This article was originally published here.
1. What is the Goal?

The plank is an isometric contraction exercise that involves maintaining a position for the maximum possible time. For the exercise to be effective it is important to feel intense muscular fatigue.

These core muscle exercises strengthen muscles and increase back and abdominal muscle tone. This complementary exercise to running, just like "stretching", ensures that you:

- Save energy while you run. 

- Increase the depth of your stride. 

- Avoid compression problems or "backache". 

- Improve tonicity, essential for momentum. 

- Slims  your figure

With core muscle exercises, you use the various muscles of the torso fully (rectus abdominus, transverse muscle, obliques, lumbars and dorsal), which helps avoid backache and improve optimum stride when you run. 
2. When And How Should You Do Core Muscle Exercises?

Just like running, you should practice the various core muscle exercises or strengthening exercises regularly and progressively. How do you define regularity? You should take 10 to 20 minutes (experienced exercisers) every morning for these exercises. Initially, you don't need to exercise on the morning of a running training session. Make time for a short session before lunchtime. 

Some essential instructions for optimal efficiency of the suggested exercises:

Breathing: when performing core muscle exercises, breath as naturally as possible by sucking your belly button in towards your spine.

Posture: keep the position for 20 seconds to 1 minute. 

Stability: Make sure you contract your abdominal muscles.

Return : after your exercise, always return slowly and gently to the starting position.

It is recommended that you do sequences (3 on average) of isometric contraction exercises (static hold) from 20" to 1 minute, with recovery time of 30", 2 to 3 times a week.
3. Some Core Muscle Exercises

There are many variations, with or without accessories for toning. These are just some examples of plank positions:

elbow plank: like a plank as shown in the photo above. Keep the back very straight, and look straight ahead.Variation: with the chest facing the ceiling, on the elbows, holding the buttocks as high as possible.

V plank: stretch the arms forward, keep the back straight and knees bent as shown in the photo at the top of the article.

side plank: on the side, the bent elbow on the ground, the other hand on the hip as shown in the photo opposite.With the legs straight (or bent if it's too difficult), lift the buttocks as high as possible, looking straight ahead for a few seconds, then switch sides.

thigh plank: in the famous chair position, the back straight and pressed against a wall. Legs must form a right angle, together or apart.

Below is a 4-week training plan with the posture you want:

The 1st week, 3 sequences of 15 to 20’’ = time spent on each session of 8 minutes.

The 2nd week, 3 sequences of 25 to 30’’ = time spent on each session of 10 minutes.

The 3rd week, 4 sequences of 25 to 30’’ = time spent on each session of 12 minutes.

The 4th week, repeat twice:

The 4th week, 2x2 sequences of 25 to 30’’ = time spent on each session of 10 minutes.

Core muscle exercises are for everyone, and everyone should do them in order to respect the tensile balance. Core muscle exercises are also considered to be types of stretching and toning, but unlike stretching, there's an additional idea of tonicity.

To be practised without moderation as soon as the need is felt.