Address flexibility first, strength second. All forms of active flexibility (dynamic and static-active) are governed by passive (static) flexibility. That means you can only kick up to the limit of how far you can stretch in the splits - holding your leg up even less so. Example: Say you can only reach 160° in the splits. You will only be able to kick up to 160° (a vertical kick is 180°). The difference between how far you can stretch, and how high you can hold your leg up, is usually about 50°. So to hold your leg 180° in the air ("vertical side kick" for example), you usually have to develop your splits to around 210° (usually called a 'negative split' or 'over split'). Work on increasing your splits, then worry about kicking high.
