High knees: how Upfit checks your form

High knees is running in place with your knees driven up. Upfit counts left-and-right pairs from your front camera and speaks up when your knees stay low or you stop.

How do I do high knees?

Stand tall with your core tight and your arms bent. Drive one knee up toward hip height, put it down, and drive the other. Keep alternating and stay light on your feet. Lower the pace or march instead if jogging in place is too much.

How do I set up my phone for high knees?

High knees is tracked facing the camera. Stand the phone upright and step back until it sees you from head to feet. Upfit measures your knee lift in shoulder widths, so it adjusts to how far you stand from the phone. Keep both knees in view the whole time.

What does Upfit check on high knees?

Upfit tracks each knee going up and coming back down, and learns the height of each knee when the leg hangs down. It then checks two things:

• Alternation. One rep needs a left knee and a right knee, one after the other. Lifting the same leg twice in a row starts the pair again, and that is not a rep.
• Knee height. The lower of the two knees in a pair has to rise clearly above its resting height, measured in shoulder widths. A low march does not pass.

There is also an idle check. If your knees stop moving for about three seconds, the coach nudges you to start again.

When does a high knees rep count?

One rep is one left-and-right pair that passes the height check. The coach counts it aloud. Missed pairs are not counted. Because there are no per-rep spoken corrections here, the coach instead checks every few seconds. If most recent pairs were too low, it asks for higher knees. If most were fine, it praises you.

What does the coach say?

These are real lines from the app's voice set:

• "Knees higher — drive them up toward your hips!"
• "That's a march, not high knees — pop those knees up!"
• "Don't stop — knees back up!"

Watch: how Upfit checks high knees

A short walkthrough of how to do it, the phone set-up, what the camera checks and what the coach says.

What can't Upfit check on high knees?

Upfit measures knee height against a resting position, not against your hips, so it cannot confirm that your thigh reached exactly parallel. It does not check your arms, your posture or how your feet land. It also does not check pace or left-right balance: no spoken cue tells you that one knee lifts higher than the other.

Where can I read more?

• What Upfit can't check, including how the form score works
• How Upfit works
• All exercise pages

Upfit is an iPhone app, out now on the App Store. It is a fitness app, not medical advice. Stop if you feel pain or dizziness.