The running physiology behind the gears

What is an effort gear in physiology terms?

A band of running intensity defined by how your body responds, not by a number on a chart. As effort rises, physiologists describe two turning points, called thresholds. Below the first, the work is mostly aerobic and you can still talk. Between the two, it's sustainable but you feel it. Above the second, it can only be held for a limited time. The three gears follow those three domains; Walk sits beneath them for recoveries and run-walk running.

The app puts the feel plainly. Easy is “Relaxed. Slower than you think.” Steady is “Working, but in control.” Hard is “Strong, but controlled.” More on lactate threshold and zone 2 training in Gears.

Why not use 220 minus age for heart rate zones?

Because formulas describe groups, not you. The best known, maximum heart rate = 220 − age, wasn't derived from original research: a review of its history traced it to an observational fit of about eleven sources, and reported that estimates of maximum heart rate carry a standard error of roughly 7 to 11 beats per minute (Robergs & Landwehr, 2002). A meta-analysis of 18,712 people proposed 208 − 0.7 × age instead, and found the older formula underestimates maximum heart rate in older adults (Tanaka, Monahan & Seals, 2001).

Even a better formula is an average, and any zone hung from it inherits the error. So Gears places your gears as pace ranges from your own running wherever it can: your Apple Health history (pace with heart rate), a race time, your fastest recent run or your Cardio Fitness. With nothing to read yet, it starts from an estimate and labels it: “Estimate: run with heart rate to sharpen it.” Every run with heart rate refines your gears, and each time you say how a gear felt, it tunes them.

Why is most running easy? The 80/20 rule

Because that's how successful endurance athletes train. Descriptive studies of nationally and internationally competitive endurance athletes converge on about 80 % of sessions at low intensity, the rest dominated by hard interval work: polarized, or 80/20, training. The same review found no convincing evidence that shifting well-trained athletes toward more high-intensity work brings long-term gains (Seiler, 2010).

Gears' plans follow that shape: mostly easy running, at most two hard runs a week and never back to back, and a lighter week every fourth week. “No training run goes more than a tenth past your recent longest.” Hold the easy days easy and the hard days can be hard. More on the personalized running plan.

What is the talk test for running?

A simple field check of effort: the point where comfortable speech gives out tracks the ventilatory threshold closely, even as that threshold moves with training (Foster et al., 2008). It's a form of perceived effort, and ratings of perceived exertion (RPE) have long complemented physiological measures like heart rate rather than replaced them (Borg, 1982).

Gears uses it in words: Easy is full sentences, Steady short phrases, Hard a few words at most. More in Easy, Steady, Hard and RPE.

Why steer by pace, not heart rate?

Because heart rate is honest but slow, and it drifts. Speed up and heart rate takes time to follow, often longer than a short rep lasts. On a long or warm run it climbs at the same pace: cardiovascular drift, a slow rise in heart rate and fall in stroke volume during prolonged exercise (Coyle & González-Alonso, 2001). A zone held strictly by heart rate would chase both.

So you steer by pace, which answers at once, and heart rate stands guard: it can only ever make a gear easier, never harder. When your heart says the pace is too much on an easy or steady stretch, the coach eases it: “Same effort, slower pace today. That's the right trade.” Indoors, heart rate leads. With no heart rate at all, pace leads alone, and the coach never pretends to know your heart rate.

The sensor matters too. In one lab comparison of wearables, a chest strap agreed with ECG most closely, and Apple Watch was the closest of the wrist devices tested (Gillinov et al., 2017). Gears works with either. More on chest strap and Apple Watch heart rate.

What signals does Gears read?

During a run: pace from GPS (or your steps indoors), heart rate from the watch, a Bluetooth strap or a Garmin's broadcast, steps per minute, and height from the barometer for hills. Around your runs, from Apple Health: workouts and routes, resting heart rate, heart rate variability, Cardio Fitness, sleep and breathing rate, among others, to place your gears and guide today's run. Every run is coached on your iPhone or watch and needs no signal. Anything kept in your account is kept only with your consent: see the privacy policy.

What does Gears never claim?

• Nothing medical. Gears is training guidance, not medical advice. It doesn't diagnose, and a readiness score or today's gear is guidance for today's run, not a health verdict. Read the health disclaimer.
• No injury prediction. The training load card shows recent training against your own usual: “A guide to getting the most from training, not a medical reading.”
• No technique it can't see. Breathing, posture and form reminders come on a timer, and the coach never claims to have seen them. Cadence is different: your steps are measured, so cadence lines are too.
• No universal numbers. There's no one right cadence. Your target defaults to 170 steps a minute and can be set from 150 to 200.
• No results promises. Gears coaches the effort. It doesn't promise a VO2 max, a race time or a weight.

Why are the coach's lines pre-recorded?

So every line is written and checked before you hear it. Each is a pre-recorded clip, and rules on your device choose which to play and when. Nothing is generated during your run, and none of it needs a signal. A blunt line plays only when the numbers back it, the coach is never harsh about what it can't see, and safety lines sound at every toughness setting. The trade-off is a fixed script: it says fewer things, and only things it can stand behind.

Who reviewed this page?

Last reviewed October 23, 2026, by the Gears team. This page has not yet had an independent review by a coach or exercise physiologist; when it does, the reviewer will be named here. Spotted an error? Tell us through support.