Lifting and Endurance in the Same Week: Does the Interference Effect Matter?

If you lift weights and train for triathlon, you've probably heard that endurance training "kills your gains". The idea has a name, the interference effect, and it has been debated since the 1980s. Some coaches use it as a reason to keep triathletes out of the gym altogether. Others ignore it entirely.

The research since then gives a more useful answer. Interference does exist, but it mainly affects explosive strength, it's most obvious when lifting and endurance work are crammed into the same session, and it concerns strength gains rather than endurance performance. For a triathlete, whose goal is to swim, bike and run faster rather than lift the most weight, that changes how much it matters.

TriForward's "Two Sessions a Week" and "The Minimum Effective Dose" cover why and how much to lift. This article is about how lifting and endurance training fit together in the same week, and what the meta-analyses say about ordering and spacing them.

What the interference effect is

The term usually traces back to a 1980 study by Robert Hickson in the European Journal of Applied Physiology, titled "Interference of strength development by simultaneously training for strength and endurance". Since then, it has come to mean that people who combine endurance and strength training gain less strength, muscle or power than people who only lift.

A few explanations have been proposed:

• Fatigue. Lifting when your legs are tired from running or riding may mean lower-quality sets.
• Competing signals in the muscle. Endurance and strength training trigger different molecular pathways, and some researchers have suggested these can partly cancel out.
• Total load and recovery. More training in total means less recovery capacity for any one type of adaptation.

These are mostly mechanistic ideas. What matters for training decisions is how large the effect is when real people train for weeks, and that's where the meta-analyses come in.

The 2012 meta-analysis: running versus cycling

A 2012 meta-analysis by Wilson and colleagues in the Journal of Strength and Conditioning Research pooled 21 studies and 422 effect sizes comparing strength training alone, endurance training alone and the two combined.

The average effect sizes were:

• Strength gains: 1.76 with strength training alone, 1.44 with combined training and 0.78 with endurance training alone.
• Muscle growth: 1.23 with strength training alone, 0.85 with combined training and 0.27 with endurance training alone.
• Power gains: 0.91 with strength training alone, 0.55 with combined training and 0.11 with endurance training alone.

Two other findings stand out for triathletes:

• Combining lifting with running, but not cycling, significantly reduced gains in muscle size and strength.
• More frequent and longer endurance sessions were linked to smaller gains in strength, muscle size and power.

Inference: if this pattern holds, a triathlete's running may interfere more with leg strength gains than their cycling. That's a reasonable reading, but it comes from comparing different studies rather than testing running and cycling head to head.

The 2022 meta-analysis: smaller than its reputation

A larger and more recent synthesis, by Schumann and colleagues in Sports Medicine in 2022, included 43 studies of healthy adults doing supervised training programmes lasting at least 4 weeks. It compared combined endurance and strength training with strength training alone.

The standardised differences were:

• Maximal strength: −0.06 (95% confidence interval −0.20 to 0.09), not significant.
• Muscle size: −0.01 (−0.16 to 0.18), not significant.
• Explosive strength, such as jump height and rate of force development: −0.28 (−0.48 to −0.08), a significant reduction.

The blunting was larger when both types of training were done in the same session than when they were separated by at least 3 hours. The authors found no significant difference by endurance mode (cycling or running), weekly training frequency (more or fewer than five sessions a week), training status (untrained or active) or age (under or over 40).

They concluded that combining aerobic and strength training doesn't compromise muscle growth or maximal strength, but that explosive strength gains may be blunted, especially when both are done in the same session.

Two points about the population: most participants were untrained or recreationally active, not competitive endurance athletes, and the abstract doesn't report a sex split. The authors' finding that training status didn't matter compared untrained and active people, not well-trained endurance athletes.

Trained people may be different

A 2021 meta-analysis by Petré and colleagues, also in Sports Medicine, asked whether training status changes the picture. It pooled 27 studies that measured lower-body one-repetition maximum (leg press or squat) and split participants by how trained they were.

• Untrained: effect size +0.03, no difference.
• Moderately trained: −0.20, not significant (p = 0.08).
• Trained: −0.35, a significant reduction (p < 0.01).

Within the trained group, the reduction appeared mainly when both types of training were in the same session (−0.66) rather than in separate sessions (−0.10, not significant).

