Training in zone 4: building the size of your 'bath'

Zone 4 is a vital part of an athlete’s training diet. Ultimately, ability to exercise in zone 4 will underpin race performance more so than any other training zone. You may hear this ability also called ‘muscular endurance’: as it is a composite of endurance ability while being able to sustain high force development (what some athlete’s feel is ‘leg strength’).

What is it?

Zone 4 falls between 75 to 85% of an athlete’s VO2max. The middle of zone 4 is the intensity you could hold for an hour: for cyclists, this is 25 mile time trial intensity, for runners, around 10-mile pace. For the sport scientist, this is an intensity is synonymous with the ‘maximal lactate steady state’.  As the term implies, it is the highest intensity you can sustain with steady lactate concentrations in the blood. It is worth noting that although blood lactate concentration is steady at this intensity other physiological systems may not be in the same equilibrium: indeed, recent research has shown that exercising at MLSS might lead to increased acidity of the blood1. To the performing athlete, this makes intuitive sense – race performance is limited at this intensity i.e. you cannot hold 25-mile power, or 10-mile race speed for a 50-mile time trial, or a marathon!

You may hear coaches refer to the point of the MLSS as the ‘second threshold’. You will remember (from reading other PBscience factsheets) that the first lactate threshold is where blood lactate concentration first starts to rise in the blood i.e. increased production above resting state. The ‘second threshold’ refers to when the rate of blood lactate clearance cannot stay in step with the production in the muscle. A lot of research, coming from the German research groups in the 1980’s, referred to this phenomenon as ‘OBLA’ (the onset of blood lactate accumulation). They observed a tendency for this point to be at blood lactate concentrations of ~4mM.


However, the absolute concentration is less important than the response across time i.e. sprint athletes with a lot of fast twitch fibres can ‘steady state’ as high as 8 to 10mM; whilst endurance athletes with greater slow twitch fibres, might struggle to reach 4mM, even at VO2max!

In the last decade, some coaches in cycling, have developed the phrase ‘functional threshold power’ in order to clarify to what this intensity refers. Functional Threshold Power (FTP) is proposed as a more practical way for athletes and their coaches to determine the upper limit of sustainable exercise – by taking the highest power achievable in performance of an hour’s work. The FTP is therefore akin to the MLSS: yet doesn’t require the same exercise testing, or blood lactate sampling procedures.

How does exercise in zone 4 impact on my physiology?

As zone 4 incorporates the maximal lactate steady state, we can assume that exercising in this zone will, in the main, be impacting on the lactate response to exercise. Exercising just below MLSS will stress the lactate production systems: so we have high rates of glycolysis (the splitting of muscle carbohydrate stores); exercising just above MLSS will stress the lactate clearance systems: so the body needs to be efficient at clearing lactate. Exercise at MLSS does indeed show ‘steady state’ in:

  • Ammonia_fatigueLactate concentration
  • Oxygen levels in the blood
  • Concentrations of chemicals controlling acidity / alkalinity
  • Blood volume characteristics
  • Breathing rates and associated gas concentrations

It is probably hard to understand why we cannot therefore maintain exercise for longer! Indeed, the only blood parameters shown to be out of balance are blood acidity (because of increasing hydrogen ions produced as lactic acid hits the blood) and blood concentrations of ammonia1. Ammonia is the end product of one system the body uses at very high rates of energy turnover: when the currency of energy ATP (Adenosine Triphosphate) has to be broken down into its very smallest form (inorganic phosphate and ammonia are the end result. Ammonia has been shown to be related to muscle fatigue, and in fact, is present when muscle glycogen stores are lowered2. Zone 4 work would use a lot of muscle glycogen, but since an athlete is able to store 400 to 600g of glycogen, it is unlikely that an hour of exercise at this intensity would lower these stores sufficiently to explain the ammonia presence. Recently, scientists are exploring if ammonia acts as a signal to the brain, inducing what is being called ‘central fatigue’3.

How do repeated exercise sessions in zone 4 make me fitter?

We’ve detailed what happens when you exercise in zone 4, now let us consider how your body will adapt if you accumulate exercise sessions at this intensity i.e. train. Adaptation to this type of exercise will come in the form of more muscle enzymes (to enable the fast rates of glycolysis); more mitochondria (to oxidise the formed lactate); more capillaries (to deliver oxygen to the muscle to help reduce lactate production and to prevent ATP breakdown to as far as ammonia; and also to remove the lactate / ammonia from the muscle bed); interconversion of your fast twitch fibres to slow twitch (helping oxidation of lactate).

It is not surprising then that sport scientists have tried to understand more about how lactate is transported around the body (after it is produced in the muscle) and eventually cleared to allow exercise to continue. In the early 1980s, physiologists began to question the involvement of lactate itself in causing fatigue. During this time, it was discovered that lactate was actively carried in and out of the muscle cell by a family of transporters4. A whole ‘family’ has now been discovered: some lactate transporters being responsible for carrying lactate out of the muscle cell (MCT4); and others working in the opposite direction (MCT1). Training studies, operating right in the middle of zone 4 at 75% of VO2max, have shown (over 50%) increases in these transporters5.

