Concurrent Training: The Interference May Not Be Where You Expect It

Combined training — that is, the combination of strength training and aerobic exercise within the same training plan — plays a central role in physical conditioning, rehabilitation, and, more broadly, in “health and fitness.” In practice, it addresses a simple need: developing multiple qualities simultaneously. An athlete must generate strength without losing cardiovascular capacity, a patient must rebuild muscle mass while improving cardiorespiratory fitness, and a recreational exerciser often seeks to improve in both areas simultaneously. Yet this combination remains surrounded by a recurring question: can endurance hinder the adaptations associated with strength training?

This question stems from the phenomenon known as “interference,” historically described as a reduction in gains in strength, power, or hypertrophy when aerobic training is combined with strength training. But this interference is not systematic. It appears to depend on numerous factors such as the type of endurance training, intensity, duration of intervals, order of sessions, total volume, training level, and likely nutritional status. In other words, combined training is not a uniform category. A long run, HIIT on a bike, or short sprints do not impose the same biological stress or neuromuscular demands. But then, in this case, when long-interval HIIT is added to a strength training program, does the interference occur at the level of muscle growth mechanisms, or does it manifest itself rather in terms of strength performance?

The Study

This is precisely what a study published in 2026 in the Journal of Applied Physiology sought to test, comparing strength training alone with a combined regimen of strength training and HIIT over 16 weeks, with a particular focus on muscle protein synthesis, satellite cells, nuclei, fiber hypertrophy, and strength gains.

To do this, the researchers recruited young men who were previously untrained and had not engaged in structured strength training, endurance training, or combined training in the preceding six months. Nineteen participants completed the entire protocol: ten in the strength training-only group and nine in the combined training group.

The weight training group performed two sessions per week for 16 weeks. The exercises selected for the lower body were the 45° incline leg press and the leg extension. The protocol increased from two to three sets after the first four weeks, with sets of 9–12RM and 60 seconds of rest between sets. Loads were adjusted to remain within the prescribed repetition range.

The combined group performed exactly the same strength training but added HIIT-style training. Two HIIT sessions were performed after strength training on Mondays and Thursdays, and two additional sessions were added on Tuesdays and Fridays. HIIT was performed at the speed associated with VO₂max, with 10 x 1 min / 1 min of passive rest for the first 4 weeks, followed by 15 intervals from week 5 to week 8, then a VO₂max retest, 15 x 1 min at the new speed from weeks 9 to 12, and 10 x 2 min / 1 min of passive rest from weeks 13 to 16.

The researchers measured maximum leg press strength, VO₂max, and muscle fiber cross-sectional area before and after the 16 weeks. They also performed biopsies of the vastus lateralis to analyze what was happening inside the muscle. Myofibrillar protein synthesis was measured. Biopsies were taken at week 4 and week 16, before and 48 hours after a standardized session, to compare acute responses in a muscle already exposed to several weeks of training. The first few weeks of weight training can be marked by muscle damage and repair processes that confound the interpretation of protein synthesis. By evaluating responses starting at week 4 and then at week 16, the researchers sought to better distinguish adaptations more directly related to remodeling and hypertrophy.

Results & Analysis

The main results show that combined training did not reduce myofibrillar protein synthesis compared to weight training alone. At week 4, protein synthesis increased 48 hours after exercise in both groups, with an estimated increase of approximately +0.379%/day in the weight training-only group and +0.425%/day in the combined training group. By week 16, the response was more moderate, especially in the weight training-only group, but no statistical difference between the two groups was observed. The addition of long-interval HIIT therefore did not appear to block the myofibrillar building or remodeling signal.

This point is central, as it puts the idea of direct anabolic interference into perspective. If the added endurance had slowed hypertrophy by reducing contractile protein synthesis, we would have expected a weaker response in the combined group. This is not what appears here. On the contrary, the molecular and cellular markers associated with muscle remodeling remain largely preserved.

The morphological results point in the same direction. After 16 weeks, the cross-sectional area of type II fibers increased in both groups, with no difference between weight training alone and combined training. The reported values indicate an increase of approximately +33% in the weight training group and +31.5% in the combined group. Type I fibers show no clear significant difference, despite gross average increases, suggesting greater variability or a less robust response under these conditions.

