The combination of normal forward running and backward running is called mixed running or alternative mixed running. Some believe that running backwards helps balance out the strain brought on by normal running. Reversing the direction works the friction of tissues oppositely . Running flat or uphill, the heel is used to push off rather than the ball of the foot as normally occurs with forward running, working the tibialis anterior muscle (pushes the heel down, raises the front of the foot) more as a prime mover than a shock absorber. When running backward downhill, the ball of the foot is used whereas forward downhill running uses mainly the heel to absorb the force. This requires more coordination and therefore develops brain power along with muscle power. While downhill backward running is essentially the reverse of uphill running, and uphill backward running of downhill running, they are different in that the fibres would fire differently due to differences in the isotonic motions. The former is an eccentric version of a concentric movement, and the latter is a concentric version of an eccentric movement. Both concentric and eccentric movements have advantages in training, which is why most weight lifters perform both for set times. With all forms there is an obvious backward lean relative to normal running's forward lean, which can shift the stress of the other muscle fibres a small degree. Other advantages to backward running are a reduction in fear related to the movement, a form of exercise that is naturally more reserved, gains in balance, and the general enjoyment resulting from entertaining activities like these. Due to constantly having to look behind oneself, or sometimes keeping the eyes fixed, people can learn to run with more neck mobility or without a straight look ahead. This can stress the neck muscles which can be dangerous if done too aggressively, but in the long term could lead to adaptations in them. Backward running adds another dimension to running, and when complemented by sidestepping in both directions (with and without crossovers front and behind intermittently) covers the essential dimensions of human movement on the two-dimensional plane. Diagonal movement as well as curving running (as is done on long race tracks like 200 m and longer in the Olympics) are additional forms of running. Backwards running allows referees in sports such as football or rugby to continuously observe an area of play without