I found when when I tried to spin (although it wasn't really spinning, cause I sucked at it) my HR went up - my HR was much lower when I pushed a bigger gear at a lower cadence. I couldn't understand the mechanics of it either.
However, after anaylsing how spinning can improve my time (son made an excellent spread sheet which showed me the 'maths' of it) I realised that being able to turn my legs more quickly was an advantage in a race - and being able to do it consistently for 40 minutes would translate to a huge improvement in a TT (huge being a relative thing).
So I have been working on my spinning on a spin bike. This is a spin bike like at the gym, so it has a fly wheel on the front.
So - the fly wheel does all the work (the pushing) when you spin... and I do the "step up" workout outlined in Gale Bernhardt's "The Female Cyclist". So what you are doing in this training workout is teaching your muscles to be quick, not making them work hard, because the fly wheeel carries your legs at the speed you set and your legs have to follow.
You warm up at a cadence of 90 (this was a yeah right moment when I read this - I never cycled at 90... however, having built this into my training programme throughout September, a warm up at 90 on the spin bike is now easy).
So, you warm up at 90 (10-15 minutes) then...
3 minutes ata cadence of 100
2 minutes at a cadence of 110
1 minute at a cadence of 120
Rest at 90 again (about 5-6 minutes) and then repeat
All I can tell you is that when doing step ups on a spin bike my HR does not go above 125 (my max is 184, my lactate threshold is 162).
When I do recovery rides on the road now, I try and stay spinning at a cadence of 90 and my HR is lower now than it was when I first tried spinning months ago.
The only time my HR goes up now when my cadence is high is when I am practicing sprinting, or at the end of a long ride when I am tired and I fall back to a bigger gear and a lower cadence.
Probably doesn't answer your "why" question but hopefully there is something of use here.



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