Limiting Factors to Endurance at Different Intensities

I'd be interested if one of our resident experts (eg 2LAP, Ricstern) could summarise what limits the length of time a rider can operate at different intensities, eg

All-out (~10 sec) sprint: ? Muscle creatine reserves

MAP: ? Lactate buildup, ATP depletion or muscle glycogen reserves

TT Power (or "MLSS"?): Lactate buildup?

between LT Power (using the 1 mmol increase definition) and TT Power: ?

below LT Power: ?


assuming for the lower intensities that carbohydrates can be ingested at reasonable rates consistent with the nature of cycling at these speeds.

Also, after reaching the endurance limit at each intensity, how long is needed to recover enough to repeat a similar effort?

I've used (I think) Ric's definitions of intensity, where "TT Power" is some sort of maximal 1hr effort and LT Power is a relatively small increase in lactate from baseline levels.

I'll start with this one, please feel free to add to/argue...

A maximal and explosive sprint. Peak power output is normaly acheived after only a few seconds after which power really drops off. 10 seconds should be seen as a maximum duration.

Its duration (time) limited by muscle creatine stores and neurological factors mainly.

The rate of creatine depletion is directly related to the intensity.

Training tends to increase the amount of work you can do during this effort (i.e. your peak power output/how far you travel in 10 seconds) rather than the duration it can be maintained.

At least 2 mins 30 seconds of complete rest. Longer if the person is active. Creatine is resynthesised using energy from the aeobic energy system. So people who are aerobicaly fitter may recover slightly faster at rest or a lot quicker when exercising relative to someone who is less fit but exercising at the same rate. Creatine supplementation may improve recovery, but the benefit from this depends upon your event (i.e. not TTs or RRs)!

Exercise at this intensity could be maintained for very long periods... how long can you walk for non stop?

Limited by glycogen depletion and dehydration in the short term.

Other physiological factors are not very well known, but may include...
Neural factors.
Muscular localised muscluar fatigue.
Cardiac fatigue.
Central fatigue.
Etc.

Most likely to be limited by boredom, need to sleep, overuse injury, local discomfort (i.e. hands, feet, saddle), etc.

People have just completed the 1200+ km PBP in france many taking in excess of 60 hours (more or less non stop) with very little (some cases <4 hours) sleep.

Depends on the effort, potentialy glycogen depletion and dehydration could be overcome during a single night. Metabolicaly and with good nutrition/hydration riders should pretty much be able to ride day in day out; however many other factors would prevent this!

Maximal Aerobic Power duration 4 to 10 minutes depending upon motivation and training status.

Limited by lactate build up, neural factors and local muscular fatigue. Lactate accumulates rapidly as this is above LT and MLSS.

Not limited by glycogen stores, because the effort is short.

ATP depletion doesn't occur and would result in death, rather than depletion the rate of ATP supply/resynthesis limits work rate (but not time).

MAP occurs at VO2 max. Training improves VO2max. VO2max describes the aerobic rate of ATP resynthesis and so limits the work rate. Improving VO2max would improve the MAP power.

Important in all endurance events, but particularly those less than 10 minutes!

10 - 30+ minutes depending upon the level of fatigue experianced.

Lactate has a half life of 15 minutes, so 1+ hour would be need to clear *all* of it. Also some people build up more lactate during an effort and some people are able to tolerate it more.

This is a good question and 2lap seems to be very knowledgeable on the subject. In only 11 months of cycling I have gone from ground zero to being able to ride 40k in 56 minutes. Now, here is my related question: what does it take to ride 100 miles in less than 4 hours? I would like to be able to do that 1 year from now. What are the limiters? Kevin

Great answers so far 2LAP, thanks. I'll be interested to hear what the limiting factors are for exercise above LT up to "TT Power", since these are probably the zones where most of us ride most of the time.

Here is where it gets confusing...

Remember that individuals and training status are all different. Motivation plays a big part too...

Between an all out sprint and MAP there is a range of intensities. For example my peak power is 1200 watts and MAP 350 watts.

These intensities can be expressed as percentages of peak power or as percentages of max power. They can be maintained for between 10 minutes (e.g. at VO2 max) to 30 seconds (e.g. when going all out). For example...

At 350 watts (i.e. 100% of MAP or VO2 max) I can cycle for over 6 minutes before colapsing.

At 110% of power at VO2max for around 2 mins 30 seconds, at 120% for just over a minute, etc. (I have exact times just not with me as I type).

