Fix Your Running Form

Trunk and hips

Maintain an erect core from neck to groin, with no forwards lean (except when you accelerate) or backwards tilt. 'Leaning forwards is a common mistake that doesn't let you breathe fully,' says Yessis. 'It throws you off balance, forcing you to land too soon and shorten your stride.'
Don't allow your core to rotate side to side or dip with each step, both of which violate the forwards-motion-only rule.
Yessis adds that the hips are especially important. 'You run from the hips, which drive your thighs forwards and backwards, so good hip function is essential. Strong and flexible hip flexors produce a longer stride, so the lunge is the best of all active stretches for running.'

Stretch: the lunge. Stand with your feet hip-width apart and take a very long step, then slowly lower your body to feel the stretch. Keep your core erect and your rear leg straight, but relaxed. Take several short steps back with the front leg to return to the starting position.