Guides · Treadmill fartlek
Treadmill fartlek: what the belt does to your surges
Three surges into a fartlek on the treadmill, you press the speed up for a 30 second push. By the time your legs feel the new pace, a good part of the push is gone. You start to wonder if you ran the surge you planned or a shorter one.
Yes, but the belt changes speed gradually, over a span of seconds that depends on the machine. Each surge starts slower than the number you pressed and each float starts faster, so a short surge can end before the belt ever arrives.
The belt does the speeding up, at its own rate
A treadmill surge starts slow and a treadmill float starts fast. On the belt you press a number and the machine climbs toward it, then comes down gradually when you press back down.
How fast that happens is a setting somebody chose. One treadmill's maker states 3 seconds for every 1 mph of change, in each direction. Whoever runs that machine can set it anywhere from 1 second to a full minute per mph. Other machines may differ, and I have not found a gym treadmill that publishes its rate.
Think of a city bus. On a long open stretch it reaches cruising speed and stays there. Between two stops a block apart, it pulls away, gets partway up, and is already braking for the next one. A 90 second surge is the open stretch. A 15 second surge is the stop a block away. The guide to treadmill interval pacing covers what the climb takes off a single fixed rep. A fartlek repeats the climb and the drop again and again.
What the climb and drop do to a fartlek
Say your float is 5.7 mph (10:32 a mile) and your surge is 8.2 mph (7:19 a mile), a change of 2.5 mph. At 3 seconds per mph the belt spends 7.5 seconds getting there. At 6 seconds per mph it spends 15.
Your average surge pace, planned at 7:19
Average pace over the whole surge at two ramp rates. These illustrate one stated factory setting and its double, not machines I measured.
| Surge length | At 3 seconds per mph | At 6 seconds per mph |
|---|---|---|
| 15 seconds | 7:55 | 8:38 |
| 30 seconds | 7:36 | 7:55 |
| 60 seconds | 7:28 | 7:36 |
| 90 seconds | 7:25 | 7:30 |
The math assumes a steady climb from 10:32 that starts the instant you press. Your machine's rate may be faster or slower.
A long surge barely feels the climb. A short one loses a big share of its time to it, and the float loses the same share coming down. At 3 seconds per mph, a 15 second float spends half its time still coming down, so it averages 9:29 instead of 10:32. Your 15/15 block was planned as 7:19 against 10:32. It runs closer to 7:55 against 9:29, so the gap between fast and easy is roughly cut in half. On paper, the pair still covers the same ground as planned. What goes missing is the contrast, which is the whole point of a fartlek.
At 6 seconds per mph, the belt touches 8.2 at the very second you press back down. Both halves of the block then average 8:38, and the belt spends the whole time climbing or falling without ever holding either speed. Your fartlek has turned into a steady run with a wobble in it.
The well-known Mona fartlek finishes on 15 second surges with 15 second floats, so its last block is exactly the one the belt rounds off. Those short pieces are the last blocks of the session, and on a treadmill they are the ones the belt is most likely to take away.
The surge number is a setting too
Once the belt arrives, it holds your number only as well as that machine can. When a lab checked four treadmills, each belt's average speed fell within 98 to 103 percent of its setting. Your 8.2 and your 5.7 each carry that machine's own error on top of the ramp. If that error stays the same from week to week, your sessions still compare. A different machine may start a new baseline. Indoors your watch estimates pace from its motion sensors, so it does not read the belt's speed either.
Run the fartlek the belt can give you
Start each surge's clock once the belt has settled at the new speed, and each float's clock once it has come down.
The shortest surges are the ones most likely to lose time to a slow ramp. How short is too short depends on your machine, so lean on longer surges indoors and treat 15 second pieces as quick lifts in effort.
If your machine has one-touch speed keys, they save you the button presses, not the wait. The belt still climbs at its own rate.
Compare sessions only on the same treadmill.
A 30 minute treadmill fartlek
This is one example, not a prescription.
- About 6 minutes easy to warm up.
- Six rounds of 90 seconds at your surge setting (8.2 in the example) and 90 seconds at your float setting (5.7), each clock started once the belt settles.
- About 6 minutes easy to finish, for close to 30 minutes in all.
Know what your two settings really deliver
This session runs on two settings, float and surge. The belt's real speed at each one is the thing your treadmill has no way to show you. Treadwell measures it on an iPhone Pro, using the phone's LiDAR and camera. With nobody on the deck and your feet on the floor, run the empty belt at your float setting and take a reading. Repeat at your surge setting. The reading tells you what each setting delivers on the empty belt once it is up to speed. It does not time the climb, so the ramp advice above still applies.
Put real paces on your surge and float
Check the belt at both of your fartlek settings and run your surges by real pace. Treadwell works on an iPhone Pro, since its LiDAR is part of every reading.
Scan with your iPhone Pro to get the appTested: in our tachometer tests the app read within about 2 percent of true belt speed.
Quick answers
Can you do fartlek training on a treadmill? Yes, but the belt changes speed gradually, over a span of seconds that depends on the machine. Each surge starts slower than the number you pressed and each float starts faster, so a short surge can end before the belt ever arrives.
What is a good treadmill fartlek workout? Pick surges long enough that the climb is a small share of each one, and count each piece from the moment the belt reaches the number. The 30 minute example on this page is one way to lay that out.
Can you do a Mona fartlek on a treadmill? Yes. Its last block of 15 second surges and 15 second floats is the part the belt rounds off, and the longer blocks lose far less. How much that block loses depends on how quickly your machine is set to change speed, and on a slow setting those surges may never reach the number you pressed.
Sources.
- The ramp as a setting: Spirit Fitness MT200 (ReCare) physical rehabilitation treadmill user manual, 2020. It gives the time the belt takes to change 1 mph, set separately for speeding up and slowing down, with a factory default of 3 seconds and a range of 1 second to 1 minute. This is one rehabilitation model, so it shows that the rate is a setting and how wide the range can be. It is not a typical gym figure.
- Gradual changes by design: the XTERRA TR260 owner's manual (2023) and the Spirit XT685 owner's manual (2011) both warn that speed and incline changes do not happen immediately and that the machine carries out a new setting gradually.
- Searched without finding a stated ramp rate: four Matrix treadmill owner's and service manuals and a Woodway medical treadmill manual.
- The Mona fartlek's shape: McMillan Running, "Tempo with the Mona Fartlek." Used only for the order of its blocks, which ends on 15 seconds hard and 15 seconds float.
- Belt speed against the setting: Willwacher and colleagues, 2021, in Scientific Reports, volume 11, article 2244. Four laboratory treadmills in known condition averaged about 98 to 103 percent of target belt speed. That range describes lab machines, not a sample of gym treadmills.
- Indoor watch pace: Apple Support article 105002 says the watch falls back on its accelerometer for pace when it has no GPS, as on a treadmill. The Garmin Forerunner 165 owner's manual likewise has the watch estimate indoor pace with its own sensor.
- The 5.7 and 8.2 settings, both ramp rates, the table, the bus and the sample session are my own illustrations. The paces come from my own arithmetic, assuming a steady climb that starts the moment the button is pressed.