Mixed

Motorized treadmill running is biomechanically comparable to overground running for most spatiotemporal, kinematic, kinetic, and muscle activity measures, with specific exceptions in sagittal plane kinematics at footstrike.

You can use a motorized treadmill for most of your training without worrying about drastically different biomechanics compared to running outside. The study shows that speed, stride length, and muscle activation are largely the same. However, be aware that your foot lands at a slightly different angle and your knee bends differently on a treadmill. If you are using treadmill data to prescribe specific gait corrections or footwear for outdoor racing, account for these small sagittal plane differences.