Few people have perfectly straight legs. Most, when viewed from the front or back, will have knees that go slightly inward (knock knees or “valgus” alignment) or outward (bow legs or “varus” alignment).
But footballers are more likely to be in the latter group, because of growth differences that have been shown to start from the age of 12. Take a look at the variation seen in the 3 players below:
Footballers are more likely to have bow legs.
Your knee alignment develops as you grow, and is finalised during your teenage years. Researchers have found that adolescents who engage in intensive sports training often focus too much on repetitive tasks. Those that involve kicking a ball result in asymmetric strengthening of the muscles on the inside of your legs, and overloading of one side of the knee which changes the angle of the top end of the shin bone as it grows, giving bow legged alignment.
Research studies in several countries have shown an extra 15mm gap between the knees (on average) in footballing teenagers compared to a matched group doing other sports. In a mild form this doesn’t present any handicap, and may indeed even be beneficial in giving greater stability in your standing leg as you shoot the ball, and provide better ability to turn rapidly.
But professional footballers are known to have a higher incidence of knee arthritis later in life (1.3x if no serious knee injury, 2.9x if those with injuries included) and since greater angles increase your knee cartilage overload it’s likely that bow-legged alignment is partly to blame.
Physios are now developing specific training methods to improve dynamic knee joint stability to counteract this overtraining effect. Youth team coaches should be encouraged to use these strategies (together with adjustment of training intensity), particularly in the most vulnerable age group entering the pubertal growth spurt, in order to reduce the risk of longer term consequences.