Zone 4 work will also force the body to produce buffers in the muscle – this will offset the acidity that comes with lactate production and the associated hydrogen ions. When muscle produces lactic acid, this then moves into the blood – becoming lactate and hydrogen ions. Many research experiments have focused on what effects this hydrogen ion formation brings to the muscle, finding that it:

  • Impairs the movement of calcium ions, which are responsible for signalling in the muscle contraction process
  • Inhibits a major enzyme controlling glycogen breakdown (rather a mouthful, phosphofructokinase!)
  • Affects cross bridges being formed in the muscle, in particular, preventing the activity of myosin ATPase, the enzyme responsible for this process.

Zone_4_bufferingBeing able to maintain muscle contraction under high levels of acidity is therefore an important characteristic of high level endurance performers. In the blood, the body’s natural buffer systems are phosphate and bicarbonate. Inside the cell, the buffers are also mainly phosphate and bicarbonate, although some proteins and amino acids can also serve in this role. Cross-sectional studies have shown that rowers who perform high-intensity interval training and race at very high intensities have higher muscle buffering values than do sedentary individuals or marathon runners6. More convincing evidence for cyclists comes from the study of Weston and colleagues7 who demonstrated not only that training at 80% VO2max for 4 weeks increased the muscle buffer capacity by 16%, but also that this change was highly related to the improvement in 40km time trial ability.

The downside of zone 4 training

An obvious question comes after reading the virtues of zone 4 training – why not do more of it, and all year around? Simply, because zone 4 training is exhausting! If training volume is high in zone 4, the athlete is at risk of accumulated glycogen depletion, which in turn will adversely affect the body’s immune system. It is incredibly potent, but must be used as part of a balanced training programme. Furthermore, while prolonged aerobic exercise conducted at intensities below zone 4 type intensities lead to a moderate rise of sympathetic activity, workloads exceeding this threshold are characterised by a disproportionate increase in the levels of the catecholamine hormones8. Therefore, the frequency of training sessions with higher lactate metabolism demands should be carefully limited in order to prevent the overtraining syndrome.

When should I use zone 4?

There are two times in the periodised year that athletes would be advised to incorporate zone 4 training into their schedule:

  • When building towards the season, for sustained race pace development
  • During the season, to maintain race pace efficiency

What zone 4 sessions are the best?

Having built a good quality endurance base, the athlete can look to build the duration of time in zone 4. Like with zone 3 work, this can be achieved at first by placing zone 4 blocks in a longer, steadier endurance session e.g. a 2 hour ride in zone 2; including 4 x 10 minutes in zone 4. This is ideal when first introducing zone 4 work. As the athlete becomes more accustomed to working at this pace, whole rides in zone 4 can be placed in the training week – the following sessions can be used during the race season:

  • Redline sessions – the athlete works above and below the maximal lactate steady state (or hour pace); building from 30 mins to one hour total. A session might use alternating 2 – 5 minute blocks, for example, at 10W below and 10W above the average power being used to represent hour pace.
  • Hour of Power – in this session, popularised by the same coaches who advocate the use of FTP as the orchestrator for the entire training zone system,  you exercise at your hour pace for again, 30 to 60 minutes. Every 2 minutes though, you change to a very small gear for 10 to 15s thus lightening the workload momentarily: enough to offload the muscular and central systems.
  • Tabata intervals9 – this session gives the cyclist a ‘double hit’ of training stimuli! It involves a frequent switch between maximal effort, and complete rest every 10 to 15s – at first glance, this may not sound like a zone 4 session (indeed, you will see it mentioned again in the factsheet on ‘zone 7’). However, once you have performed the session and look back at your training data, the average responses (heart rate, power) will be very similar to a zone 4 ride.

There are many variations on the above themes. One common strand is how the intensity is varied, taking the athlete above, and below the average intensity of your current MLSS. Why does this work? This allows the systems that clear metabolites (such as the lactate, H+, NH4) to be challenged in a stepwise manner – think of it like turning on the tap of your bath: adding those into the system (your bath tub). In order to keep the water at the same level, your body has to adapt and produce a bigger drain!


Message to take home:

You cannot race without zone 4 training. It is the best intensity to use to build your race pace efficiency and your use of the glycogen fuelling processes. It is the most common ‘limiter’ of endurance performance, not only for performances lasting one hour (e.g. 25 mile time trialling) but also for efforts that require you to repeat hard / recovery efforts (e.g. road racing,  criteriums, track). Three abilities are developed by zone 4 work – the ability to burn glycogen (the tap), the ability to tolerate it (the size of the bath), and the ability to match production with clearance. Use it wisely though, always focusing on nutritional strategies to replenish your glycogen stores post exercise.


1.   Baron et al. Int J Sports Med 2003, 24, 582-587.

2.   Broberg & Sahlin. J Appl Physiol 1988, 65, 2475-2477.

3.   Brouns et al. Int J Sports Med 1990, 11 Suppl 2, S78-84.

4.   Halestrap & Price. Biochem J 1999, 343 Pt 2, 281-299.

5.   Dubouchaud et al. Am J Physiol Endocrinol Metab 2000, 278, E571-579.

6.   Parkhouse et al. J Appl Physiol 1985, 58, 14-17.

7.   Weston et al. Eur J Appl Physiol Occup Physiol 1997, 75, 7-13.

8.   Urhausen et al. Sports Med 1995, 20, 251-276.

9.   Tabata et al. Med Sci Sports Exerc 1996, 28, 1327-1330.