What nobody has countedEmerging evidence
No study counts calf pain in treadmill users; US emergency data for home exercise machines count injuries, 66% of them from treadmills, not overuse pain. In runners in general, calf strains were 4.2% of new injuries and Achilles tendon pain 10.3% in a large review, and runners aged 40 and over had more calf, Achilles and hamstring injuries than younger runners. Across 10 studies of 5,397 athletes, mostly in football codes, older age and a previous calf strain were the strongest risk factors for a calf strain.
So the map on this page is built from lab studies of slopes, speed and the belt, studies of the conditions, and NHS, NHS inform, NICE and NHS trust pages, and it says so.
Sources 1, 2, 3, 4
What the slope and the belt do to the calfEmerging evidence
On a treadmill, walking up 9 degrees (about 16%) raised calf muscle activity to 175% of level walking in the upper calf and 136% in the deeper calf, alongside bigger rises at the hip, and the changes grew with speed. In 16 adults walking at 0%, 5% and 10%, the deep calf muscle's activity, with the thigh's, explained most of the extra effort of the slope, and in 6 men the calf muscle worked at longer lengths uphill with substantial stretch in the Achilles tendon. In 19 runners, uphill running raised the modelled load on the Achilles, faster running made no significant difference there, and about 10 more steps a minute lowered it.
The belt itself: in one lab study of 18 runners, peak Achilles force was 12.5% higher running on a treadmill than overground at the same speed, which is why a review suggests treadmill running may suit a return from a shin stress fracture but not from Achilles or calf problems. Other measures were comparable but not identical. These are small studies of healthy runners, and none measured calf pain.
Sources 5, 6, 7, 8, 9, 10, 11, 12
Shoes and foot strikeEmerging evidence
In a modelling study of 19 women, those who landed on the forefoot had 11% more Achilles load over each step and a 15% faster loading rate. Running in minimal shoes or barefoot raised how fast the Achilles was loaded, and a forefoot landing in minimal shoes raised how much. In a trial of 99 runners, full minimalist shoes brought more shin and calf pain, and in 553 leisure runners, low-drop shoes went with more injuries in regular runners (hazard ratio 1.67) but fewer in occasional ones.
So a sudden switch to running barefoot, in flat shoes or with a forefoot landing on the belt shifts work to the calf (our reading).
Sources 13, 14, 15, 16
How the causes are told apartEmerging evidence
An NHS trust leaflet says the upper calf (gastrocnemius) is more often injured in sudden, explosive movements such as sprinting, lunging or jumping, and the lower calf (soleus) after slower repetitive activities such as steady running; expert clinicians describe lower calf strains as a tightness that creeps up and is hard to pinpoint, and in elite Australian footballers 84.6% of calf strains were in the soleus. A cramp is a sudden painful tightening that can last from a few seconds to 10 minutes. The cord above the heel, stiff in the morning and sore after exercise, is Achilles tendinopathy, often after a change in load; a sudden snap, then flat-footed walking, can be an Achilles rupture. Both calves aching for a day or two after a new slope is ordinary muscle soreness.
Tightness that builds at the same point in every session and eases within minutes of stopping can be exertional compartment syndrome. Calf pain that comes on after a set distance and goes within a few minutes' rest is the pattern of intermittent claudication, and in 53 people with spinal stenosis or narrowed arteries, calf symptoms eased by standing still pointed strongly to the arteries, while most single symptoms told the two apart poorly. In a young athlete, calf pain with tingling in the foot can, rarely, be popliteal artery entrapment. One swollen, throbbing calf with red, blue or darkened skin can be a clot.
So calf pain on the treadmill is most likely after a jump to a steep slope or the 12-3-30, a faster pace or longer sessions, a switch of shoes or foot strike, a return after a break, from about 40, after a past calf strain, and, for the ache that eases on standing still, in people who smoke or have diabetes (our reading).
Sources 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29
When a scan or test helpsEmerging evidence
Most calf strains are diagnosed from the story and an examination; an NHS trust keeps a scan for a severe injury or one that isn't improving, and in elite footballers severe damage to the tendon sheet inside the muscle on an MRI (magnetic resonance imaging) scan meant a longer return (31.3 against 19.4 days). A suspected clot needs an ultrasound scan, and the NHS says referral should be within 24 hours.
For a suspected Achilles rupture, the calf squeeze test had a sensitivity of 0.96 and a specificity of 0.93 in 174 people, and hospitals confirm it with a gap and the squeeze test. Narrowed leg arteries are checked by comparing the blood pressure at the ankle and the arm. Compartment syndrome may need a pressure test.
Sources 17, 30, 29, 31, 22, 26, 25
How it usually goesEmerging evidence
Mild calf strains may take a few days and more severe ones weeks or months, an NHS trust says. In Australian football, calf strains from running took about 12 days longer to recover than others, and the risk of another muscle strain stayed raised for 15 weeks after return, with a recent calf strain raising the odds of another about 13 times. In new runners, injuries took a median of 71 days to recover.
With claudication, the NHS says most people's symptoms stay stable with treatment and some improve, and an NHS trust leaflet says walking distance usually improves over 3 to 6 months of regular walking. No one can promise a date.
Sources 17, 19, 32, 33, 26, 34
