I have a fairly old ruptured disc in my lumbar and I have noticed that it seems to be affecting my running form. After the injury, I noticed that when I run more stress is being placed on my quads/hip flexors. This is evident in the noticeably increased fatigue in my quads after training sessions as well as an increase in the size and development of my quads. Also, my acceleration has been affected by the injury, as times in short sprints are worse than before, and perhaps because of the acceleration being weakened my top speed is also slightly affected. Speed endurance however has improved, probably compensating for the less intense accel/top speed, as times in longer sprints like 200m are the same or improved in training (proof of Charlie’s theory on the energy envelope). My question is, why is my injury causing the change in the amount of stress placed on the quads/hip flexors? My lower body strength has decreased since the injury because I have had to drop back squats from my training because they are too painful to perform, would this have any affect on the amount of stress on my quads when running? Could my body be instinctively changing muscle activation when running, causing the increased stress? My theory is that my body is forcing my hips to be locked in a certain position and not allowing any spine flexion or extension to try to protect from further injury/pain causing poor acceleration and sub-par top speed and resulting in subconcious tightening of the quads/hip flexors to try to take the stress of the lower spine and consequently the posterior chain. Any thoughts or opinions would be appreciated! I would especially like to get feedback from the experts!