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In order to attain an ambitious target, let's now take a look at the role played by your body weight.
You surely already know that there is no point in weighing yourself every 5 minutes! You should weigh yourself when you get up in the morning, before breakfast and after having urinated. This should be done several mornings in succession in order to get a reliable average reading.
What's more, weighing yourself just after a run is not advisable:you will probably not want the unexpected surprise of a weight gain when you have rehydrated yourself! Indeed, a run can result in a moderate loss of water of between 0.5 and 1 L per hour, whereas this fluid loss can exceed 2 L depending on the intensity of effort and the external temperature.
Another factor that can result in a temporary weight loss: energy consumption.
Your body draws on its glycogen reserves during any exercise that requires endurance. This results in a slight loss of weight that is equally temporary because the weight is regained as soon as your reserves are built up again…
The well-known body mass index (BMI) can give you an initial indication for estimating your fitness weight. Using a calculation that combines your height and your weight, you can work out whether your body shape matches the standard or if you are overweight.
The calculation is as follows: weight in kg/(height in metres squared).Example:72/(1.81x1.81)= 21.98. If, as is the case here, the result lies between 18.5 and 25, your BMI is considered to be "normal".
If it exceeds 25, you are overweight and, if it is below 18.5, you are underweight.
If you are new to running, this measure can be helpful to motivate you to train and get back in physical shape. This being said, in order to obtain a more reliable and precise measurement, you need to know the proportion of fat and muscle mass of your body by consulting a specialist.
When running, surplus weight (fat mass to be more precise) can have several negative repercussions on the body: increase in heart rate and hence oxygen consumption, increase in body temperature during exercise, increase in energy expenditure as well as an increase in the pressure exerted on the bones and joints.
Weight is a key factor in defining the oxygen consumption of sportsmen and women, particularly when it comes to identifying the threshold at which the maximum volume of oxygen consumed is attained (VO²max). For a runner, this maximum value is associated with a specific speed: the Maximum Aerobic Speed. Developing the MAS in training therefore leads to an increase of the running speed at which your volume of oxygen is maximised.Reducing fat mass reduces your need for oxygen and reduces the energy expenditure associated with your running stride, which therefore leads to an improvement in this speed.
Indeed, an American survey by the National Runner's Health Study on 4700 users showed that a 2 kg change in weight can lead to a difference of almost 1 minute on a 10 km run!
Finding a balance, in other words, your fitness weight, therefore contributes to optimising your running capacity and preserves your body's energy levels when exercising. The fitness weight should not be too close to the minimum recommended level for your fat mass ratio. This is the case for the "racing weight" reserved for high-level athletes whose medical condition is monitored regularly.
However, take care not to become underweight! Excessively reducing fat reserves can have a negative effect at the end of a long race because the body taps into this energy reserve when your glycogen reserves have been exhausted? It is therefore advisable not to fall below 10% of fat mass as a proportion of total weight for men and 15% for women.
For all amateur runners, the genuine fitness weight must depend on a general feeling of well-being on a daily basis and during your exercise sessions!
Whereas, in certain sports, gaining weight is a key part of increasing power and body protection, in running, a reasonable loss of weight can improve your potential. It up to you to find the correct middle ground where you feel good when running!
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