I've always been unable to touch my toes. This is becoming increasingly problematic for my yoga. Of course focusing on stretching my hamstrings helps and I'm making progress but I'm concerned that my regular cycling — which I'm told tightens the hamstrings — is counteracting my yoga efforts. Is this likely to be the case?
Cycling has been shown to shorten the rectus femoris. This happens because its range of motion is smaller than in for example running. The same thing happens to any muscle that is used across a shorter range of motion, so I presume that the exact same thing happens to the hamstring.
There is another factor that plays a role there as well. The hamstring has two functions (and thus two distinct parts, at least the biceps femoris). One part of the biceps femoris only bends the knee, the other part both bends the knee and extends the hip. When cycling, the knee-bending part works at shorter ranges, while the hip-extending part is stretched because we reach forward for the handles. This can eventually lead to impalances, as one part of the muscle becomes tighter than the other.