Sprinters run slower around a bend than on a straight, and the difference is not a matter of technique alone. Turning requires force that must come from somewhere.
Why a curve costs speed
Moving along a circular path requires a continuous force directed towards the centre. On a flat track that force comes entirely from friction between shoe and surface.
Friction is limited, and the same contact must also provide forward propulsion. The two demands compete for the available grip.
The tighter the radius and the faster the runner, the larger the required force becomes. Athletes are effectively speed-limited by the geometry.
What banking contributes
Tilting the surface inward means part of the ground's normal reaction points towards the centre of the curve. That component supplies turning force without consuming friction.
The steeper the bank, the more of the requirement it covers. On a correctly banked curve an athlete can run closer to straight-line mechanics.
Banking is calculated for a target speed, so it is optimal at one velocity and imperfect at others. Slower runners feel the slope more than faster ones do.
Why indoor tracks are banked steeply
Indoor circuits are shorter, which means the bends have a much tighter radius. The turning force required at sprint speed rises sharply as a result.
Without pronounced banking, athletes would be unable to hold competitive speed through the curve. Steep bends are a necessity of the shorter lap.
Records set indoors are consequently kept in a separate category. The conditions differ enough that direct comparison would be misleading.
How lane position interacts with the bend
Outer lanes have a larger radius, so the turning force required is lower and the curve is easier to run. They also offer less visual information about rivals.
Inner lanes require the tightest turn, which costs the most speed, but allow a runner to see the field. Lane allocation therefore trades physics against information.
Staggered starts equalise distance but cannot equalise curvature. The advantage and disadvantage of each lane are structural rather than incidental.
What athletes change on the curve
Runners lean into the bend so that the resultant of gravity and ground force passes through the body's centre of mass. The lean angle rises with speed.
Arm action becomes asymmetric to accommodate the lean, and the inside leg works through a shorter range. Stride mechanics differ measurably from the straight.
Transitioning off the bend requires re-establishing an upright posture without losing rhythm. That change of state is where races are frequently decided.
