What are some exercises or drills (perhaps plyos) that can increase the angle of the knee joint and teach the power production of the triple extension during accel and top flight phase?
1: relax
2: get stronger in the key muscle groups
3: med ball starts
Again the GPP Essentials video provides several snapshots in progression toward reaching certain levels in this regard.
Yeah, I think this is really a matter of general strength and power. It will naturally fall into place as the athlete gets stronger.
and what are the key muscle groups?
I would say back, glutes and quads with reference to extension, but the hamstrings are key too.