Regardless of an individual’s background most people will have a dominant or stronger side of the body. As this side is more dominant, strength, co-ordination and muscle size may vary from the opposite limb. Most exercise programmes related to strength development contain bilateral (both sides of the body) movements such as the Bench Press, Squats and Deadlifts.
If one side of the body is stronger and effectively “out of balance” compared to the opposite then this may accentuate the difference from one side to the other. Typically, this may be shown in that if one leg is stronger than the other during a squat movement and becomes more dominant during the movement, shifting the centre of gravity and causing rotation at or around the pelvis which may place stress on the joints further up or down the body. The negative impact of these imbalances may be highlighted typically in movement flaws and poor technique of bilateral movements. Ultimately these poor movements result in tight overactive muscles and weak underactive muscles with the worst case scenario being injury.
Unilateral training focuses specifically on the opposite sides of the body. The aim of these types of movements is to highlight any potential strength imbalance from side to side and help to level these imbalances out by loading the body equally. The advantages of this type of training protocol can help develop core and postural strength as the body has to stabilize itself against rotation around its centre of gravity through movement. Consider the stabilization the torso musculature has to provide to prevent torso rotation during a single arm row or how much instability there is in a single leg squat compared to a bilateral squat where rotational forces can be stabilized on the opposite leg.
Bi-lateral movements have a greater advantage in that additional load can be placed on the body so for force development they are a vital part of a conditioning programme. This has led to a rather blanket assertion that they are the be all and end all of training. However, this is a limited view point in that uni-lateral movements are just as important as they help address strength imbalances from side to side and can refine technique and issues with range of motion and mobility.
Uni-lateral exercises are challenging in their own right as they demand a greater amount of segmental muscle control and integration of muscular movement compared to bi-lateral movements. A balanced training programme aims to build postural balance between all the muscles of the body. Posture dictates function; therefore function dictates dysfunction- if posture is not correct and there is a strength deficit from one side to the other then dysfunction and injury may occur. Bi-lateral exercises do have their own place in resistance training hierarchy if performed correctly at the right time, as does unilateral training.
Individuals who respond well to including uni-lateral exercises in to their training include those who play in sports that are one side dominant such as kicking or throwing. The training of single limbs is functional from an athletic development point of view as it allows an individual to develop force production in positions where we experience most movements in life as well as sporting situations. For instance, whenever we move or run we are have to balance and change direction of one leg.