When a runner's trunk and pelvis position breaks down, send them back to the wall to assist stability. Cue them to press into the wall, which provides compression sensation to the mechanoreceptors in the spine, via the upper limb. This causes reflex stabilisation, permitting better mobilisation of the lower limb.
Maintain an isometric stance on the ground.
Start with isometric holds (great for achilles rehab) for 10-15 seconds.
Progress to alternating march.
Then to quick switches.
Add a "load and lift" - also part of achilles rehab (except for those who don't tolerate load at length), to check for single leg triple flexion force absorption competency - a great acceleration mechanics drill.
To drill acceleration mechanics