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Footwear

Midsole

What is a midsole?

The midsole is the foam layer between the outsole and your foot. It absorbs impact and sets how stiff or soft a shoe feels, and it's the part that quietly dies long before the tread looks worn.

Midsole at a glance

Common materials
EVA and polyurethane
Trail shoe life
300 to 500 miles
Fails
Invisibly, before the tread

What the midsole is doing

Every step puts a few times your body weight through your foot, and the midsole is what stands between that and your joints. It's foam, usually EVA or a polyurethane, compressed into a shape that also decides how the shoe flexes.

Why stiffness is a choice, not a quality

Stiffness here is a design choice rather than a quality signal. A soft, flexible midsole lets your foot move naturally and works well on smooth trail with a light load. A firm one resists twisting, which is what you want carrying 35 lb over talus, and it feels dead and clumpy on a gentle path. Neither is better.

Why midsoles fail invisibly

The awkward truth about midsoles is that they fail invisibly. Foam compresses permanently with use, and once it's packed out the shoe stops absorbing impact even though the tread looks fine. People replace shoes because the outsole wore through and quite often the midsole died a hundred miles earlier, which is what they were feeling in their knees.

What this means when you're choosing

  • Stiffness matched to your load rather than to the price
  • Visible compression lines or wrinkles along the foam, a sign it's packed out
  • Whether a polyurethane midsole is present, which lasts longer but weighs more than EVA

The common mistake

Replacing shoes when the tread wears out

The foam usually dies first and gives no visible signal. If distances that never used to bother you leave your knees and feet aching, that is the midsole telling you, and it will have been telling you for a while.

Common questions

How do I know when a midsole is worn out?

Press your thumb into the foam along the side. Fresh foam springs back; packed-out foam stays dented and shows deep creases. If your feet and knees ache after distances that never used to bother you, that's usually the answer too.

Does a stiffer midsole mean a better boot?

It means a boot for heavier loads and rougher ground. On easy trail with a light pack a stiff midsole just makes walking harder, so stiffness is a fit-to-purpose decision.

Where this comes up

Guides on Tom's Hiking where midsole changes which product is right for you.

More in footwear

Lug depth

Lugs are the raised blocks on the sole of a boot or trail shoe, and lug depth is how tall they are, usually 3 to 6 mm. Deeper lugs bite into soft ground; shallower ones give more contact area on rock and hard trail.

Outsole

The outsole is the rubber layer on the bottom of a shoe, the part touching the ground. It carries the lug pattern and the rubber compound, which together decide grip, and it's usually the first part of a shoe to wear out visibly.

EVA foam

EVA is the light, springy foam most hiking and running midsoles are made of. It cushions well and weighs little, but it compresses permanently over time, which is why shoes go flat after a few hundred miles.

Stack height

Stack height is how much material sits between your foot and the ground, measured in millimetres at the heel and forefoot. More stack means more cushioning and less ground feel; less means more feedback and a lower, more stable platform.

Heel-to-toe drop

Drop is the height difference between a shoe's heel stack and its forefoot stack, given in millimetres. Zero drop means the foot sits level; 8 to 12 mm tilts you forward slightly and shortens the load on the calf and Achilles.

Rock plate

A rock plate is a thin stiff sheet, usually TPU or a nylon composite, embedded in the forefoot of a shoe. It spreads the load from a sharp rock across the whole foot instead of letting it press into one point.

Shank

A shank is a stiffening insert running through the arch of a boot, made of steel, nylon or composite. It controls how much the boot twists and bends, which is what lets a boot support a heavy load and edge on rock.

Rand

A rand is a strip of rubber bonded around the lower edge of a boot's upper, most often at the toe. It takes the abrasion from rock and roots that would otherwise wear through fabric or leather, and it protects your toes from impact.

Gusseted tongue

A gusseted tongue is sewn to the sides of the boot with fabric panels instead of hanging free. It seals the gap along the laces so grit and water can't get in, and it stops the tongue sliding sideways as you walk.

Vibram Megagrip

Megagrip is a Vibram rubber compound formulated for grip on wet and dry rock. It's softer and stickier than standard hiking rubber, which is why it holds on slabs and greasy roots, and why it wears out faster.

Gaiters

Gaiters are fabric sleeves worn over the top of a shoe or boot to keep grit, snow and water out. Low trail gaiters cover the collar and stop scree filling your shoes; tall gaiters run to the knee and seal the leg against deep snow and wet brush.

Microspikes

Microspikes are elastomer harnesses with short chains and spikes, roughly 10 to 15 mm, that stretch over a shoe for traction on packed snow and ice. They are the middle rung between yaktrax-style coils and full crampons, and they are not mountaineering equipment.

Crampon

A crampon is a metal frame carrying 10 or more points up to 5 cm long that binds rigidly to a stiff-soled boot, so a climber can stand on hard alpine ice and steep snow. It is the tier above microspikes, and it is technical equipment that needs an ice axe and training.

Insulation weight in grams

Insulation weight is the mass in grams of one square meter of the insulating sheet inside a boot or glove. A 400g boot uses a heavier sheet than a 200g boot, so it is warmer, but it is nothing like twice as warm.

Vibram Arctic Grip

Arctic Grip is a Vibram outsole technology built for traction on wet ice. It stays pliable at temperatures where ordinary hiking rubber goes hard, which is the reason a winter sole grips where a three season one skates.

Flotation

Flotation is how well a snowshoe keeps you on top of snow instead of sinking into it. It is a function of deck surface area measured against your loaded weight, which is your body plus clothing, boots and everything in your pack.

Heel lift

A heel lift is a small wire or plastic bar behind the heel of a snowshoe that flips up so your boot rests on it during a climb. It stops your calf being held at full stretch on sustained steep ground.

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