REASON:
They are being asked to do too much during early swing as the calf isn't strong enough to generate enough force for propulsion.
FIX:
Calf raises can help.
And this.
REASON:
They're weak and fatigue easily, creating a feeling of 'perceived' tightness.
FIX:
Hip flexion strength work can help.
REASON:
They are genuinely tight and there's a barrier to hip extension.
FIX:
Eccentric loading into hip extension can help.
And this.
REASON:
An anterior pelvic tilt and lumbar extension is utilised as a strategy as opposed to true hip extension - leading to an over-reliance on the hip flexors.
FIX:
Learn to hinge on a neutral pelvis to achieve full hip extension without anterior pelvic tilt.
And this.