40/20s are a type of interval session, commonly used in cycling training. 40 seconds of hard effort (130% of functional threshold power) are followed by 20 seconds of rest or very easy riding (active recovery). Typically 40/20s involve 4x5 minutes of intervals with 10 minutes between each set. This session has a reputation for being tough so it’s important to consider all aspects of fueling — before, during and after.