It differs from instrument to instrument Oliver. Maybe your violin has a week sound and needs more tension. The "mittel" strings are designed to suit the average instrument. Usually your best bet is medium tension. Some instruments need less tension to sound good and some need more. Only try them if you are dissatisfied with the action from your instrument. Too much pressure on the bridge can actually make the instrument difficult to vibrate and have a counter effect. Higher tension strings are thicker and don't respond as fast either.