Some of the most efficient ways to exercise are weight lifting about 24 repetitions of each exercise to complete exhaustion and interval training like some kind of sprinting.
Lower weight higher repetition weight lifting (meaning about 24 reps) done to complete muscle exhaustion (lift until you can't lift a single time more) can actually build muscle even faster low repetition with a heavier weight. A study on this is here: http://www.plosone.org/article/info%3Adoi%2F10.1371%2Fjournal.pone.0012033 N. A. Burd, et al. “Low-Load High Volume Resistance Exercise Stimulates Muscle Protein Synthesis More Than High-Load Low Volume Resistance Exercise in Young Men,” PLoS ONE, vol. 5, no. 8, p. e12033, 2010.
Interval training works so well partly because you move fast turning on more fibers. Each muscle fiber in a skeletal muscle either contracts fully or not at all. You lift heavier weights or move faster by contracting/engaging more muscle fibers. So fast motions in interval training engages as many fibers as heavy lifting but also trains your heart, lungs, circulatory system, and intra-muscular glucose storage. And intervals take less time than long cardio sessions or heavy weight room workouts with lots of long breaks. Exercising efficiently for less time could also reduce injuries, less time exercises means less chance to get injured and less time to cause a repetitive stress injury while still having the positive effects of intense interval training.