Everything we do requires energy — from shaking someone's hand to mowing the lawn to changing the channel. Even when no physical activity is present, there are a number of metabolic processes occurring that need energy to continue. The energy pathways within the body that enable any type of energy-requiring work often release a number of undesirable by-products as they take place. One of these by-products is lactic acid, and the lactic threshold refers to the point at which this chemical begins to rapidly increase in the bloodstream.
Ordinarily, a state of equilibrium exists in the body in which the amount of lactic acid produced is equal to the amount being removed. During moderate activity, this equilibrium is able to be maintained; however, when more intensive physical activity is in the works, an imbalance may occur. This imbalance is primarily due to the vast amount of energy needed to perform certain physical tasks.
Energy cannot come from just anywhere; the body must produce it utilizing one of its energetic pathways. These pathways can provide more endurance-based energy, as in the case of a person running a marathon, or shorter-term energy boosts, for example, a powerlifter attempting a new maximum weight. The amount of lactic acid produced and other possible by-products is dependent upon which pathways are utilized and varies according to exercise demands.
The human body is a system of checks and balances working together to maintain an equilibrium most desirable for well-being. The lactic threshold represents the body being out of sorts with this equilibrium. Normally, lactic acid is removed at roughly the same rate as it is produced, allowing for a balance. With extreme exercise, the body must create energy at a faster-than-standard rate, causing a spike in lactic acid so dramatic that the body is unable to remove it from the bloodstream quickly enough to maintain desirable levels — this is the onset of a lactic threshold.
Those who are athletes or have experienced intensive exercise have also likely experienced the lactic threshold. The lactic threshold is characterized by a burning sensation deep within the muscle fibers. There is evidence that this burning is not actually caused by lactic acid itself but rather other acidic by-products; however, the medical community mostly agrees that this sensation is the initial onset of a lactic threshold.