Ya some do, although it seems that it is starting to turn.<p>Remember - I said sports medicine was pragmatic not correct ;)  While muscles are being used heavily, a ton of lactic acid is made faster than it can be recycled by the body. So no it&#x27;s not causing fatigue there is a high correlation between lactic acid and muscles being fatigued. Lactic threshold is still a very interesting concept, the point at which you recycle lactic acid at the same speed as it is created. What the heck lactic acid does is maybe less important to sports performance...