What nobody has countedEmerging evidence
No study counts ankle pain caused by treadmills. In US emergency departments, treadmills caused 66% of injuries from home exercise machines in 2007 to 2011, though they are about a quarter of the machines sold; of treadmill injuries, 36% were to the leg and 21% were sprains or strains, but the study doesn't say which were ankles or how they happened. In runners in general, the ankle took about 1 in 5 new injuries (19.0%), Achilles tendon pain 10.3% and ankle sprains 5.8%. In 1,929 recreational runners, 5.2% developed Achilles tendon pain around an event, and a past episode raised the odds about 6 times.
So the map on this page is built from lab studies of the belt and slopes, studies of the conditions, and NHS, NHS inform and NHS trust pages, and it says so.
Sources 1, 2, 3
The belt itselfEmerging evidence
Across 33 studies of 494 healthy adults, treadmill running was largely comparable to running outside, but the foot landed about 10 degrees flatter (low certainty) and the ankle worked slightly harder in 2 studies; an older study of 22 runners found the same flatter landing. In 18 runners, peak Achilles force was 12.5% higher on the treadmill, with a 15.6% faster loading rate. That is one study, and the review suggests treadmill running may not suit a return from Achilles problems. Treadmill walking was largely comparable to walking outside in 22 studies.
These are small studies of healthy people, and none counted pain. A sudden switch from the road to the belt may load the Achilles more (our reading).
Sources 4, 6, 7, 5, 8
The slopeEmerging evidence
Walking up 9 degrees (about 16%) on a treadmill raised calf muscle activity to 175% and 136% of level walking, and the changes grew with speed. In 19 adults walking up to 20 degrees, peak ankle bend roughly doubled, from about 10 degrees on the flat to 21, and people with stiff ankles worked their shin and calf muscles harder. On a 10% slope, the Achilles stretched a lot with each step, walking or running. In 19 runners, uphill running raised modelled Achilles and shin load, and uphill runners land further forward on the foot.
These are muscle-activity studies and one modelling study read from its abstract, in healthy people; none measured ankle pain. The 12-3-30 workout (12% incline, about 7 degrees) hasn't been studied by name. So a steep slope asking more of the calf, the Achilles and the front of the ankle, more so with speed, is our reading.
Sources 9, 10, 11, 12, 13, 14, 15
Shoes, foot strike and step rateEmerging evidence
In 20 adults, Achilles load was highest with a forefoot landing, then midfoot, then heel. In a trial in 99 runners, full minimalist shoes brought more shin and calf pain, and in 553 leisure runners, heel drop made no overall difference, though low-drop shoes went with more injuries in regular runners. Quicker steps lowered modelled Achilles load in 19 runners; in 45 runners, 5% to 10% more steps a minute mainly eased the knee and hip, and at the ankle only push-off work fell.
Sources 16, 17, 18, 13, 19
How the causes are told apartEmerging evidence
The cord above the heel, stiff on the first steps and worse uphill, is Achilles tendinopathy; NHS inform lists a change in load among the causes, and it is common outside sport too. A pinch at the front of the ankle as your shin tips forward, worse on a steep slope or squatting, often after old sprains, is anterior ankle impingement; that a steep belt brings it on is our reading of the ankle bend studies. Pain behind and below the inner ankle bone after more walking or running is usually the tibialis posterior tendon, and behind the outer ankle bone the peroneal tendons. Outer sprains are the commonest kind, and on a flat belt a roll usually means a misstep or a fall (our reading).
One spot on a bone that hurts with every step, later at night, can be a stress fracture. Those of the inner ankle bone are rare (0.6% to 4.1% of stress fractures) and often need surgery; fibula stress fractures healed in about 7 weeks; and navicular stress fractures cause vague pain on top of the midfoot and are often diagnosed months late. So ankle pain on the treadmill is most likely after a jump in slope, speed or time, a switch of shoes or foot strike, a return after a break, a past Achilles episode or old sprains (our reading).
Sources 20, 21, 22, 23, 10, 24, 25, 26, 27, 28, 29, 30, 31
When a scan helps, and how it usually goesEmerging evidence
Not usually at first. After a twist or fall, clinicians use the Ottawa ankle rules, which ruled out ankle and midfoot fractures well across 27 studies of 15,581 people; the NHS says you'll usually have an X-ray if a break is suspected. For Achilles pain, a Dutch guideline advises against scans to predict how it will go. For one sore spot on a bone, an early X-ray can miss a stress fracture, so an MRI scan may be needed.
NHS inform says new ankle pain should begin to settle within 6 weeks, and Achilles tendinopathy takes several months or more. Most sprains feel better after 2 weeks, NHS inform expects recovery by 8 to 12 weeks, and a hospital leaflet says pain and swelling can last 3 to 6 months; not managing hops and landings 2 weeks after a first sprain predicted an ankle that keeps giving way. Inner ankle tendon pain usually settles within 3 to 12 months; a foot stress fracture takes 6 weeks to 6 months to heal. No one can promise a date.
Sources 32, 33, 34, 35, 36, 20, 37, 38, 39, 40, 24
