Trainees often do three or four sets of an exercise. With the 4X method you take your 15-rep max but do only 10. Rest 35 seconds, and do 10 more—and so on for four sets, going to failure on the last. It’s a great “density” method for increasing muscle size via sarcoplasmic expansion.
A good variation is to go to failure on set 3, then use set 4 as a unique mass stimulus. For example, after your 35-second rest between sets 3 and 4, reduce the poundage by about 20 percent and do an X-centric set on your last one.
X-centric training is raising the weight in one second and lowering in six. Shoot for seven reps, which will give you 49 seconds of tension time. Plus, the slow eccentric stroke on each rep stresses the myofibrillar strands, creating microtears that can trigger fiber remodeling for new growth.
Another variation is X-celeration. You can use the same weight as on the previous three sets or lighten it. Either way you rep out with fast-but-controlled reps. Fire out 1.5-second reps, and get as many as you can. This “speed” method can help activate dormant fast-twitch fibers and/or at least enhance muscle pump.