High-density exercise involves either high intensity
for short periods or moderate intensity for longer periods.
A very high intensity exercise for a short interval
(e.g., a sprint or a low-repetition, very-high-weight weightlifting
exercise) is high density.
A moderate-intensity exercise for a moderately
long interval (e.g. moderate-weight weightlifting exercise repeated
to failure or nearly to failure) is also high density.
A low-intensity exercise for a very long interval
is simply an endurance exercise.
The classic high-density workout is the 8x8 routine.
The participant uses a weight approximately half of his or her
one-rep maximum, doing 8 sets of 8 repetitions each, with rest
intervals of 15 to 30 seconds between sets. The routine targets
slow-twitch and fast-twitch A muscle fibers.