If you work as hard as you can (e.g. sprint maximaly) your legs will start to burn and after that muscles will start to fail to contract with enough force to maintain the power after 30 to 45 seconds.

The main thing that limit performance in these intensities, lactate accumulation (or rather associated acidosis) this prevents your muscles contracting as forcefully (i.e. you can't maintain the power).

For these efforts you make huge demands of your aerobic and anaerobic energy systems; so increasing the capacity of either would allow you to increase your endurance at these 'supra maximal efforts'.

Ergogenic aids like bicarbonate, may buffer the lactate and alow you to perform for longer.

Lactate itself doesn't cause fatigue rather changes in the muscles associated with the lactate cause fatigue.

As the fatigue is caused by metabolic sorces, strength training won't improve endurance for these efforts.

As lactate is the limiting factor (and the highest concentrations will be accumulated during a single sprint of say 1 minute) similar rest is needed as the MAP.

10 - 30+ minutes depending upon the level of fatigue experianced.

Lactate has a half life of 15 minutes, so 1+ hour would be need to clear *all* of it. Also some people build up more lactate during an effort and some people are able to tolerate it more.

Times do depend upon individual differences and motivation.

Efforts just above LT can be maintained for long periods (e.g. 4 hours) and at MLSS for upto 60 minutes.

The longer efforts (>1.5 hours) will be limited by nutrition and hydration. And all efforts at this intensity by lactate accumulation, oxygen uptake, localised muscular fatigue, discomfort (e.g. hand, saddle, etc.), psychological factors, neural fators, etc.

As you are riding at an intensity below MLSS (a constant intensity at which lactate concentrations don't increase (e.g. they are raised but have plateuxed) over time (e.g. 20 minutes)), initialy lactate won't accumulate a great deal. As you fatigue and have to work harder there will be some build up of lactate. This will result in your legs not being able to contract at an intensity needed to sustain the power.

When riding below LT, the amount of oxygen you use is directly related to your intensity. Above LT the amount of oxygen you use slowly creeps up over time until it reaches your VO2 max. At this point you have reached your maximal aerobic capacity and cannot sustain the rate of exercise aerobicaly so have to work anaerobicaly (i.e. building up lactate) or slow down. The causes of this 'creep' or slow component are not well known, but may be due to fatigue/recruitment of specific fibre populations within the muscles.

Depending upon the cause of fatigue, the efforts could be repeated the next day. Glycogen depletion and dehydration will take longest to correct. Oxygen debt will very clear quickly and lactate will take the usual amount of time.

Efforts could be between 1 hour and 4 minutes. Above MLSS lactate will accumulate reletivly quickly, the rate of this accumulation is greater the greater the intensity above MLSS.

Dehydration and nutrition will not be a problem. Unless the longer efforts (>20 mins) are performed in a hot climate.

Lactate accumulation will limit performance.

Increasing oxygen cost for the power output will limit performance. (see post above).

Similar to the MAP, 10 - 30+ minutes depending upon the level of fatigue experianced.

Lactate has a half life of 15 minutes, so 1+ hour would be need to clear *all* of it. Also some people build up more lactate during an effort and some people are able to tolerate it more.

Although psychological factors and central fatigue (related to Central Nervous System) may prevent you from doing the effort again.

TT POWER: When mentioned TT power on another thread, it has been what can be maintained for roughly an hour. Obviously the power at which you ride a TT at does depend upon its duration.

A 5 mile TT could be riden at just below MAP (my PB being 10.08 at 18 years old) while a 100 mile TT would need to be ridden at below MLSS.

TT power being a coaching rather than sport science term.

You need to make a small jump in performance so that the power you can hold for 56 minutes can be held for 4 hours!!!

You also need to think about comfort, nutrition and hydration. Training @ VO2 max, LT and TT power will get you there.

Another interesting question is... what limits the power output at each intensity rather than duration?

This is perhaps a more important question as it determines what kind of training should be done. Should we strength train for 200 mile rides, lose weight for hill climbs, do aerobic training for anaerobic events?

The differences between elite athletes and novices is not so much the length of time they can maintain an effort, rather the amount of work (or average/peak power) they perform in the same time.

Ultimate limitors for endurance exercise are comfort, injury, psychology, VO2 max, lactate (be it LT, MLSS, buffering capacity, etc), nutrition and hydration.

Hope I haven't killed this thread (or made too many mistakes!).

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looks fine to me, but after a day of reading through papers and an hour of hill climb intervals, things can start to look fuzzy!!!

