Concurrent Training Interference Effect: What the Research Actually Shows

A landmark 2012 meta-analysis by Wilson and colleagues quantified what coaches had observed for decades: combining endurance and resistance training in the same program attenuated hypertrophy by roughly 31% and power development by roughly 17% compared with resistance training alone, while pure strength was largely spared. This is the concurrent training interference effect, and understanding its mechanisms, magnitude, and moderators is essential for any athlete who must develop multiple physical qualities at once.

The interference effect describes the blunting of strength, power, or hypertrophy adaptations when endurance training is performed concurrently with resistance training. The phenomenon was first formally documented by Hickson (1980), who found that adding endurance work to a strength program eventually compromised strength gains that strength-only training would have produced.

Importantly, interference is largely unidirectional: endurance training impairs strength and power adaptations more than strength training impairs endurance adaptations. This asymmetry is a recurring finding and shapes how concurrent programs should be sequenced.

It is worth distinguishing acute from chronic interference. Acutely, a hard endurance session transiently suppresses anabolic signaling and leaves residual fatigue for hours. Chronically, repeated exposure can shift muscle fiber characteristics toward a more oxidative, fatigue-resistant — but less forceful and explosive — phenotype. Programming choices target both: spacing sessions addresses the acute conflict, while controlling total endurance dose addresses the chronic one.

The dominant mechanistic explanation is molecular. Resistance training that drives growth works largely through the mTORC1 pathway, which initiates muscle protein synthesis. Endurance training activates AMPK, an energy-sensing kinase that promotes mitochondrial biogenesis and, in doing so, can acutely inhibit mTORC1 signaling (Baar, 2014).

The simplified hypothesis is that AMPK and mTORC1 are reciprocally antagonistic: a strong endurance stimulus elevates AMPK and temporarily suppresses the anabolic signaling that resistance training is trying to maximize. While the real biology is more nuanced than a simple switch — the antagonism is partial, transient, and dose-dependent — the AMPK-mTOR framework explains why high endurance volumes near a lifting session are most disruptive.

A handful of studies anchor current understanding of the interference effect and how to manage it:

The trajectory of this literature is notable: early work established that interference exists, while more recent work has increasingly shown it is a programming problem rather than an inevitability. The modern consensus is that how you combine the two modes matters far more than whether you combine them.

Magnitude depends heavily on how the concurrent training is structured:

These figures represent poorly-optimized concurrent training. Well-designed programs that separate sessions, control modality, and manage total endurance volume can shrink the interference effect substantially, sometimes to negligible levels.

Research consistently identifies several variables that determine how severe interference becomes:

• Endurance volume and frequency: Higher volumes and more frequent endurance sessions worsen interference. This is the single most important lever.
• Recovery between modes: Greater time between endurance and resistance sessions reduces the acute signaling conflict. Same-session, endurance-first arrangements are the most disruptive to subsequent lifting quality.
• Training status: Less-trained individuals tolerate concurrent training better; highly-trained athletes operating near their ceiling are more affected.
• Energy availability: Low energy availability amplifies the catabolic environment and worsens interference.

Not all endurance training interferes equally. The literature shows a clear hierarchy driven by mechanical and metabolic overlap with lifting:

Cycling repeatedly shows less interference with lower-body strength than running, primarily because it lacks the high eccentric loading that damages the same muscle fibers being trained for strength.

The research supports several concrete strategies to preserve strength and power while still developing endurance:

• Separate sessions by 6+ hours when possible, or place them on different days.
• Lift before endurance if both must occur in one session, so resistance quality is not compromised by prior fatigue.
• Choose low-impact modalities (cycling, rowing) when the priority quality is lower-body strength or power.
• Cap endurance volume to the minimum that achieves the aerobic goal; volume is the strongest moderator.
• Protect energy availability with adequate carbohydrate and total intake, especially around training.

Periodization offers another layer of control. Rather than maximizing both qualities every week, many concurrent athletes emphasize one quality per training block while maintaining the other with minimal effective volume. A strength-emphasis block keeps endurance to two short maintenance sessions; an endurance-emphasis block drops lifting to heavy, low-volume work that preserves strength without adding fatigue. This block-based prioritization sidesteps much of the day-to-day signaling conflict that drives the interference effect.

One of the most practically important findings is that explosive power is more vulnerable to interference than maximal strength. Power depends heavily on rate of force development and high-velocity neuromuscular function, both of which are sensitive to the residual fatigue and altered fiber characteristics that endurance training can induce. Athletes whose sport demands power — sprinters, jumpers, throwers, court-sport athletes — should therefore be especially conservative with concurrent endurance volume and prioritize modality and sequencing choices that protect velocity.

The interference effect is real but manageable. Its magnitude in the literature reflects unoptimized programs; the underlying AMPK-mTOR conflict is dose- and timing-dependent, which means thoughtful structure can shrink it dramatically. The practical hierarchy is clear: control total endurance volume first, separate sessions in time, lift before you run when forced to combine, favor cycling or rowing over running for leg-dominant athletes, and monitor neuromuscular readiness objectively so accumulating interference is caught early rather than discovered as a plateau.

For the multi-sport athlete, the takeaway is empowering rather than discouraging: you do not have to choose between endurance and strength. You have to choose how to arrange them. Volume, timing, modality, and energy availability are all within your control, and adjusting them turns a 31% penalty into a rounding error.