Why is running over the same distance more exhausting than cycling? When we run 1 kilometer, we feel completely drained, but if we switch to cycling, it feels like a piece of cake. Both sports ultimately involve moving the human body over the same distance, and the energy expended comes from physical effort (primarily from the legs). Cycling doesn't actually do any work, but it increases the rider's additional weight. So why do we feel that running is much more tiring than cycling?
Some argue that the total work (or energy expended) required to "move mass" is the same whether cycling or running on a flat surface along the same route. However, this claim is not reliable. The work involved doesn't lie in moving mass itself, but in overcoming what physicists call "dissipative forces" such as wind and friction during the movement.
To understand the physical implications of cycling and running, let's make two semi-quantitative estimates. Measurements show that cycling at a constant speed on a relatively light bicycle requires about 1 Newton of force to overcome wheel axle friction and about 2 Newton of force to overcome tire rolling friction. At a speed of 8 km/h, wind resistance is also about 1 Newton. Thus, the total force required to overcome these "dissipative forces" is approximately 4 Newton. For a 1-kilometer ride, the total work required is about 4,000 joules. If the speed is doubled (to 16 km/h), wind resistance will increase by four times, and the total work required will be about 7,000 joules. If the speed is doubled again (to 24 km/h), the wind resistance will increase to 9 Newton, and the total work required for a 1-kilometer ride will be about 12,000 joules.
The situation with running is more complex, involving at least two main movements: horizontal movement of the body and vertical jumps of the center of gravity with each stride. It is the latter that consumes a large amount of energy for runners. For a 70-kg runner, if they jump up about 15 centimeters with each stride, the energy consumed per stride will be about 100 joules. This energy is ultimately converted into heat and sound energy when the foot hits the ground. Assuming 925 steps per kilometer, the total energy expended on impacting the ground will be about 92,500 joules. Compared with the total energy expended on cycling, the difference is obvious.
We can also examine this from another perspective: both sports consume energy from the athlete's aerobic respiration. According to exercise physiologists, the energy consumption of running is about 260,000 joules per kilometer, which is nearly three times the energy consumed in overcoming external "dissipative forces"! This extra energy is used to overcome "internal friction" in the body, such as sweating during running, which indicates that the body generates a large amount of heat. Therefore, runners will feel increasingly hot and even sweat profusely. In contrast, cycling consumes very little of this energy. If cycling at a speed of 8 km/h, the cyclist will pedal about 32 times per minute (with a cadence close to that of a casual walker), and their upper body will remain largely inactive except for the periodic pedaling of their legs. If the bicycle and road conditions are good, their aerobic respiration consumption will only be slightly higher than that of a walker, and less than one-tenth of that of a runner.
The above estimates and measurements show that cycling is indeed much more energy-efficient than running over the same distance.