Compound vs. isolation exercises: which is best?

It’s a battle of the ages in bodybuilding: isolation vs. compound exercises. Much has been written about the benefits and downsides of either, but never has a scientific study actually compared the 2 head on in a realistic training setting. Until now. Paoli et al. (2017) compared the effectiveness of isolation and compound exercises for muscle growth, strength development and cardiorespiratory fitness (‘endurance’).

36 Soccer players embarked on their first quest for gains with one of the following programs consisting of only single-joint (SJ) or only multi-joint (MJ) exercises. The programs were 8 weeks, but to ensure they weren’t complete noobs, they also did a 4-week familiarization phase. Note that researchers often use the more objective classification of single- vs. multi-joint exercise, as virtually no exercise truly isolates 1 muscle group, but practitioners often use the isolation vs. compound exercise classification, as they don’t care how many joints are involved, only which muscles are targeted.

As you can see, the training splits were a bit weird and the groups trained somewhat different muscle groups each day. For example, leg curls were the isolation equivalent of deadlifts. Leg curls train the hamstrings in a very different manner than deadlifts and don’t train the glutes, forearms, quadriceps or back muscles. Not exactly a perfect comparison. Similarly, leg extensions were used to replace leg presses. Also, the biceps curls and triceps extensions were placed on different days than the multi-joint group trained their arms. Overall, the single-joint program neglected the glutes, erector spinae and arms. The goal with all of this was to equate total work, but since only whole-body fat-free mass (‘muscle growth’) was measured (with DXA scans), it begs the question what the growth patterns looked like.

The training programs also differed in other ways. Both groups trained to failure with 4 sets for each exercise, but the SJ group performed sets of 12-18 reps with 1.5-2 minutes rest in between sets, whereas the MJ group performed sets of 6-8 reps with 2.5-3 minutes rest between sets. The goal here was to mimic practical training settings and equate total work, but it does induce several confounders into the comparison of single-joint vs. multi-joint exercises.

Design limitations aside, let’s look at the results. To quote the researchers: “Both groups decreased body fat and increased fat free mass with no difference between them. Whilst both groups significantly increased cardiorespiratory fitness and maximal strength, the improvements in MJ group were higher than for SJ in VO₂max, bench press 1 RM, knee extension 1 RM and squat 1 RM. In conclusion, when total work volume was equated, RT programs involving MJ exercises appear to be more efficient for improving muscle strength and maximal oxygen consumption than programs involving SJ exercises, but no differences were found for body composition.”

These results are worth going into in greater detail.

Greater bench press gains in the multi-joint group were to be expected. They trained the bench press after all. Greater squat gains aren’t a surprise either. They trained the leg press, which is much closer to a squat than a leg extension. Interestingly, however, the multi-joint group also gained more strength on the leg extension, even though it was the single-joint group that directly trained this lift. This is a finding in favor of compound exercises for ‘overall strength development’ and lends credence to the saying that compound exercises are more ‘functional’ than isolation exercises. However, here’s where the odd design confounds things. The multi-joint group trained with heavier weights (%1RM) and rested longer between sets. We know both of those factors are good for strength development. Still, it’s interesting that these factors appear to be more important for strength than specificity of movement pattern.

Better cardiorespiratory fitness in the multi-joint group is again not a surprise. It doesn’t take much endurance to complete a set of leg extensions. Deadlifts and leg presses are a different beast. Those will stress your respiratory and cardiovascular system considerably more.

The most important results from a bodybuilding point of view were that both programs resulted in the same body composition change… or did they? If you look at the actual data in the table below, you can see there was actually a trend for greater fat loss in the multi-joint group, only júst short of being statistically significant. Not only that, there was an equally strong trend for greater fat-free mass gains in the multi-joint group. And yes, it is possible to lose fat and gain muscle at the same time. Basically, the multi-joint group outperformed the single-joint group on every measure.

Conclusion

At face value, this study found multi-joint exercises are equally beneficial for bodybuilding but better for strength and endurance development, given the same total work output in the training program. However, the confounded study design makes it questionable if these results were due to the choice of single- vs. multi-joint exercise or the difference in training programs. The higher training intensity and longer rest in the multi-joint group may have been responsible for the greater strength development. Moreover, a closer look at the results shows that the multi-joint group probably achieved more muscle growth and better fat loss to boot. Again, however, this may have been due to exercising the arms, glutes and erector spinae more, not because compound exercises are inherently superior for muscle growth than fat loss. Overall, this is a very interesting study, but it leaves us with as many questions as answers.