The athlete has a sturdy body.
However, we can find difference in the muscle between a marathon runner and a 100m sprinter.
This is because they usually do the different type of training.
The 100m splinter, besides the running, uses the weights to apply big load to the muscles for the training.
To see how the muscle changes by weight training, five males who usually do not train much are asked to do weight training using the dumbbell.
The training was to extend only one arm with the dumbbell and the other arm did nothing.
After four months of training, difference was seen in muscle thickness of the right and left arms.
The photograph below is a cross sectional images of the arms taken by the MRI (the figures cut into round slices).
The cross-section images are taken every 1cm from the shoulder to the elbow.
The bone is seen in the center of the oval shaped arm.
The muscle above the bone, triceps brachii which works to extend the elbow, became thicker after the training.
The cross section area of the muscle was measured from end to end and the muscle volume was calculated.
The volume increased 30% in the average of all five males.
The lower photograph is a vertical section image of the triceps brachii muscles which was taken along the bone using a supersonic wave.
The muscle is made of slim long cells called the muscular fibers and in the case of triceps brachii muscule fibers forms diagonal lines.
The upper photo is a person wo does not train very much and the lower is that of a bodybuilders who has thick muscles. The diagonal lines of the body-builder's has greater inclination than that of the untrained person.
The muscle is the source of every physical movoment not only sports activity.
The well developed muscle of the athlete shows intensity of their exercise.
On the contrary, the muscle of a bedridden person shrinks and it may becomes difficult even to stand up.
The muscle clearly appears daily life and exercise intensity of a person.