12 Jun Threshold

The LT/VT threshold refers to the exercise intensity corresponding to the increase in blood lactate above resting levels (LT) and the associated change in gas exchange (VT). Both of these variables are powerful predictors of performance. 

Why is this ‘point’ important?

It is at this ‘point’ that if exercise goes above the LT, there is a non-linear increase in metabolic, respiratory and perceptual stress, which results in rapid fatigue due to changes at a muscular level. Therefore, an improvement in LT/VT with training is a clear marker for enhanced endurance capacity – you can work harder before you reach this point. 

This threshold is typically found anywhere between 50 and 80 percent of an individual’s VO2 max. Lots of studies have hypothesized that the LT represents the optimal intensity for improvement of endurance fitness. Training at LT should provide a high quality aerobic training stimulus without the accumulation of lactate, which would compromise training duration. Generally speaking, training at intensities close to or slightly above the existing LT/VT is important in eliciting significant improvements in this parameter.

Endurance training results in the following adaptations within the skeletal muscle, which all in turn, help to increase the LT/VT threshold, and thus performance:

• Increases in sodium-potassium pump concentration
• Increases in lactate transport capacity and possibly myoglobin concentration
• Marked increase in the oxidative capacity of skeletal muscle – due to increase in size and number of mitochondria per unit area and an increase in the concentration of enzymes of the Krebs Cycle, and the electron transport chain. 

The last point is worth exploring in more detail. 

Figure 1. Mitochondrial density and exercise (Fitstar, 2016).

In very basic terms, following training, there is a large increase (50 – 100%) in the number of mitochondria in the muscles involved in the activity. Consequently, at the same work rate, the oxygen uptake is shared by a greater number of mitochondria, and the ADP concentration does not have to rise to the same level as before training to achieve the same rate of oxidative phosphorylation after training. The lower level of ADP after training results in less stimulation of PFK and a reduction in CHO turnover, and the greater number of mitochondria increases the capacity to use fat as fuel. This results in less lactate formation.

A third of the Classes @ Cycle Collective focus on improving a riders Threshold