Inference: most age-group triathletes count as at least moderately trained. The two meta-analyses aren't in direct conflict. Schumann's finding of little effect on maximal strength may reflect mostly untrained participants, while Petré suggests interference shows up more clearly once people are already trained, and mainly when sessions are combined.

What happens in well-trained endurance athletes

A 2012 trial by Rønnestad, Hansen and Raastad in the European Journal of Applied Physiology tested the question directly in endurance athletes.

• Endurance group: 11 well-trained cyclists with no strength-training experience added heavy strength training twice a week for 12 weeks, on top of a high volume of endurance training.
• Comparison group: 7 people who weren't endurance-trained did the same lifting without endurance training.

Both groups increased one-repetition maximum, thigh muscle size and squat jump performance, but the relative improvements were larger in the lifting-only group. Peak rate of force development, a measure of explosive strength, improved only in the lifting-only group.

The groups differed in more than endurance training, and the sample was small, so this isn't proof of cause. But it is consistent with the meta-analyses: endurance athletes still get stronger from heavy lifting, just by less, and explosive qualities are the most affected.

The question that matters more: does lifting hurt endurance?

For a triathlete, the more important question runs the other way. Does strength training hold back your endurance performance?

The evidence says no. A 2014 review by Rønnestad and Mujika in the Scandinavian Journal of Medicine & Science in Sports concluded that combining strength training with endurance training improves running economy in runners and, with heavy strength training, improves cycling economy and performance. TriForward's guide "Heavy Lifting for Bike Power" covers the 2025 meta-analysis of 17 cycling studies, which found heavy lifting improved time-trial performance with no loss of VO2max.

Inference: for a triathlete, interference is a cost paid on strength gains, which are a means to an end. If lifting still gets you meaningfully stronger, and your endurance performance improves or holds, the interference effect isn't a reason to avoid lifting. It's a reason to schedule it sensibly.

Order within a session

If you have to combine lifting and endurance training in one session, does the order matter? A 2018 meta-analysis by Eddens, van Someren and Howatson in Sports Medicine pooled 10 studies of healthy adults training for at least 5 weeks.

• Lifting before endurance exercise produced larger gains in lower-body dynamic strength than the reverse order, by 6.91 percentage points (95% CI 1.96 to 11.87).
• Order made no difference to muscle growth or static strength.
• Order made no difference to maximal aerobic capacity or other outcomes.

For a triathlete, there's a trade-off. Lifting first favours strength gains. But the endurance session is usually the priority, and doing a hard run or ride on tired legs after heavy squats may cost more than the extra strength gain is worth. TriForward's "Two Sessions a Week" also covers evidence that prior cycling reduces strength capacity more than prior running.

Spacing between sessions

Both large meta-analyses point the same way: interference is clearer when both types of training are in the same session. Schumann found the effect was smaller when sessions were at least 3 hours apart, and Petré found that trained people who separated sessions showed no significant reduction in lower-body strength gains.

Inference: three hours is the shortest gap tested in this way. Morning and evening sessions, or different days, give more separation, and are a reasonable choice where your week allows. There's no strong evidence on how much spacing beyond that matters.

Does losing explosive strength matter to a triathlete?

Explosive strength was the one quality the 2022 meta-analysis found clearly blunted. In these studies, it usually means jump height, rate of force development (how quickly you can produce force) or similar fast, short efforts.

For a sprinter or a team-sport player, that would be a serious cost. For a triathlete, it matters less, but it isn't irrelevant. Short, fast force production plays a part in a few triathlon situations:

• Surges and climbs on the bike, where you need a burst of power above threshold.
• Running economy, where stiffer, springier muscle-tendon behaviour is one proposed reason strength and plyometric training help runners. TriForward's "Jumps or Iron?" covers that evidence.
• Race starts and short accelerations, in the swim and on the bike.

Inference: if explosive qualities are a goal for a particular block, for example plyometrics to support run economy, it makes sense to give those sessions the most separation from endurance work and to do them fresh, rather than after a run or ride. If heavy strength for the bike is the main aim, the evidence suggests maximal strength gains are largely preserved, so the explosive blunting is a smaller concern.

Why triathletes are a harder case than the studies

Most of the studies above combined lifting with one endurance sport, typically two or three endurance sessions a week. A triathlete may do six to ten swim, bike and run sessions a week, plus lifting. The 2012 meta-analysis linked more frequent and longer endurance sessions with smaller strength gains, while the 2022 meta-analysis found no clear difference between more and fewer than five weekly sessions. Neither tested the volumes many age-groupers train.

