The interference effect of concurrent training is now widely known. Many fitness professionals now advise athletes and even members of the general population against using endurance training as part of their training if their goals are to increase strength, develop power and bring about hypertrophy. But are these statements based on solid ground?
Here is a new study that shows that the truth may be a little bit more complicated than was previously assumed.
The study: Neuromuscular and Cardiovascular Adaptations During Concurrent Strength and Endurance Training in Untrained Men, by Mikkola, Rusko, Izquierdo, Gorostiaga and Häkkinen, in International Journal of Sports Medicine, 2012
What’s the background?
Why study concurrent training?
Resistance-training and aerobic exercise both cause muscular and cardiovascular adaptations, which differ depending on various training parameters, including intensity, volume, and frequency, among others. Resistance-training primarily leads to increases in strength, muscle size, rate of force development (RFD) and muscular power. Aerobic exercise primarily leads to increases in maximum oxygen consumption and time-to-exhaustion in incremental or constant-load endurance tests. However, it has been observed on many occasions that performing both resistance-training and aerobic exercise concurrently in a training program appears to lead to inferior gains in most if not all of the main resistance-training adaptations in comparison with a program comprising solely resistance-training. This phenomenon is called “the interference effect”.
Does the interference effect affect all adaptations equally?
A recent meta-analysis by Wilson (2012) reported on the effects of concurrent training vs. resistance-training only for muscular hypertrophy of the upper and lower body, strength of the upper and lower body, and power of the upper and lower body. In their meta-analysis the reviewers in fact found that gains in muscular hypertrophy and strength were not significantly different between resistance-training-only and concurrent training groups. However, they found that power was significantly lower in the concurrent training group than in the resistance-training-only. This indicates that power is more sensitive to the interference effect than either strength or hypertrophy.
However, the reviewers found that when the results were analyzed by type of endurance exercise and by body part, running was found to lead to an interference effect on lower body strength and hypertrophy while cycling was not. Exactly why there should be a difference between running and cycling in respect of the interference effect is unclear, although it is noteworthy that previous studies have noted reductions in muscle mass in recreationally active individuals undertaking marathon running training. It may be a result of the extensive muscle damage caused by the large number of eccentric actions that occur during running.
In any event, the reviewers concluded that power is the adaptation that is most strongly affected by the interference effect. They recommend that athletes whose sports depend on power and RFD should avoid concurrent training. However, they recommend that athletes whose sports depend on strength and hypertrophy can make use of concurrent training so long as they do not use running.
What did the researchers do?
The researchers wanted to investigate the effects of 21 weeks of concurrent training in comparison with resistance-training-alone or endurance-training-alone in 44 healthy adult males. Importantly, the researchers designed the resistance-training program to enhance explosive strength, deliberating testing whether concurrent training has an adverse effect on power even when power is being emphasized. They also designed the endurance training program carefully to be based around cycling, which based on the review by Wilson (2012) noted above, should yield a small if any interference effect.
The resistance-training program was performed 2 times per week and involved 7 exercises, including the bilateral leg press, the knee extension exercise, the bench press or lat pull-down exercise, the triceps pushdown or biceps curl, the sit-up or trunk extension, knee flexion exercise or calf raises, and leg adduction or abduction exercise. The bilateral leg press and knee extension exercises were performed using two different workout protocols. Most (80%) of the workouts were performed with heavy loads while a small minority (20%) were performed explosively with lighter loads (50 – 60% of 1RM).
The endurance-training program was also carried out 2 times per week using a bicycle ergometer with progressively increasing intensity and volume. The concurrent training involved the combination of both individual training programs.
Before and after the 21-week intervention, the researchers measured 1RM leg press strength and also used a dynamometer to measure maximal unilateral isometric knee extension torque and maximal unilateral isometric knee extension RFD. The researchers also measured EMG activity of the vastus medialis during this test. Additionally, they measured the cross-sectional area of the right quadriceps muscle using magnetic resonance imaging (MRI). Finally, they measured both VO2-max and maximal cycling power, during an incremental cycle ergometer test, but only for the concurrent and endurance groups, unfortunately.
Maximal 1RM strength, knee extension force and RFD
The researchers found that the strength group improved 1RM strength by 21%, the concurrent training group improved 1RM strength by 22%, and the endurance group improved 1RM by 1%. They found that the strength group improved maximal isometric force by 20%, the concurrent group improved maximal isometric force by 28%, and the endurance group improved maximal isometric force by 4%. They found that the strength group improved maximal RFD by 38%, concurrent group reduced maximal RFD by 7% and the endurance group reduced maximal RFD by 2%. These findings are shown in the chart below.
The only significant difference between the concurrent training group and the strength group was found in respect of the training effect on RFD, which was also significant only in the strength group.
Muscle cross-sectional area
The researchers found that the concurrent group increased muscular cross-sectional area by 11% while the strength group increased by 6% and the endurance group increased by 2%. The increase in the concurrent group was significantly greater than that in the strength group.
This is a very interesting finding, as it suggests that the interference effect may not be quite as simple as was previously assumed to be the case. Rather, it seems that if the modality of the endurance exercise is carefully selected and some explosive strength training is included, that muscular cross-sectional area can actually be enhanced through concurrent training in comparison with resistance-training only.
The researchers measured the effects of the training interventions on VO2-max and maximal cycling power. They found that the concurrent training group increased both measurements significantly more than the endurance group, as shown in the chart below:
What did the researchers conclude?
The researchers concluded that the results of their study do not support the prevailing interference hypothesis of concurrent training, which suggests that endurance training reduces strength, muscle mass and power gains. Rather, the researchers concluded that concurrent training appeared to lead to significantly greater increases in muscle mass than resistance-training only. They noted that the increase in maximal isometric force displayed a similar, non-significant trend.
The researchers also concluded that despite these beneficial effects in respect of muscle mass and maximal isometric force, the concurrent training group displayed a reduction in maximal RFD in comparison with the strength group. Finally, the researchers concluded that the addition of strength training was beneficial for improving both VO2-max and maximal cycling power in comparison with endurance-training only.
What were the limitations?
The study was mainly limited in that it was performed in untrained individuals. Therefore, different results might occur in trained individuals. It is therefore difficult to assess whether endurance training, using cycling as a modality, could be expected to be additive in a hypertrophy program for resistance-training athletes or bodybuilders.
What are the practical implications?
Personal trainers can be confident that adding low-impact aerobic exercise, such as cycling, will not jeopardize gains in strength or hypertrophy for their clients. In fact, it appears likely that it may in fact increase their muscular gains.
Where bodybuilders and physique athletes decide to use cardio, they should select low-impact aerobic exercise, such as cycling, for this purpose. Whether cardio is beneficial for hypertrophy trained individuals is unclear at the present time.