Which is harder to do? 100 straight squats and 100 straight push ups, or 10 sets of 10 of each alternated with each other? The answer depends on how you look at the question. For pure muscular strength endurance it’s harder to do the reps straight. However for cardiovascular and general endurance (inclusive of some muscular strength endurance with higher heart and lung function). It’s harder to do the 10 sets of 10 alternated. Why?
Because the act of changing from one exercise to the other, changing direction and momentum requires more cardiovascular endurance. Also you force the heart and lungs to work harder by forcing them to pump blood and oxygen to different parts of the body alternately creating greater over all circulation. You also take advantage of the fact that from set to set, while your muscles are being worked they are still fresher and can move at a faster pace with cleaner reps and more speed. This is why conditioning routines like pyramids, circuits, intervals and things like the deck of cards routine are so effective in building endurance.
Your breathing and cardio goes through the roof and over the course of the entire workout, you’ve done enough volume to stimulate muscular endurance and hormonal and chemical change in the body. As a wonderful side benefit, you don’t effect your max strength as much as straight rep or long slow style conditioning workouts do. Because you can use faster pacing and in effect bursts of muscular effort, you’re still teaching the body to operate at a high level of strength, but now blended with endurance together.
Take this into account in your workouts. By all means, when you’re building a base of muscular and cardio endurance it’s okay and appropriate to do straight rep sets. Everybody should go though them at some point in their training career to build their benefits and build that muscular memory into your strength and endurance, but most of the time your endurance training should be on some type of an interval. The most bang for your buck, and the most change in your life. - Bud Jeffries