Practical session planning for age-groupers

These suggestions combine the meta-analyses above. They're inference for triathletes, not a tested schedule:

1. Separate lifting from endurance sessions by at least 3 hours where you can, ideally more.
2. Lift on easy days or after easy sessions, not before key bike or run workouts. An easy swim followed by a gym session later in the day is a common fit.
3. If you must combine them, decide which is the priority. Lift first if strength is the goal for that block, such as in winter. Do the endurance work first if it's a key session.
4. Be careful with running volume during a strength block. The 2012 meta-analysis linked running, more than cycling, with blunted strength gains.
5. Don't expect maximal explosive gains. If jump height or sprint power is a priority, give those sessions extra recovery.
6. Accept a smaller strength gain as the price of endurance training. Triathletes don't need to match the strength gains of people who only lift.

A triathlon week

The following example is illustrative, not drawn from a study. A 38-year-old woman training for an Olympic-distance race trains eight hours a week. She wants to keep two strength sessions in winter.

• Monday: easy swim at lunch, heavy lower-body lifting in the evening.
• Tuesday: hard bike intervals.
• Wednesday: easy run.
• Thursday: easy swim in the morning, second strength session in the evening.
• Friday: rest.
• Saturday: long ride.
• Sunday: long run.

This places each strength session on an easy day, at least several hours after the endurance session, and at least a day before the hardest sessions.

Common mistakes

• Avoiding the gym because of the interference effect. The evidence suggests strength still improves, and endurance performance doesn't suffer.
• Lifting heavy straight after a long ride or run. That's the combination most likely to blunt strength gains and compromise technique.
• Putting heavy legs the day before a key session. Soreness, especially early in a programme, can wreck the next day's workout.
• Judging the programme on jump height. Explosive strength is the quality most blunted by endurance training.
• Treating the 3-hour figure as a hard rule. It's the threshold the meta-analysis tested, not a precise cut-off.

What we don't know

• How much interference well-trained triathletes experience, since no study has tested people training three sports.
• Whether women and men differ, because the meta-analysis abstracts don't report sex splits.
• The ideal gap between sessions beyond the 3-hour comparison.
• Whether swimming interferes with leg strength gains, which hasn't been studied in this way.
• Whether the mechanistic explanations are right. The molecular evidence doesn't yet explain why interference is selective.

References

• Schumann M, Feuerbacher JF, Sünkeler M, Freitag N, Rønnestad BR, Doma K, Lundberg TR. Compatibility of concurrent aerobic and strength training for skeletal muscle size and function: an updated systematic review and meta-analysis. Sports Medicine, 2022;52(3):601–612. doi:10.1007/s40279-021-01587-7
• Petré H, Hemmingsson E, Rosdahl H, Psilander N. Development of maximal dynamic strength during concurrent resistance and endurance training in untrained, moderately trained, and trained individuals: a systematic review and meta-analysis. Sports Medicine, 2021;51(5):991–1010. doi:10.1007/s40279-021-01426-9
• Wilson JM, Marín PJ, Rhea MR, Wilson SMC, Loenneke JP, Anderson JC. Concurrent training: a meta-analysis examining interference of aerobic and resistance exercises. Journal of Strength and Conditioning Research, 2012;26(8):2293–2307. doi:10.1519/JSC.0b013e31823a3e2d
• Eddens L, van Someren K, Howatson G. The role of intra-session exercise sequence in the interference effect: a systematic review with meta-analysis. Sports Medicine, 2018;48(1):177–188. doi:10.1007/s40279-017-0784-1
• Rønnestad BR, Hansen EA, Raastad T. High volume of endurance training impairs adaptations to 12 weeks of strength training in well-trained endurance athletes. European Journal of Applied Physiology, 2012;112(4):1457–1466. doi:10.1007/s00421-011-2112-z
• Rønnestad BR, Mujika I. Optimizing strength training for running and cycling endurance performance: a review. Scandinavian Journal of Medicine & Science in Sports, 2014;24(4):603–612. doi:10.1111/sms.12104
• Hickson RC. Interference of strength development by simultaneously training for strength and endurance. European Journal of Applied Physiology and Occupational Physiology, 1980;45(2–3):255–263. doi:10.1007/BF00421333