Braking is surprisingly cheap. Downhill running often demands less metabolic energy because gravity supplies energy that muscles must generate themselves when carrying the body uphill against its pull. The distinction is mechanical. Whereas climbing requires muscles to perform positive mechanical work, descending asks them to absorb energy, chiefly through eccentric contractions in which active muscles lengthen under load.
Force is not fuel. During eccentric contraction, muscles can resist substantial loads while consuming less adenosine triphosphate, or ATP, than they need to produce comparable force while shortening. Uphill running reverses matters. Its concentric contractions require repeated chemical expenditure to lift the body's center of mass, increasing gravitational potential energy with every gain in elevation along the route. Downhill, gravity pays first. Muscles still spend ATP to control movement and support the body, but absorbing mechanical energy costs less than supplying it; braking does not mean recovering that energy as usable fuel.
Cheap has its price. The high forces associated with eccentric loading can produce muscle damage and delayed soreness, especially after unfamiliar descents, even when breathing feels easier than on an ascent. Steepness changes the bargain. As the slope increases, stronger braking and altered movement can raise metabolic demand again, so a steeper descent does not guarantee a smaller energy bill. Easy breathing proves little. At the bottom, the lungs may register a bargain while the thighs carry the receipt, having absorbed the energy that gravity supplied throughout the descent.
