Running on an even, straight surface, whether it�s a road, grass or pavement, uses the same muscle groups over and over. This is not so good.
The lower limbs, the legs, are not simple pivotal joints, as these are often too simply described. The thighs, the knees and various joints in the feet and ankles have many degrees of movement. Stabilizing movement for these degrees are many muscle groups, working in synergy, which activates and stabilizes the legs and the body.
The lower limbs have more than a 100 different muscles. The muscles in the limbs perform the movement, and stabilize the joints in the most appropriate position. The muscles damp and inhibit shocks. The body cushions most of the shocks by an appropriate action of muscles, not only by the quality of the shoes or running surface. Appropriate running shoes and surface help reduce shocks, but without trained muscles, running can be extremely harmful (as you can test for yourself: wear the best shoes and run some steps with locked knees � unpleasant!).
When running on a straight and even surface, the same muscle groups are used time and again. As a result, these muscles become fatigued. Stabilization of leg joints is lessened, as is shock absorption, you will not notice it but every step at this state is harmful. Your running speed is the last thing to be influenced, because it is easy to notice. Therein lays the danger: activity continues while the muscles become fatigued and do not perform the complex action of stabilizing, working synergistically and absorbing shocks.
When we run on uneven terrain it forces us to adjust our stride to the changing surface. In some cases, steps become shorter. In others, the heel should be raised in order to go up or down an obstacle. The ankle joint should be sensitive and free so the sole of the foot will be able to make full contact with the ground, and avoid spraining.
The constant changes during field running spread the activity over many muscle groups, lessen fatigue and reduce shocks, especially to the knees.
An uneven running surface is an acquired taste, it can be dangerous if you are not gradually used to it. Even trained runners need several weeks in order to get used to field running over a changing terrain. Slopes, inclinations and rugged terrain necessitate a goal-oriented table of running that is done mostly on tough terrain.
As a rule, when going up, you should lower your speed, in order to keep the pulse within the limits of the running table. In rugged terrain you should lower your speed, because the feet are lifted higher than usual, to prevent tripping, which requires greater effort. In slopes you should keep your body straight up, do not lean forward! Speed bursts, which are possible on a straight road, should be limited in the field because of the risk of falling.Running on rugged terrain keeps the knees and ankle joints from overuse injuries.