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How Do I Stop The Burning Pain In My Legs When I Train


Have you come to triathlon from team sports ? Are you naturally fast, but cannot maintain a good pace over race distance? Do you legs always start to burn after a few minutes training or racing? Read on...

At our lake last summer (www.openwaterswim.co.uk) I was asked the following question by a rugby player who swims with us regularly.

Question: I'm new to triathlon but have played rugby for a few years. I am really strong for the first 10 minutes of any training session, but then my legs burn up and I slow down dramatically. I have a busy life style and only train about 4-5 hours a week. Can I stop this happening?

Answer: Team games are stop and go sports. In rugby, the maximum distance you will run in one go is anything up to 100m sprint, usually a lot shorter. The sport involves powerful bursts of energy, interspersed with longish recovery periods. A professional rugby player is estimated to cover about 8km in 80 minutes, of this only few minutes will be all out sprinting, the rest will be jogging, walking or slower pace running. On top of this all rugby games have 10-15mins rest at half-time.

Playing rugby, the body has become accustomed to running very fast over short distances, with longish recovery periods. You must now teach your muscles to sustain a slower work rate, but maintain their power output for longer periods of time.

Triathlon is different from team sports. A typical Olympic distance event, will take most age group triathletes over 2 hours. Thus, to do well in competitions, your body must perform at the limit of its ability for over two hours.

A basic knowledge of the different energy systems should help you to understand, why your "muscle burn" is occurring.

The quickest form of energy is called adenosine triphoshate, (better known as ATP). It is a high energy compound, providing energy for muscle contraction. ATP cannot be obtained from the blood or other tissues for that matter, which means it is rapidly used up, and can only power the muscles for a few seconds at a time. Since ATP continuously needs to be resynthesised, it is only really good for immediate energy.

For more prolonged activity our bodies must burn off the food we eat to provide energy. Typically fats are preferred for energy at slower work rates and carbohydrates at higher effort intensities. The rate that fats and carbohydrates are converted into energy is measured by the rate of oxygen consumption. Thus if someone has large efficient lungs powered by a big strong heart, he will be capable of consuming high volumes of oxygen. Having a large oxygen utilisation capability (often expressed as VO2max) is no good if your muscles are not trained to use oxygen efficiently.

In a triathlon 99% of the energy supplied to the muscles is transported by oxygen (aerobically). Consequently it is vital that you develop your aerobic system to the maximum.

The other main energy source comes from the breakdown of glycogen. But the by-product of this is lactic acid. If all the energy for exercise is provided aerobically, the muscles do not produce lactic acid. However, above a certain threshold of intensity, commonly called the anaerobic threshold (AT), the pace of exercise uses up more energy than can be supplied aerobically. The deficit is supplied by glycogen and results in the accumulation of lactic acid.

When you are sprinting on the football field, and the heart and lungs temporarily can not meet your muscles energy demands aerobically, the deficit can be made up from the break down of glycogen. Thus the body has a temporary method of keeping up a high work rate even when it is in oxygen debt.

All debts must be repaid in full

When we use glycogen for energy (anaerobically), we pay for it with a build up of lactic acid in the muscles. When this happens, it inhibits the provision of energy aerobically. It is very important to understand this.

Triathletes must attempt to maximize oxygen utilization and minimize lactic acid build up. Footballers on the other hand, need maximum bursts of energy for short periods of time and so anaerobic energy is a vital top up for them when the aerobic system can not keep up with demand. But footballers unlike triathletes have a chance to recover within a game, and thus allow some of the lactic acid to dissipate.

Going back to your original question, I would suggest that your anaerobic system is highly developed, allowing you to make many short fast sprints and tolerate large quantities of lactic acid build up.

Since you find it easy to sprint, it is only natural that you set off in training at too fast a pace, even though it may be slower than your sprinting speed on the rugby field. The lactic acid build up in your muscles, is ultimately your undoing, and prevents the transportation of oxygen. As you struggle on, this leads to the burning sensation you describe in your bike training, and then results in your dropping back into the pack.

The heart is the mechanical pump for the energy system, and your heart rate can be used to determine how hard you are working and whether or not you are working aerobically or anaerobically. If you do not have a heart rate monitor (HRM), I suggest buying one as they are very good tools to aid your training. However, you should never rely totally on the HRM. Listen to how your body feels, and monitor at what speeds the burning sensation starts. Although more difficult to do initially, listening to you body while training will make you a better athlete than if you just rely on a heart rate monitor, to fix your training and race pace.

Research as has proved that, by training at, or close to your anaerobic threshold (AT), you will become more efficient, and able to hold a faster pace without building up lactic acid. If you also work close to the limit of your maximum rate of oxygen consumption (VO2MAX), you will develop your heart and lungs, i.e. make the system bigger, so that you are able to pump more oxygen to the muscles.

In order to make your aerobic system bigger and more efficient, it is important to have a rough idea of your AT.

There are various ways recommended to calculate this. It is not necessary to go to great expense and have blood samples taken while training at different intensities. A great starting point is training at 80-90 % of your maximum heart rate. Unless advised otherwise by a physician, you may want to run to exhaustion, to find out your maximum heart rate. A much easier, but less accurate way, is to subtract your age from 220. To develop a tolerance to lactate build up in the muscles, a 40 year old with a theoretical max of 180, should train between 144 and 162 HR. Over time and with training, you should find that you are able to hold a slightly higher heart rate, and race faster, before that burning sensation sets in.

To simplify the advice, my suggestion is for you to raise your lactic threshold and try to improve your VO2 max. This means interval training for about 30 mins a week in each discipline, with short rest periods. Long rest times means more recovery time, lactate levels will drop and oxygen consumption rates will be lower.

Suggested Training:

Swim: 15x 100m with 1/3 rest, at 1,500m race pace. eg: if you swim 1,500m in 30minutes aim to do each 100m in 2minutes, take 40 seconds rest between each interval (Note: It is difficult to monitor HR while swimming)

Bike and Run: 6x5min @ 80-90% Max HR. Max rest 4 mins. Min rest 2 mins

Note: Start at about 4 mins rest and try to reduce it by 30 seconds each week, until 2 mins is reached.

As you train for only 4-5 hours a week, it is important that your AT sessions do not go over 1/3 of your total training time. If you do the three sessions that I have outlined you will be just under 1hour 30 of hard training, which will be ok so long as the rest of your training is easier aerobic work,(around 60-70% max HR). If the percentage of hard training is too high you will be susceptible to injury or over training.


These are sample sessions, even if you vary them the principles are the same.

1. Try to do 3 AT sessions a week.
2. Keep AT work to 1/3 of total training time
3. Max HR attained during the hard efforts should be no more than 90% of max HR.
4. Aim to get rest times down to about 30% of hard interval training times.

Why not go beyond the article and have us train you 1-to-1 to meet and exceed your running and triathlon goals - contact us.