As an Accredited Sports Scientist and operator of a sports physiology practice (Adelaide Human Performance), I’ve recently posted several blogs on Adelaide Cyclists about fitness and training. My last post discussed how general training zones based on fixed percentages of maximum heart rate or maximal aerobic power output are a reasonable starting point for controlling training workloads, but to get maximum benefit from fitness training, training zones need to be personalised based on an individual’s response to exercise. Training at the correct exercise intensity is essential, because it’s the main determinant of training load, where training load is the product of the intensity and duration of exercise. If the training load is too low, the cardio-respiratory, metabolic and muscular systems are not worked hard enough to stimulate functional improvements and increase fitness. If the training load is too high, the body cannot adapt fast enough and the rate of fitness development is reduced – or possibly reversed! This means low-intensity exercise must be performed for a longer duration to provide an effective training load, whereas an appropriate training dose can be achieved from short bouts of high-intensity exercise. However, high-intensity exercise generates higher levels of fatigue, meaning the provision of adequate recovery becomes increasingly important. So what is the right mix between short high-intensity workouts and long low-intensity training sessions, and how should a training program be structured to balance an effective training load with adequate recovery? There is no one-size-fits-all answer to this question, but the physiological basis of the training zones and the training practices of elite endurance athletes both provide insight into the ideal training load.
Fitness benefits from T1 (recovery) training
The T1 training zone represents a very light training intensity. At this level of exertion, the aerobic energy system can easily meet the energy demands of exercise, and activity from the anaerobic energy system is negligible. This means the T1 exercise intensity is generally too low to provide an adequate training stimulus, unless the duration of exercise is very long (~3+ hours). So with the exception of training for extreme endurance events, the T1 training zone is mainly used for recovery sessions, and for pre-training warm-up and post-training cool-down. The duration of recovery sessions depends heavily on an athlete’s overall training volume, but most sessions are in the range of ~30-60 min.
Benefits from T2 (light aerobic) training
Most endurance training should be completed using the T2 training zone, as this exercise intensity is sufficient to stimulate aerobic fitness, but only produces low lactic acid levels and minimal fatigue. T2 training sessions involve continuous-type exercise performed for moderate (~30-90 min) to long (~90-180 min) durations. T2 exercise requires relatively low levels of muscular force, which means slow twitch muscle fibres can meet the force production demands. Slow twitch muscle fibres use the aerobic energy system almost exclusively, so fat is used as a fuel source for energy production, meaning muscle glycogen (carbohydrate) is used very efficiently. As T2 exercise emphasizes utilisation of slow twitch muscle fibres, these fibres are encouraged to grow stronger and develop more blood capillaries. Slow twitch muscle hypertrophy increases the capacity for force production, and increased capillary numbers enhances delivery of oxygen and fats for energy production. The emphasis on slow twitch muscle fibre usage also encourages increases in the number of mitochondria, which enhances oxygen extraction and utilisation within the muscle fibre. Each of these adaptations improves aerobic power (VO2max) and muscular endurance. Short (~20-45 min) T2 training sessions can also be programmed for recovery purposes, although this may be best suited to athletes that train less than 6 days per week.
Benefits from T3 (moderate aerobic) training
The T3 training zone provides a higher exercise intensity than the T2 zone, so physical exertion and muscular force demands are higher. This means more slow twitch muscle fibres are recruited to increase the force of muscular contractions, so aerobic energy demands (and the associated training stimulus) are increased relative to T2 exercise. However, some fast twitch muscle fibres are also used to meet the force production requirements, so T3 training introduces a moderate contribution from the anaerobic energy system. As lactic acid is a by-product of anaerobic energy production, T3 training is associated with moderate lactic acid levels. Fast twitch muscle fibres are very poor at using fat as a fuel, so muscle glycogen is the main fuel used for energy production. The increased reliance on glycogen as a fuel source means that muscle glycogen levels are depleted faster during T3 exercise, which is why T3 training intensities are more fatiguing and can’t be sustained as long as T2 efforts. T3 training still involves continuous-type exercise, but the session duration is ultimately limited by glycogen levels. Moderate duration training sessions (~30-90 min) provide an effective training stimulus without excessive depletion of muscle glycogen, meaning the T3 training zone provides a time-efficient workout for aerobic fitness development. The fitness benefits from T3 training are essentially the same as those from T2 exercise, although there is less emphasis on muscular endurance (because training sessions are shorter) and more emphasis on developing aerobic power and fatigue resistance (because exercise intensity is higher).
