Answer
If we use a lower gear, the force is applied by the chain farther from the axis. Then for a given torque, the required force is less. This allows our leg muscles to provide the required force more comfortably.
Work Step by Step
The torque is proportional to the distance between the point where the force is applied and the axis of rotation. For a bicycle chain on a gear, the axis of rotation is the center of the gear.
If we try to use a high gear, the force is applied close to the center of the gear. Then for a given torque, we need to apply more force which requires our leg muscles to apply a larger force. This can be quite arduous for the leg muscles.
If we use a lower gear, the force is applied by the chain farther from the axis. Then for a given torque, the required force is less. This allows our leg muscles to provide the required force more comfortably.
