There is the tendency for new athletes to avoid what we call “heavy days.” They either avoid them entirely or execute them incorrectly.
What they fail to recognize is that cross training is a strength and conditioning program. While people sometimes characterize CrossFit by its programming of mixed-modal workouts for time (“metcons”), this is a limited view. Days devoted to strength training are essential to CrossFit, cross training, and any other discipline focuses on forging a broad fitness. Therefore, heavy days are integral to the prescription of constantly varied workouts.
For example, heavy days are necessary to build top-end strength and power. Power output decreases with time, meaning that an athlete’s work capacity in very short time domains sets the theoretical limit for his or her entire curve (see image below). It is possible to have high levels of short-duration power and little power elsewhere (e.g., a powerlifter), but it is impossible to have low levels of short-duration power and higher levels of longer-duration power. Therefore, heavy days are essential to a general physical preparedness program and should be used at least once a week or once every two cycles (where a cycle is three on, one off).
Image and article taken from here.
Heavy days are not the only time athletes drive strength adaptations. Even within a metabolic-conditioning workout, depending on the task and the capacity of the athlete, any number of exercises may build strength. Push-ups for novices build pressing strength similar to a bench press, and attempting a 95-lb. thruster for a new CrossFit athlete builds squatting strength. As an athlete’s strength increases, push-ups and 95-lb. thrusters tend to favor other adaptations such as stamina, and greater loads are necessary to further increase top-end power.
Heavy days can be completed with most any weightlifting or gymnastics movement, such as weighted dips and pull-ups and lifting odd objects (e.g., sandbags, axles). More often than not, however, a barbell is best because it is impossible to match the barbell’s ease and range of loading with other equipment. Heavy days may also include all variations of standard barbell movements (e.g., hang, power, from a deficit, pulling from pins/bumpers).
Tyler