Anaerobic capacity and speed reserve
Evidence: moderate
Anaerobic capacity is the energy a runner can supply without oxygen. The anaerobic speed reserve, the gap between aerobic speed and sprint speed, profiles middle-distance runners well and explains why raw top speed matters even on the track.
Anaerobic capacity is the total work a runner can do from energy sources that need no oxygen: stored phosphates and glycolysis. It matters most over the seconds to a couple of minutes when oxygen delivery cannot keep up with demand, which is why it separates milers and 800m runners from pure marathoners. The aerobic system still does most of the work in distance racing, but the anaerobic contribution decides surges, finishes and the shortest events.
Measuring it: the oxygen deficit
The reference measure is the maximal accumulated oxygen deficit, or MAOD. A runner exercises to exhaustion at a supramaximal intensity, around 120% of VO2max, and the oxygen the body demanded but could not supply is calculated by extrapolation; that deficit estimates the anaerobic energy released (Medbo et al. 1988). MAOD is the closest thing to a gold standard for anaerobic capacity, and it tracks event specialism: sprinters and middle-distance runners record larger deficits than long-distance runners (Medbo et al. 1988). It is demanding to measure, needing several supramaximal tests, which limits its everyday use.
VO2 kinetics
How fast oxygen uptake rises at the start of an effort sets how much energy must come from anaerobic stores before aerobic supply catches up. The lag at the onset of hard exercise creates an early oxygen deficit, and above the lactate threshold a slow upward drift in oxygen cost adds to it (Jones & Burnley 2005). Training speeds the kinetics, so a fitter runner reaches steady state sooner and draws down less of the anaerobic reserve for the same pace (Jones & Burnley 2005). Faster kinetics and a larger anaerobic capacity are partly independent qualities. See VO₂ kinetics for the detail.
Anaerobic speed reserve
The anaerobic speed reserve, or ASR, is the speed band between the slowest pace that elicits VO2max (maximal aerobic speed, the velocity at VO2max) and a runner’s maximal sprint speed (Sandford et al. 2019). It captures something VO2max alone misses: two runners with identical aerobic ceilings can differ widely in how much faster they can go for a few seconds.
Why top speed matters on the track
Middle-distance races are run above the velocity at VO2max, inside the speed-reserve band, so the size of that band shapes performance. Among the world’s best 800m men, maximal sprint speed and ASR sorted athletes into distinct profiles, a speed-based type with a large reserve and an endurance-based type with a small one (Sandford et al. 2019). A runner with more top-end speed sits at a lower fraction of their maximum for any given race pace, which leaves room for a surge or a kick.
ASR also offers a tidier way to set training intensity. Expressing a supramaximal interval as a fraction of a runner’s speed reserve, rather than as a fixed percentage of aerobic speed, equalises the physiological demand across athletes with different sprint speeds (Sandford & Laursen 2021). Much of the prescription evidence so far comes from other sports rather than running, so treat the running application as promising rather than settled (Sandford & Laursen 2021). The practical lesson stands: developing maximal speed through short, fast intervals raises the ceiling that distance pace sits beneath.