Reasoning

The question asks why cycling is less toiling (requires less energy) than running for the same distance and time.

• Running: When you run, your center of mass (COM) oscillates vertically with every step. You constantly perform work against gravity to lift your body weight, and this potential energy is largely dissipated as heat/impact when you land. This internal work constitutes a significant portion of metabolic cost.
• Cycling: On a bicycle, your center of mass remains at a relatively constant height. You do not have to repeatedly lift your body weight against gravity. The wheels support the weight, and the energy is primarily used to overcome rolling resistance and air drag.

Therefore, the primary factor that makes running more strenuous is the work done against gravity due to the vertical oscillation of the body.