Depends what youre trying to achieve Martie? Doing HIIT properly is not pleasant, far from it. If you can sustain the activity for more than 30 seconds then its probably not HIIT. I end my 50km bike rides with 2 x 20 second bursts of HIIT. This is 100% flat out empty the tank of what little energy I still have left and pedal as fast as I can go. I feel physically ill as I pull up in my driveway and get off the bike. Id definitely give yourself a day in between doing the HIIT activity as dykask suggested.

You need to up the speed and or resistance on the rowing machine. You shouldn’t be able to do HIIT for 90 seconds. You should be 100% anaerobic and not aerobic, and that is sustainable for about 30 seconds. It should help deplete your muscle glycogen stores. Not sure it will build all that much lean tissue. You want to build lean tissue than that is resistance training. Weights and reps. If you’re right on the limit of resistance training then your muscles should feel sore and Id be doing very light work the next day or two to recover and probably having a little bit more protein.

When I do an easy bike ride I don’t crave protein. When Ive pushed right on my limit for a 2 hour ride I usually crave protein afterwards. Good luck with it.

@marty. Im by no means an expert in any of this physiology stuff. Just enough to be dangerous. What are you actually trying to achieve with all this exercise? Muscle tone? Weight loss? Fitness? Mental health and well being? Exercise is good for all of these things but the weight loss should be treated as a minor benefit. Proper diet will account for probably 90% and exercise 10%.

I have a real bike not an exercise bike. Riding my bike is my “happy place”. So I ride for all of the reasons above.

Aerobic just means you need oxygen (breathing) to burn your fuel. Anaerobic means you burn the fuel without oxygen (breathing not required).You have 3 basic energy systems. Two are anaerobic. ATP-PCr. This is stored within the cells and is absolutely immediately supplied. 5 to 20 seconds worth. Then you flip over to lactic acid LA system and start consuming glycogen (stored glucose) in the muscles. 30s to 2 min at max effort. Then you swap over to an aerobic system. It draws on stored glucose (glycogen) but needs oxygen to burn it. This lasts for about 1-2 hours. Then you swap over to burning fats. This process is a lot slower than burning glycogen.

So if you are maxing out you really shouldn’t be able to last for more than 30 seconds before the effort drops. Then you should be able to last about 2 minutes. After that you are spent. Operating at max effort empties your glycogen stores. During recovery you go to the “fat bank”, to top up the glycogen stores again. Operating at such a level, basically beyond 100% is very unpleasant. I basically finish off my bike rides doing 2 x 20 second bursts. It is not pleasant but its good for the soul. I only do this once per week.

Id be alternating between resistance training and HIIT to give your system time to replenish. With a days break in between.

The link has a bit about the 3 energy burn systems.

https://semiprocycling.com/energysystems