Cycling is one of the most accessible and joint-friendly cardio options for HIIT training. Whether on a stationary bike, spin bike, or road bike, the lower-impact nature of cycling makes it suitable for people with knee or hip issues that prevent running. You can still achieve maximum heart rate responses without the ground impact that makes running problematic for some.
For indoor cycling HIIT on a stationary bike: warm up at easy resistance for 5 minutes, then alternate 30 seconds at maximum effort (high resistance, high cadence) with 30 seconds easy pedalling, for 15-20 minutes, then cool down. A more structured protocol is the "tabata bike": 20 seconds all-out with resistance cranked up, 10 seconds easy, x 8 rounds. These sessions are brief but absolutely brutal done correctly.
For outdoor road cycling, hill repeats deliver the best HIIT stimulus. Find a moderate hill (1-3 minute climb) and sprint up at maximum effort, then recover on the way down. Repeat 5-8 times. Flat sprint intervals work well too: sprint 200-400m at maximum power, soft-pedal to recover for 2 minutes, repeat 6-8 times. Road cycling HIIT provides the added benefits of fresh air and varied scenery that keep sessions mentally engaging - an advantage over the stationary bike for long-term adherence.