Snicklefritz

My new exercise program has been proceeding nicely, particularly over the last two months with significant fitness gains and body measurements. (see the thread on post-workout soreness)

I've worked up to 5 days total of exercise, 2x/week with the personal trainer. The other 3 days include one day for aerobic work , one day for speed work, and one day of whatever I happen to feel like doing. i then take 2 complete rest days.

I'm wondering how to modify the intervals on my speedwork day when my cardiovascular system feels like it needs more of a challenge, but my muscular-skeletal system can't quite keep up. Here's an example
I started off doing this:
5' warm up, 2x[2' on, 3' rest] the 2' on is around 6 out of 10 on the RPE scale, then do 4x[1' on, 2' rest] where the 1' = max effort.

If I do this on a treadmill I go at about an 8 mph pace for the 1' max intervals, or what amounts to a 7.5 min/mile, and it feels HARD, but I can recover from it very quickly during the rest period. The problem is, running on the treadmill feels awkward at 8 mph. The muscles on the side of my leg and also on the front of my leg start to feel a bit stiff. So even though I feel like my cardio system can push harder, I don't want to screw up my muscles if they are getting stiff.

A similar thing happens when I do the speedwork on a rowing machine. If I go much above 45 strokes/min then I start to feel slightly stiff on the shins or on the side of my leg. It's not that bad, but I thought if I'm feeling this, maybe that is a sign to be careful. I don't want to risk injury. Is that a sign to just stay at that level for a while to work out of it? Or a sign to stretch more before I workout? Or back off to some level lower than that for a while? How can I continue to develop my cardiovascular system, while still allowing my musculo-skeletal system to catch up?