running, biking, and swimming depends upon the ability to maintain the correct athletic position throughout the event.
The foundation begins with Planks, Bridges, & Squats and the ability hold to the correct form through a variety of movements that introduce instability. It is not enough to simply enough to perform these basic exercises but you must force the body to adapt to complexity and a wider range of movement.
A training progression starts with simple forms of these exercises, and then adds complexity, repetition, duration over time.