To give a bit of background, the ground reaction force (GRF) is generated on your body in opposition to the force you apply to the ground. It shifts with position changes, producing certain movements at joints which then activates muscles to control or counter those movements. You can conceptualize it like this. When a GRF acts at a joint, it causes the limb section “downstream” from it to move towards the same plane. For example, when the GRF travels behind the knee, it causes the shin to also move towards the back of the knee. This makes your knee bend, which then prompts your quadriceps to eccentrically contract as a counter-force. Also, the closer the GRF is to a joint’s center of rotation, the more mechanically advantageous it is for that joint to move.
Now looking at the diagram above, you can see that the GRF line is furtherin front of the hip center of rotation (symbolized by the dot) during uphill running compared to flat-level running. This means that uphill running is actually LESS mechanically advantageous for hip movement. Your hip flexors would need to contract hard concentrically to drive up the hill, while your hip extensors will contract eccentrically to counter. All this results in your hip muscles getting a more brutal workout for their troubles!
And why should we care so much about the hip? Well, the muscles around it play a huge role in generating POWER in your strikes. Stronger hips also improve general athleticism and may prevent the development of chronic pain issues as you progress in your sport.
It’s probably safer for you