Exercise Types and Crossfit

Types of Exercise
Long and slow - metabolic conditioning. Generally should be below 60% HRM. Similar to recovery workouts, but those are usually short and slow, less than 30 minutes, on the day after a hard weight or interval training.  Metabolic conditioning is at least 30 minutes, but can be quite long, depending on conditioning.  Builds aerobic (heart) and metabolic (mitochondrial density).  Do not combine with weights in same workout.  Good to use different muscles from those trained in weights. Stronger By Science article.

Weights, skill work (short and intense with long rests).  Need to rest enough between sets to bring heart rate down and to recover muscle energy. Let HR return to near-resting before each set. Weights are usually periodized so not trying to hit PRs every time. Important to stop far before exhaustion.  Missing weights for several weeks does not result in a decrease in strength (PainScience article). 

Interval training (HIIT, short intense with short rests) training at ~80% HRM. Includes sprinting. Can work close to exhaustion. May be added to weight lifting at end of workout as a "drop set". 

A Note on Crossfit
Distinguishing the public-facing sport of Crossfit from the reality in a Box can be difficult.  Crossfit from the outside, despite introducing functional fitness, looks like a very regressive and simplistic training style characterized by unimodal "short and hard" training, whereas anyone who studies exercise science knows that athletes train at different intensities, not just 100% all the time. And from the inside its clear that's what Crossfit does, with Box coaches setting up interval circuits in their gyms, etc. But all of the official workouts from the Crossfit.com homepage to the Open, to the Sanctionals to the Games look like unimodal exercise.