Hybrid racing bets against the interference effect. The 2026 evidence mostly agrees

Fitness racing is built on a proposition that exercise science spent forty years doubting: that a person can train hard for strength and hard for endurance at the same time without one ruining the other. The doubt has a name — the interference effect — and in 2026 it got its most comprehensive audit yet.

An umbrella review published in Sports Medicine pooled 17 meta-analyses covering 144 individual studies and 1,492 healthy participants [s1]. Compared with endurance training alone, concurrent training produced significantly greater strength adaptations (standardised mean difference 0.59, p < 0.001) [s1]. Compared with resistance training alone, concurrent training significantly improved aerobic capacity (SMD 0.77, p = 0.02) while strength, power and hypertrophy outcomes were comparable [s1]. Aerobic gains from concurrent training were comparable to those from endurance training alone [s1].

Read plainly: in the pooled data, adding running or cycling to a lifting programme did not cost measurable muscle or strength, and adding lifting to an endurance programme did not cost aerobic fitness.

The review also tested training order. No significant effect of sequence was found, though the authors note trends favouring resistance before endurance for strength (SMD 1.69, p < 0.001) and hypertrophy (SMD 0.83, p = 0.36) [s1]. The hypertrophy p-value there is not close to conventional significance, and the authors describe the sequence findings as trends rather than conclusions [s1].

The caveat that matters most

The review's own conclusion limits its reach: the findings support concurrent training "for recreationally trained individuals," and data from highly trained to elite athletes remain scarce [s1]. The search was conducted to 28 February 2025 and the protocol was registered as CRD42025646460 [s1].

That is the crux. Interference, when it has been observed, tends to appear at high endurance volumes in people who already have a lot of strength to lose. An umbrella review of recreationally trained participants is the wrong instrument for detecting it.

What the race actually asks for

The first laboratory study of HYROX gives a sense of the load being argued about. Eleven recreational athletes (median age 33, median VO₂max 51 mL/min/kg, median 18 months of HYROX experience) completed a simulated open-division race [s2]. Median finish time was 86.5 minutes, of which 51.2 minutes was running and 32.8 minutes was stations [s2]. Peak heart rate reached 185 bpm, mean heart rate was 170.9 bpm, and 79.5% of the race was spent between 90% and 100% of maximum heart rate [s2]. Peak blood lactate was 8.5 mmol/L at stations and 7.7 mmol/L on runs, with a mean of 6.3 mmol/L across the event [s2]. Peak rating of perceived exertion hit 18 at stations [s2].

Finish time correlated with VO₂max (ρ = −0.71, p = 0.01), with endurance training volume (ρ = −0.68, p = 0.04), and with body fat percentage (ρ = 0.67, p = 0.03) [s2]. The authors' reading is that performance leans on aerobic capacity, endurance volume and body composition rather than maximal strength [s2]. With eleven participants of relatively homogeneous fitness, performing a simulated race alone indoors on a treadmill rather than in competition, this is a small and preliminary study and the authors say so [s2].

The competitive record points the same way. Across 39,696 individual PRO and ELITE results from the format's first seven seasons, running consistently accounted for roughly 50% of total race time and showed the closest correspondence between discipline-specific and overall ranking [s3]. Strength-determined stations produced more rank reshuffling and had larger absolute time costs for slower performers [s3]. Improvement over the seasons was driven mostly by faster running: about 8 minutes of the top-100 men's ~13-minute gain and about 10 minutes of the women's ~17-minute gain [s3].

Where interference still shows up

Two 2026 studies sharpen the picture in different directions.

