There are four basic leg classes available:

Implant
The lowest model is implanted in your existing, organic legs and greatly improves power, speed and endurance.

Optimized for Power
Especially interesting together with other parts, gives you enormous Power, up to 20 times of organic legs.

Optimized for Speed
Built very light (titanium and compound materials) and not as strong (ratio 2x) but gives you great speed.
You may constantly run 35 km/h !

Combo
The alround-choice, medium power and speed.