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2LAP. Thanks again, you certainly haven't killed the thread - heaps of interesting information. If I can trouble you for a couple of follow-up questions ...

1. With nutrition as a limiting factor, is there likely to be an intensity above LT but below MLSS at which endurance is limited by glycogen stores being used at a faster rate than they can be replenished by eating on the bike (at which point the dreaded bonk / hunger flat hits?). Any idea how far above LT this generally happens?

2. If I understand your last point properly, MLSS effectively falls the longer a rider exercises above LT since increasing amounts of aerobic capacity are required to hold a given intensity over time? And once you reach the point where MLSS has fallen to the current intensity, lactate starts accumulating and forces you to slow down?

3. I do a 200km "challenge" ride held annually which includes lots (~3600m) of climbing. I generally find that by the last long climb, regardless of how far within the limit I've tried to take the preceding 160km, my HR tops out about 40 bpm below max and my speed falls accordingly, yet I don't have that absolutely empty feeling of a hunger flat, nor am I getting cramps etc. What might be causing this and what would be the most appropriate training to overcome it?

Thanks again for the comprehensive replies.

No, I don't think this happens, as most people should be able to meet the nutritional demands of performing at this level by eating and drinking on the bike. It would be quite simple to estimate the number of kcals you are using (particularly with power meters) and then to try to replace an amount close to this on the ride. Also if you start a ride but you are already slightly glycogen depleted, then you will be playing catch up from the start. That is why recovery is very important.

Actualy, MLSS (like MAP and LT) is a fixed work rate and defined as (off the top of my head) the power output/intensity that corresponds to no significant rise in blood lactate concentration over a long time period (for example 20 minutes). The lactate at this point may be quite high say 3 to 6 mmol for example, whereas in LT which is measured in a ramp test 1 to 2 mmol. The difference in power output about 20% at a guess.

You are correct in your point though; as you fatigue the LT, MLSS, MAP, etc. all effectivly occur at different/lower levels. (i.e. after riding for 100 miles a rider is never going to get their best LT or MAP values in a test). (i.e. you are less fit following training than you were before!). Hence lactate would start to accumulate at lower power outputs towards the end of a ride.

If you ride at a given power output for a period of time as you fatigue that power output will get closer to the maximum you can sustain. When the power that you need to produce exceeds what you are able to produce, you have to produce a smaller power output or stop. This is Fatigue!

This is a really interesting question!!!!

As your ride is long an undulating, at times you will be working close to MAP (or even over it) and at other times under LT (or even freewheeling/resting). Therefore you don't fit into any of the above categories well, but can choose some of the factors at will!!!

This is most likely to be caused by local muscular fatigue and neurological factors (hence you can't press that hard on the pedals to raise your HR) as your nutrition and hydration is OK. At the start of the ride you will be recruiting a number of motor units (i.e. groups of muscle fibres) and as these fatigue additional units will be recruited. By the end of the ride you will have few 'fresh' motor units to recruit and so have to slow down or won't be able to lift the pace. This has muscular and neural origins and means that you can't contract the muscles forcefully even when you can still produce ATP at a good rate (i.e. not glycogen depleted).

Also, your central nervous system also fatigues, which means that it doesn't provide the same number of nerve impulses that it could at the start of the ride. You can measure this change in a lab using EMG. It is caused by changes in the neurones themselves and also by hormonal changes in the body and brain.

Training for this is relativly easy...

*Increase VO2 max using short intervals.

*Increase MLSS by riding at 1 hour TT power for long intervals (e.g. 2 x 20 mins).

*Increase LT by riding for 1 to 2 hours at LT.

*Ride longer rides upto ride distance (e.g. 75% of ride distance).

*No sprint training needed (i.e. little training at powers greater than 100% MAP and no training of <10 sec sprints!).

The results of this training will be...

(1) If you maintain the same pace as before, the whole ride will be completed at a lower relative intensity (i.e. lower % of MAP or LT, etc). You will complete the ride in a much fresher state... you will fatigue fewer motor units, use more fat in preferance to glycogen, accumulate less lactate, etc.

(2) You will ride the ride at the same reletive intensity, however as you are fitter you will ride a lot faster. You will feel as bad at the end, but will enjoy having received a new PB or beating some of the other riders!!!

(3) A mixture of 1 and 2. You ride a little faster the whole ride, and feel a little fresher at the end.

All this training needs to be built into some kind of periodised plan (i.e. you can't do MAP/VO2 max sessions all year!).