I’ve been doing HIIT for about 4 weeks now, 3 times a week.
I currently do 3 x 30sec bursts with 2 min rests on an exercise bike at resistance level 16 (I grind to a halt at level 20!) plus I do 3 x 30sec bursts on the treadmill at 14.5km/h on an inline of 3.0.

I’m at the stage where I feel I can up the intensity a little but should I increase the time (i.e. pedal/run for longer) or increase the resistance/incline, or perhaps increase the number of intervals?

Any suggestions?

PS please don’t ask me to increase all three options at once!