Zone 4 -5a represents an athlete’s functional threshold, the output they can sustain for 45-75 min, or a distance of about 10-21.1 km, depending on fitness/conditioning. At this exercise intensity, the anaerobic system begins to play a greater role in energy production, with carbohydrates being used as the predominant fuel source and lactate produced as a byproduct of energy metabolism. While lactate itself does not limit exercise performance, the associated increase in the acidity level within the muscle fibres and the depletion of carbohydrate stores (glycogen) limits the duration that exercise in this zone can be sustained for.
Zone 4 training is typically completed using intervals of 5 – 20 min. Rest periods between intervals allow the muscles to recover enough to produce an effective power output in the subsequent effort but are short enough to prevent the muscles from undergoing a full recovery, thereby promoting the following adaptations:
- Increased metabolic efficiency, so lactate and acidity can be buffered and utilised quickly, avoiding their accumulation, and glycogen use can be minimised
- Increased running economy through an improved muscular force output at a lower energy cost.
Note: The large variance in duration/distance that different athletes can sustain exercise in zone 4 is affected by the fitness and conditioning level of a runner. Ultra runners often struggle to maintain zone 4 threshold effort efforts, while 10 km and half marathon runners may find extended zone 4 intervals relatively comfortable.