Isometric stretching: additional sets for strength gains?

[dcclarke]

When doing isometric stretching I follow the below routine:

Warmup:
Dynamic stretch front/side/back (1 set of 10 reps)

Isometric Stretch

I do this whilst holding onto a chair because the tense and relax method doesn't increase my range of motion without holding onto something (probably because my legs aren't strong enough to support that position alone).

I apply 3-4 tensions (10 secs) and when I can't go any further I hold the last tension for up for 30 secs.
This is then repeated with a further 3 sets to achieve my current maximum range of motion and I hold the final tension for as long as I can (usually around 1 - 1 & half mins.

My question is: is it worth doing a couple more additional sets for the purpose of strengh gains or should I just focus more on holding that last stretched position for longer or both?