A human traveling on a bicycle at , using only the power required to walk, is the most energy-efficient means of human transport generally available.
Air drag, which increases with the square of speed, requires increasingly higher
power outputs relative to speed. A bicycle in which the rider lies in a
supine position is referred to as a
recumbent bicycle or, if covered in an aerodynamic
fairing to achieve very low air drag, as a
velomobile. According to a study a human at requires about 60
watts to walk at on firm and flat ground, while according to a calculator at kreuzotter.de the same person and power output on an ordinary bicycle will travel at , so in these conditions the energy expenditure of cycling is about one-third that of walking the same distance. Uphill and downhill speeds vary according to the slope of the incline and the effort of the rider. Uphill cycling requires more power to overcome gravity and speeds are therefore lower and/or the heartrate is higher than during flat riding conditions. With medium effort a cyclist can pedal 8–10 km/h up a gentle incline. Riding on grass, sand, mud, or snow will also slow a rider down. Without pedaling downhill a bicycle rider can easily reach speeds of 20–40 km/h down a gentle 5% slope and speeds exceeding 50 km/h on steeper inclines.
Energy output How much power humans can generate and for how long varies with physical form. The
specific power may be expressed in watts per kilogram of body mass. Active cyclists can produce from 1.0 W/kg (novice female) 2.2 W/kg (average untrained male), 3.0 W/kg (male, fair or female, good [fitness]), and 6.6 W/kg (top-class male athletes) at their
functional threshold power (about one hour). 5 W/kg is about the level reachable by excellent male or exceptional female amateurs. Maximum sustained power levels during one hour are recorded from about 200 W (
NASA experimental group of "healthy men") to 500 W (
Eddy Merckx on ergometer 1975). For a day's pedalling these figures can roughly be halved, for a minute's duration doubled.
Energy input The energy input to the human body is in the form of
food energy, usually quantified in
kilocalories [kcal] or
kilojoules [kJ, which is equivalent to kWs or kilowatt-seconds]. This can be related to a certain distance travelled and to body weight, giving units such as . The rate of food consumption, i.e. the amount consumed during a certain period of time, is the input power. This can be measured in kcal/day or in J/s = W (1000 kcal/d ≈ 48.5 W). This input power can be determined by measuring oxygen uptake, or in the long term food consumption, assuming no change of weight. This includes the power needed just for living, called the
basal metabolic rate BMR or roughly the
resting metabolic rate. The required food can also be calculated by dividing the output power by the
muscle efficiency. This is 18–26%. From the example above, if a 70 kg person is cycling at 15 km/h by expending 60 W and a muscular efficiency of 20% is assumed, roughly 1
extra food is required. For calculating the
total food required during the trip, the BMR must first be added to the input power. If the 70 kg person is an old, short woman, her BMR could be 60 W, in all other cases a bit higher. Viewed this way the efficiency in this example is effectively halved and roughly 2
total food is required. Although this shows a large
relative increase in food required for low power cycling, in practice it is hardly noticed, as the extra energy cost of an hour's cycling can be covered with 50 g nuts or chocolate. With long and fast or uphill cycling, the extra food requirement however becomes evident. To complete the efficiency calculation, the type of food consumed determines the overall efficiency. For this the energy needed to produce, distribute and cook the food must be considered. ==Typical speeds==