Benefits from T4 (hard aerobic) training
The T4 training zone provides a higher exercise intensity than the T3 zone. Given physical exertion and muscular force demands are higher, more fast twitch muscle fibres are needed compared with T3 exercise, so there is an increased reliance on anaerobic energy production. In fact, the T4 training zone represents an athlete’s anaerobic threshold, where lactic acid production exceeds the maximum capacity for lactic acid removal. This means that small increases in exercise intensity result in large increases in blood lactate level, and muscular contractions become impaired due to increasing levels of acidity inside muscle fibres. This is called metabolic acidosis. The effects of metabolic acidosis mean that T4 training is the highest exercise intensity that can be sustained on a continuous basis. If exercise is performed above the T4 intensity, impaired muscle function from metabolic acidosis forces a reduction in exercise intensity. This is why athletes ‘blow-up’ during short races or time-trials if they push too hard for too long.
Although there is an increased reliance on anaerobic energy production during T4 exercise, it doesn’t mean aerobic energy production becomes less important. Aerobic energy production continues to increase, so T4 training actually provides a very strong aerobic stimulus. As the demand for energy increases in parallel with exercise intensity, the cardiorespiratory system is forced to work progressively harder to supply the oxygen requirements for energy production. This means that T4 exercise pushes the aerobic system to near-maximal levels, thereby providing a strong stimulus for functional improvements within the cardiovascular system (to improve oxygen delivery). However, the high energy demand also means that fat cannot be transported into muscle cells fast enough to contribute to energy production, so carbohydrate (from muscle glycogen and blood glucose) is the only fuel source available to the aerobic and anaerobic energy systems. This means that muscle glycogen supplies are used very quickly, so T4 exercise can only be sustained for up to ~60 min before glycogen depletion limits exercise performance. Fitness benefits from T4 training include: aerobic power development, enhanced fatigue resistance, and improved speed-endurance.
Fitness benefits from T5 (maximal intensity) training
The T5 training zone provides a higher exercise intensity than the T4 zone, which means the aerobic energy system is working at 90-100% of maximum, and there is also a very strong reliance on anaerobic energy production. As by-products from the anaerobic energy system contribute to the development of metabolic acidosis, the duration of exercise is limited to ~1-20 min. This means that T5 sessions involve interval training rather than continuous exercise, so short (1-5 min) or moderate (5-10 min) intervals are interspersed with periods of rest or active recovery to permit continuation of high-intensity exercise. T5 training causes rapid glycogen depletion and high levels of fatigue, so adequate recovery following T5 sessions is essential. Fitness benefits from T5 training include: aerobic power development, enhanced fatigue resistance, and improved top-end speed (from anaerobic fitness improvements).
Programming training load
Using the training practices of elite endurance athletes as a benchmark, Seiler and Tonnessen (Sportscience, 2009) suggest that a training program should ideally consist of ~80% low-intensity exercise (corresponding to zones T1-T3 as defined in this article) and ~20% high-intensity work (zones T4 and T5). However, the relative break-down between training zones also depends on the duration of the endurance event being prepared for, so longer events (e.g. road cycling vs. track endurance, or a marathon vs. a 10 km race) will have a greater emphasis on T1-T3 training. For example, elite male marathon runners (event duration ~2.25 h) have been observed to use a training zone break-down of 80% T1 or T2, 8% T3, 6% T4, and 4% T5 (Billat et al. 2001, Medicine and Science in Sports and Exercise), whereas elite road cyclists (event duration ~3-6 h) have been shown to use a training zone break-down of 78% T1 or T2, 20% T3, and 2% T4 or T5 (Zapico et al. 2007, Journal of Medicine and Physical Fitness). This group of cyclists was also observed to increase their emphasis on higher-intensity training as the cycling season progressed, so their training break-down shifted to 70% T1 or T2, 22% T3, and 8% T4 or T5. This demonstrates that the relative breakdown between training zones should change as a training program progresses from base-fitness development towards race-specific preparations.
Most training should be completed at T2 or T3 intensity, with up to 1-2 sessions per week including T4 or T5 intensity work. High-intensity workouts (zone T4 or T5) or long duration sessions should be followed by a lower intensity session (zone T2 or T3) or rest day. Training volume should gradually increase each week, and recovery weeks should be programmed every 3-4 weeks.
Add a Comment