Satellite cells, involved in repair, remodeling, and the potential addition of new nuclei to muscle fibers, were not reduced by combined training. Their content increased primarily in type II fibers over the course of the intervention. Similarly, the number of nuclei increased in both type I and type II fibers, with even higher values in type I fibers in the combined group. This suggests that the cellular environment necessary for fiber volume expansion or maintenance is not compromised by this type of combined training.

Where interference really appears is in strength. Both groups became stronger, but the progress was greater with weight training alone. The weight training group gained approximately +88 kg on the 1RM leg press, compared to +55 kg for the combined group. After the intervention, absolute strength was higher in the weight training group, with an estimated difference of 55 kg. Type II fiber hypertrophy was similar, and protein synthesis was preserved, but maximum strength developed less when HIIT was added.

The most cautious interpretation is therefore that the interference observed here is not primarily morphological or related to protein synthesis. Rather, it may concern the neuromuscular adaptations necessary for maximum strength: motor recruitment, coordination, the ability to rapidly generate tension, reflex adaptations, or the efficiency of neural drive. The study does not directly measure these variables, so this explanation remains a hypothesis supported by the logic of the results and by the literature discussed by the authors, but it helps explain why two groups can hypertrophy comparably while diverging in terms of maximum strength.

Finally, the improvement in VO₂max appears only in the combined group, which is normal since only this group performed structured endurance training. This confirms the value of the protocol for simultaneously developing muscular and cardiovascular adaptations, even if maximum strength increases less than in a program focused purely on weight training.

Practical Applications

In this specific protocol, the addition of long-interval HIIT did not hinder the primary mechanisms related to muscle hypertrophy, but it reduced the magnitude of maximum strength gains.

If the priority is lower-body hypertrophy in an untrained individual, adding structured HIIT does not necessarily seem problematic, provided that the volume of strength training is maintained, progression is controlled, and protein intake is sufficient. In this study, both groups performed a comparable volume on the leg press and received post-workout protein supplementation. These conditions are important: it is not possible to extrapolate the results to a disorganized, under-recovered, or nutritionally inadequate program.

If the priority is maximum strength, greater caution is warranted. Even with similar hypertrophy, weight training alone produces better gains on the leg press in this context. For a strength athlete, a weightlifter, a powerlifter, or any situation where 1RM is central, the frequent addition of HIIT on the same limbs may therefore represent an adaptive cost. This cost is not necessarily muscular in the sense of fiber growth, but it may relate to the quality of neural adaptations and the specificity of force production.

For health, rehabilitation, or general fitness profiles, the results are generally favorable for combined training. It allows for the development of cardiovascular capacity while enabling type II fiber hypertrophy. This is particularly relevant when the goal is not exclusively to maximize 1RM, but to simultaneously improve muscle function, body composition, and cardiorespiratory fitness.

However, we must stay within the scope of the study. The participants were untrained young men. The results therefore cannot be directly applied to women, older adults, trained athletes, high-level strength sports, or different HIIT protocols. The follow-up period was 16 weeks, with a small sample size and limited molecular measurements. The endurance protocol used long, high-intensity intervals, not prolonged continuous training or very short, repeated sprints.

In conclusion, keep in mind that combined training is not automatically anti-hypertrophy. Under controlled conditions, with long-interval HIIT and a maintained volume of strength training, muscle growth mechanisms remain active. However, when the goal is to optimize maximum strength, the presence of a high aerobic volume on the same limbs appears to hinder progress. Programming must therefore be based on the hierarchy of goals: building muscle and cardio in parallel is possible; maximizing pure strength will require greater specificity.

Reference

Conceição M, Vechin FC, Telles G, Lixandrão M, Ribeiro N, Riani Costa LA, Hevia-Larraín V, Scarpelli MC, Damas F, Libardi CA, Camera DM, Hawley JA & Ugrinowitsch C. Concurrent training with long-interval HIIT does not impair skeletal muscle protein synthesis or hypertrophy: little evidence of an “interference effect”. J Appl Physiol 140:2, 415-427, 2026.