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What Causes Our Muscles To Fatigue?

You are just 40 minutes into your regular gym session, steadily training away on the cross trainer.  Normally you love the cross trainer, you could go for hours on this machine … well perhaps one hour.  However, today your legs are burning, you feel like your heart is ready to burst and you are probably going to cough up a lung at any stage.  You taper the intensity right back, but unfortunately your legs still seem to be giving way on you.  Why are you so tired?  Are you not as fit as you thought you were?  This poses the question in your mind … what causes your muscles to fatigue on you? 

There are a range of different reasons that our muscles fatigue, and a combination of these may be the reason for your muscles fatiguing during exercise of a medium to long duration.  Traditionally carbohydrate depletion (glycogen) and dehydration have been seen as the key reasons contributing to fatigue … however there are other reasons
that may be halting your performance.


When you exercise, especially in a warmer environment, your muscles generate heat, which is carried by your blood to the surface of your skin via capillaries.  Your body needs to cool itself … so to counter this heat your sweat glands release sweat in order to cool both the surface of your skin as well as the blood underneath.  This process is an important temperature regulation mechanism.  However the down side is that with this loss of fluid through sweating your body becomes dehydrated

If you body becomes dehydrated this will affect your exercise performance.  Even mild dehydration, as little as 2% of your body weight, has been shown to impair your athletic performance. 

So what can we do about it?  As most of you will know, to maintain the optimal fluid levels, or at least to aspire towards them, we must replace the fluid loss we incur by consuming the appropriate amount of liquid before, during and after exercise.  However, water is not necessarily the best option if you are looking to rehydrate, or maintain hydration, during your exercise bouts…  A beverage containing glucose and sodium is most effective during longer exercise sessions.  Most sports drinks in store these days will contain these two agents in the most appropriate quantities for quick absorption.

So what is the take home message?  In a nut shell … keep your fluid levels up and you will decrease the risk, or lessen the impact of dehydration induced fatigue.

In warmer environments, or with more intense exercise
, overheating can also be a risk factor contributing to fatigue.  However, generally this is something that endurance trained athletes are more at risk of during exercise than the regular gym goer.  Remember Craig Barrett, the 50km walker at the Commonwealth games a few years back? … overheating, combined with severe dehydration can have an enormous effect on physical performance. 

Again, drinking correct fluids (as this aids in cooling the body) and avoiding training during the heat of the day should suffice for the most of us in reducing the risk of overheating…  The gym is normally air conditioned anyway!

Depletion of Muscle Fuels      
Along with Dehydration, depletion of muscle fuels or glycogen depletion can play a large part in fatiguing exercise performance.
  Glucose is the main fuel source for your muscles in short term intensive exercise lasting longer than 10 seconds and less than 3 minutes, and is provided by a process called anaerobic glycolysis (the breakdown of glycogen to glucose).          

Exercise performed at a lower intensity and for a longer duration is also reliant on glycogen, as well as a couple of other energy resources, such as fatty acids and amino acids.  It has been shown in various studies that depletion of glycogen can reduce performance in short, medium and longer duration exercise. 

To reduce the likelihood of fatigue due to glycogen depletion it is especially important to ensure that pre exercise glycogen levels are high
, and also that post exercise, glycogen is replaced sufficiently.

A substantial meal rich in carbohydrates (eg pasta) eaten 2-3 hours prior to longer exercise sessions, and a small carbohydrate snack eaten up to an hour before exercising should ensure that your glycogen levels are fully saturated.  After exercise, a combination of carbohydrate, protein and sufficient fluids are required at regular intervals to ensure you recover fully from your exercise. The first hour is most important in your recovery process … if you fail to re fuel you will probably feel it in your next session!       

The take home message …. Don’t go off to do your gym workout hungry, or having not eaten.  Otherwise you are bound to feel flat and tired.         

Central Fatigue           
Recent research has indicated that mental fatigue, sometimes referred to as central fatigue, can be a cause of impaired physical performance during exercise.  While this will not affect your muscles directly, as it results from impaired function of the central nervous system, it can reduce your capacity to perform.  The onset of central fatigue is seemingly related to a correlation between certain amino acid levels in the brain, and it has been shown that supplementation of branched chain amino acids can improve performance in sports and endurance events.

Take home message…  a bit more research required here … but it may not be that you are just lazy when you cut your exercise session short!  It could be due to central fatigue!  But lets not use this as an excuse to cut every session short!

Exercise Intensity
The exercise intensity that you perform at also has a large impact on the level and onset of fatigue your muscles and body feel.  If you are exercising at a level that is too intense …  and your aerobic system can not supply the required energy … you will start to work the lactic acid or anaerobic system.  The lactic acid system is capable of releasing energy without the requirement for oxygen.  However, if you are exercising at a level which is above your lactate threshold (level at which you begin to start working anaerobically) … your exercise performance will become limited by the lactate build up and the associated acidity of your muscles (due to increased Hydrogen Ion levels in the muscles). 

Sounds a bit complicated I know, but I am sure you have all experienced the ‘burn’ in your muscles whether from continuous sprinting during a game of soccer, rugby or some other pursuit.

Make sure that you are exercising at the appropriate intensity for your current fitness level.  Always warm up slowly before undertaking any intensive exercise.  If you are unsure about the intensity you should be performing interval or tempo training consult your fitness instructor, or have them perform a fitness test on you.


Lose The Beer Belly

Yikes, you have just looked at the calendar and it is August already ... only a few months until summer kicks in.  Where did I put my Speedos?  Where did this beer belly come from?

Have you been doing nothing over the winter period?  Or perhaps you training just isn't working?  Do you want to lose the beer belly so you can get back in your togs over summer?  The time to start is definitely now!

Remember ... spot weight reduction doesn't work ... (ie you can't just lose weight around your stomach), and no matter how many sit ups you do no ones going to see a six pack until you lose the weight from the stomach region!

So what to do then?

1.  Increase your cardio
The first place to start is to look at your cardio.  If you are doing nothing lets try top start off with 3-4 visits a week to the gym for 30 minutes or so of continuous  exercise, exercise that maintains your heart rate at an elevated level for a continuous period.  Get started burning those calories!  If you are looking for a gym to attend click here to search for one in your area.  To view some of the profiled gyms click here.

2.  Mix up intensity and duration
Have you been going to the gym religiously, 4 days a week, doing the same session day after day... but still can't get the results that you desire?  It is time to mix things up ... break out of the habit ... if you keep doing things the same you will get the same result.  Try playing with the intensity and duration of your sessions.  Perhaps one longer session per week of over an hour.  Or include an interval session on the bike where you do high intensity bursts of5 times  5 minutes.  If you are lost for ideas... it may be time to talk to your gym staff, or even a personal trainer for some motivation.  Click here to search the personal trainers listed in your area.

3.  Take it down
One of the most important things that you can do is to write down the training that you are doing.  Often it feels as if you are training hard and regularly, then when you look at your completed workouts you have had two good weeks, a week and a half off, one good week, another slack one ... you get the idea! 

Write down your training ... what you aspire to do, and what you actually do ... then you can analyse where you may be falling down ... you will also find that it motivates you to train more regularly!

Good luck with your weight loss training ... and roll on Summer!