Hi joconnor,
Probably as a result of your rugby background, you look like you are carrying your head low and your shoulder high. This is a defense mechanism in rugby, whether you have the ball or are running at the guy with the ball. You’re trying to protect the “knock-out button” on your chin. I’d suggest that as a result this is keeping your hips low and back as you drive up in the first 15m or so, and carries on as you come up (or don’t come up fully) to full speed. American football guys have same issues. Low head carry keeps hips low, high shoulders lock your hip’s rotation about long axis of spine so that they can’t function properly, which effects knee drive and optimal knee height.
If you put your back to your nearest door-jam and prepare to get your height measured, you will instinctively hit a good tall position through the long axis of your body, usually including a natural lowering of your shoulders. When running at whatever pace, every time your foot drives down through hip/leg extension to GC, your head/hips should be acting as if you are preparing to get measured. You can also practice this during the Running A drill that I mentioned in the other thread for practicing leg/hip extension. As a concept, it is really all one thing…drive the leg and elbow down, while at same time keeping head and hips high. When I first began to change this (because I grew up playing American football) I had to consciously re-tall myself on every single contact…until it progressively became a habit. This will add to the elasticity of your entire body.
This is all good for fast running/sprinting, but can be applied to acceleration as well. Differences are that your tall, straight long axis is rotated forward…and your foot is on the ground for majority of the time you are driving hip/leg to extension.
Long story short…my suggestion, taller head and hip, drive limbs down.