Positive and negative work generation during walking and during stair negotiation.

Blue represents the phases during which negative work is generated by the right leg (braking). Red represents the phases during which positive work is generated by the right leg (propulsion). In walking, the right leg both brakes and propels within one stride. In stair negotiation, however, the right leg generates predominantly negative work throughout stair descent (T_UL), and predominantly positive work throughout stair ascent (T_L). The gait cycle during stair negotiation follows the definition in [1].