A controlled trial randomised 41 moderately trained men to concurrent training with resistance work at 0%, 15% or 40% velocity loss, or to endurance training alone, over eight weeks [s4]. All concurrent groups increased muscle mass, with the 40% velocity-loss group gaining most, while the endurance-only group gained none [s4]. The 15% and 40% groups made greater one-repetition-maximum gains than endurance alone [s4]. But the aerobic result inverted: maximal aerobic speed improved in every group, with the largest gains in the endurance-only group, and within the concurrent groups the lower the velocity loss the larger the aerobic effect [s4]. The authors' interpretation is that fatigue generated during the lifting session attenuated endurance adaptation [s4]. Endurance training alone, meanwhile, produced no strength gains and significantly reduced rate of force development at 400 ms (p = 0.01) [s4]. Forty-one participants split four ways is a small trial.

A second meta-analysis asked whether sprint interval training is a gentler endurance stimulus to combine with lifting. Across nine studies and 177 participants, adding sprint intervals to resistance training produced no significant difference versus resistance training alone in lower-body strength (SMD 0.01, p = 0.94), upper-body strength (SMD −0.06, p = 0.83), jump performance (SMD 0.11, p = 0.11) or sprint performance (SMD −0.01, p = 0.95), while significantly improving maximal oxygen consumption (SMD 0.78, p = 0.001) [s5]. A sensitivity analysis found greater jump gains with sprint protocols of 10 seconds or shorter (SMD 0.41, p = 0.025) [s5]. Nine studies is a thin evidence base for a general claim.

Put together, the 2026 literature suggests the interference question has partly changed shape. The cost is showing up less as blunted muscle growth and more as blunted aerobic adaptation when lifting fatigue is high — the reverse of the classic worry.

The unmeasured variable

One small study raises a mechanism the training literature rarely quantifies. Eight male HYROX athletes aged 23 to 32 wore ambulatory sleep monitors after resistance and endurance sessions [s6]. Despite reporting good subjective sleep quality, they averaged about 6.6 hours of total sleep time and spent about 5% less time in REM than non-athlete reference values [s6]. After resistance sessions, sleep onset latency was longer (29 versus 10 minutes, p = 0.003) and wake after sleep onset shorter (31 versus 48 minutes, p = 0.008) than after endurance sessions, with corresponding differences in cardiac arousals [s6]. Eight participants over four monitored nights is a pilot, not a finding.

What to watch

The umbrella review's own request is the thing to track: trials in highly trained and elite athletes [s1]. Fitness racing has now produced tens of thousands of well-documented competitive performances [s3] and a laboratory literature that still measures eleven people at a time [s2]. The gap is where the interference question will actually be settled.

Sources

1. [s1] Held S, Wolf L, Rappelt L, et al. Maximizing Adaptations in Concurrent Training: An Umbrella Review of Meta-analyses. Sports Medicine, 28 February 2026. https://doi.org/10.1007/s40279-026-02401-y
2. [s2] Acute physiological responses and performance determinants in Hyrox©. Frontiers in Physiology, 31 March 2025. https://pmc.ncbi.nlm.nih.gov/articles/PMC11994925/
3. [s3] Rappelt L, Wiedenmann T, Held S, et al. Longitudinal performance development in PRO and ELITE HYROX competitions across the first seven competitive seasons. Frontiers in Physiology, 8 July 2026. https://doi.org/10.3389/fphys.2026.1847569
4. [s4] Tundidor-Duque RM, Loturco I, Paéz-Maldondado JA, et al. Velocity Loss During Resistance Training: Implications for Concurrent Training Adaptations. Scandinavian Journal of Medicine & Science in Sports, March 2026. https://pmc.ncbi.nlm.nih.gov/articles/PMC13002141/
5. [s5] Ferraro-Farro D, Bandeira-Guimarães M, Blanco-Rambo E, et al. Does Sprint Interval Training Cause Interference in Concurrent Training? A Meta-Analysis Study. International Journal of Sports Medicine, 17 March 2026. https://doi.org/10.1055/a-2820-4527
6. [s6] Buoite Stella A, D'Andrea F, Deodato M, et al. Strengthening recovery, enduring sleep. European Journal of Applied Physiology, 28 February 2026. https://pmc.ncbi.nlm.nih.gov/articles/PMC13287113/