Concurrent training interference effect

Strength training develops muscle size and strength, while aerobic exercise builds cardiorespiratory endurance. Many people want to do both. Examples include people preparing for a marathon who also want to build strength, and people seeking muscle growth who also want to improve fitness and manage their weight. Including both types in one training plan is called concurrent training.

However, observations suggest that combining the two may produce smaller gains in strength or muscle growth than focusing on just one. This is the interference effect, named for the idea that the two types of training interfere with each other.

The starting point was a 10-week experiment by Hickson (1980, *European Journal of Applied Physiology and Occupational Physiology*). It compared a strength-only group, an endurance-only group, and a group that did both. The combined-training group increased its maximal oxygen uptake by a similar amount to the endurance-only group, but its strength gains were smaller than those of the strength-only group. This gave rise to the term “interference,” which has long been discussed in fitness and exercise coaching settings.

It is worth noting that the training volume in this experiment was quite high. Endurance training took place 6 days per week and strength training 5 days per week, with the combined group completing both programs. Keep in mind that these conditions were much more demanding than an everyday exercise plan.

A systematic review and meta-analysis by Schumann et al. (2022, *Sports Medicine*) pooled 43 studies and organized the results by outcome.

• Maximal strength — There was almost no average difference with combined training (standardized mean difference approximately -0.06).
• Muscle size (hypertrophy) — There was likewise almost no average difference (approximately -0.01).
• Explosive strength (power) — This was lower on average with combined training (approximately -0.28). The difference was particularly apparent when aerobic and strength exercise were performed back-to-back in the same session.
• Muscle fiber cross-sectional area — There may be a small difference. It was more apparent when running rather than cycling was used for aerobic exercise, and in slow-twitch (type I) fibers. You can learn about the different fibers in Muscle fiber types.

These average findings do not mean there is no difference under every condition. Differences may be larger when training volume is very high, when the two types of exercise are performed back-to-back without time to recover, or when the goal is explosive strength.

There are two main explanations.

Molecular signaling hypothesis. Strength and endurance exercise activate different signaling pathways within muscles. This model proposes that signals activated by endurance exercise may suppress signals that promote muscle growth. Although it was presented as a leading hypothesis for some time, there has also been ongoing discussion of how long-term human studies have not produced results as clear-cut as the model predicts.

Acute fatigue and recovery hypothesis. Fatigue remaining after aerobic exercise reduces the quality of the strength training that follows. The explanation is that less weight can be lifted and there is less neuromuscular recruitment, weakening the training stimulus itself. This fits well with the observation that differences in power were larger when the two types of exercise were performed back-to-back in the same session.

Current discussions suggest that the latter view—that how total training volume and recovery are distributed plays a greater role—has considerable explanatory value. In either case, interference is closer to “gains may be smaller” than to “muscles shrink.”

If you want to do both types of exercise, here are some points to consider.

• Set your priorities. If strength is your top priority, arrange your training so that you are fresher for strength exercise. If endurance is your goal, do the reverse.
• Leave time between sessions. If you need to do both on the same day, allow enough rest between them, or split them across different days if possible. Research findings on which training order and interval are better are not yet consistent, so choosing what fits your condition and schedule is a practical approach.
• Manage the total amount. Increase the duration and frequency of aerobic exercise gradually rather than suddenly. You can read more about total training volume in Training volume.
• Choose the type of aerobic exercise. Running involves greater impact and more loading while muscles lengthen, so it may require more recovery (Delayed onset muscle soreness). Cycling and walking tend to leave less residual fatigue in the leg muscles.
• Eating and resting are part of the plan, too. When energy is insufficient, both types of training are more likely to become less effective (Energy availability and relative energy deficiency in sport).

If you are combining the two types of exercise and persistently feel heavy and sluggish, the weights you can usually lift decrease noticeably, or your sleep becomes lighter, these may be signs that your training volume has outpaced your recovery. In that case, reduce the amount for about a week and see how you feel (Overtraining and recovery).