I have been involved with martial arts training for five years now. Although, when warm, I can achieve a front split, I am still very tight and unable to achieve a side split. What's more... when doing side leg raises, I experiencing a very painful muscle ache in the opposing leg. What's going on??

As for the side split, probably because your hamstrings are too weak.

Leg raises - if your back and stomach are not strong enough you will experience problems. But as your leg muscles are not connected it is probably the downward force in the opposing leg or weakness in the back or stomach.

I would recommend going to see your doctor or an applied kinesiologist to specifically identify the problem as the ideas I have suggested are merely speculation on what you have stated.