The toe box is the front section of a shoe that surrounds the toes. Its width and volume determine how much room your toes have to splay and swell, which matters greatly in trail running and on long descents and ultras, where cramped toe boxes cause blisters, black toenails, and discomfort. Many trail and zero-drop shoes feature a wide, foot-shaped toe box for natural toe splay.
Lugs are the raised rubber knobs or studs on the outsole of a trail-running shoe that bite into soft, loose, or uneven ground for traction. Deeper, more widely spaced lugs grip best in mud, snow, and steep terrain (and shed debris), while shallower lugs suit hard-packed trails and feel smoother. Lug depth, shape, and spacing — plus rubber compound — determine a shoe's grip.
A rock plate is a thin, stiff protective layer built into a trail-running shoe's midsole or between midsole and outsole that shields the foot from bruising on sharp rocks, roots, and rugged terrain. It adds underfoot protection valued on technical trails, at the cost of a little weight and some ground feel and flexibility. Not all trail shoes have one.
Stack height is the thickness of material (midsole, plus any plate and outsole) between your foot and the ground, measured in millimeters, usually given at the heel and forefoot. Higher stack ('maximal') shoes offer more cushioning and protection but can feel less stable and reduce ground feel; lower stack shoes give better ground feel and stability but less cushioning. The drop is the difference between heel and forefoot stack.
A zero-drop shoe has no height difference between the heel and forefoot (0 mm drop), keeping the foot level as if standing barefoot, which proponents say encourages a more natural foot strike and posture. Zero-drop shoes can have plenty of cushioning or very little; switching to them increases load on the calves and Achilles, so a gradual transition is important to avoid injury.
Heel-to-toe drop (offset) is the difference in stack height between a running shoe's heel and its forefoot, measured in millimeters — for example, a 10 mm drop sits the heel 10 mm higher than the toe. Lower drops (0–4 mm) encourage a more midfoot/forefoot strike and load the calves and Achilles more; higher drops (8–12 mm) shift load toward the heel and knees. It's a key fit-and-feel spec, best changed gradually.
A soft flask is a collapsible, flexible water bottle — usually made of soft TPU — that fits in the front pockets of a hydration vest and collapses as you drink, which reduces sloshing and saves space. Lightweight and packable, soft flasks (often with bite valves or wide caps for mixing) are the standard way trail runners carry water and electrolyte drinks up front.
A hydration vest is a lightweight, snug-fitting running pack worn like a vest that carries water (via soft flasks in front and/or a reservoir in back), plus food, layers, and mandatory gear. Designed to move with the body without bouncing, it's the standard way trail and ultra runners stay hydrated and self-sufficient on long outings and in races with required-gear lists.
Electrolytes are minerals — chiefly sodium, plus potassium, magnesium, and chloride — that carry an electrical charge and are essential for fluid balance, nerve signaling, and muscle function. Runners lose them (especially sodium) through sweat, so replacing electrolytes during long or hot efforts via drinks, tablets, or food helps maintain performance and helps prevent both cramping and the overhydration condition hyponatremia.
An energy gel is a small, portable packet of concentrated carbohydrate (sugars/maltodextrin), sometimes with electrolytes or caffeine, used to quickly fuel endurance activity and stave off the bonk. Gels deliver fast-absorbing carbs on the move and are a staple of marathon and ultra fueling, usually taken with water and at regular intervals to maintain energy without overwhelming the stomach.