How The Treadmill Differs From Outdoor Running?
The issue to be aware of regarding treadmill running lies with the muscles that are engaged on a treadmill – or more specifically those muscles that are less engaged.
The Hip Flexors are actively engaged on the treadmill to bring the thigh up and through when running. Given the direction of the belt they do more work as they must first decelerate the backward motion of the leg and foot, and are also less assisted by the muscle shortening reflex. The antagonist muscle group of the Hip Flexors are the Hip Extensors which consist of a number of muscles including the glutes and hamstrings (biceps femoris, semitendinosus and semimembranosus).
These Hip Extensor muscles are responsible for the movement of the thigh backwards which is where the issue with a treadmill can arise. Because the direction of the belt assists in moving the foot backwards some of the work of the Hip Extensors is done by external forces (by the treadmill belt) so the Hip Extensors become less engaged in creating extension.
This means that on a treadmill the agonist Hip Flexor works more than when running on a non-moving surface and the antagonist muscle group works less. Over time, and without compensatory work this can lead to an imbalance between these muscles and the risk of injury – the most common of which is hamstring strains when trying to run hard as the weaker Extensors try and retard the stronger Flexors.
So, whilst treadmills definitely serve a purpose, avoid doing all your running on a treadmill. Balance this with some outdoor running and/or some appropriate strengthening exercises. Whilst isolation exercises are good the simplest Hip Extensor exercise is simply running backwards up a steep hill.