Most marathon runners have a complicated relationship with the gym. They know they should go. They've read the articles. They've seen the Instagram posts of elite runners doing single-leg squats and box jumps. And yet, when it comes to choosing between an extra easy run and a strength session, the run almost always wins.

The reasons are familiar. Strength training feels like it belongs to a different sport. The gym is intimidating if you don't know what you're doing. And there's a persistent worry, rarely spoken aloud but always present, that lifting heavy will add bulk, slow you down, or leave your legs too tired for the sessions that actually matter.

The science says otherwise. Not vaguely, not in the "it probably helps" way that most training advice is delivered. The evidence that strength training improves marathon performance is robust, specific, and increasingly difficult to ignore. And the mechanism isn't what most runners think it is.

It's not about getting stronger (not exactly)

The most common framing of strength training for runners is injury prevention. Strengthen your glutes, stabilise your hips, protect your knees. There's truth in this, but the evidence is more nuanced than the headlines suggest. Retrospective studies consistently find that muscle weakness, particularly around the hips, is a characteristic of injured runners. But prospective studies, the ones that follow runners forward in time to see who gets hurt, have struggled to show that strength training alone reliably prevents injuries, especially when the training is unsupervised. A 2025 scoping review in Translational Sports Medicine noted that injury rates among runners haven't decreased over the last several decades, despite strength training becoming more popular.

This doesn't mean strength training is irrelevant to injury risk. It means the injury prevention framing is too narrow. The real performance case for strength training is about something more specific and more measurable: running economy.

Running economy is the oxygen cost of running at a given pace. Two runners with identical VO2max values can perform very differently in a marathon if one of them uses less oxygen at race pace. It's the efficiency of the machine, not the size of the engine. And strength training, particularly heavy resistance work and plyometrics, consistently improves it.

A 2024 systematic review and meta-analysis in Sports Medicine examined the effect of strength training programmes on running economy across middle- and long-distance runners. The findings were clear: strength training with loads at or above 80% of one-repetition maximum improved running economy, with typical improvements in the range of 2-4%. For a three-hour marathoner, that translates to roughly two to four minutes. Not from running more. Not from running harder. From making each stride cheaper.

The mechanism is primarily neuromuscular. Heavy strength training improves the rate of force development and the stiffness of the muscle-tendon unit. This means each ground contact produces more elastic energy return and wastes less energy on unnecessary muscle activation. You're not building bigger muscles. You're building muscles that fire more efficiently and store energy more effectively with every stride.

The durability effect

There's a second benefit that's only recently been quantified, and it may matter even more than the economy improvement in fresh conditions.

A 2025 randomised controlled trial published in Medicine and Science in Sports and Exercise examined what the researchers called "running economy durability," the ability to maintain running efficiency as fatigue accumulates. After ten weeks of concurrent strength and endurance training, the strength group maintained better running economy after 90 minutes of running and showed a 35% improvement in time to exhaustion at high intensity. The control group, which only ran, saw an 8% decline over the same period.

This is the finding that should change how marathon runners think about the gym. Marathon performance isn't determined by how efficient you are at kilometre five. It's determined by how efficient you are at kilometre thirty-five, when your muscles are damaged, your glycogen is depleted, and your form is deteriorating. Strength training doesn't just make you more economical. It makes you more resistant to the loss of economy that accumulates over the course of a race.

If you've ever watched a marathon and seen the contrast between the runners who hold their form in the final kilometres and those who are visibly falling apart, you're watching the durability effect in action. Some of that is aerobic fitness. Some of it is pacing discipline. And some of it, the research now suggests, is the structural resilience that comes from consistent strength work.

What to actually do in the gym

This is where most articles either get too vague ("do some squats and lunges") or too ambitious (a powerlifting programme disguised as runner-friendly). The research on heavy loading and running economy is compelling, but it was conducted in controlled settings with supervised athletes. For most runners managing strength work around four or five weekly runs, a job, and a life, the priority isn't peak neuromuscular adaptation. It's consistency, low interference with your running, and exercises that directly support the demands of the stride.

The movements that matter for runners are simpler and more targeted than most gym programmes suggest. Calf raises are non-negotiable: the calf-Achilles complex absorbs two to three times your body weight with every stride, and it's the most common site of running injury. Beyond that, the core set covers the muscles that keep you moving efficiently: ankle dorsiflexion work, knee extension (leg press, step-ups), knee flexion (hamstring curls, Romanian deadlifts), and hip abduction and adduction (lateral band work, Copenhagen adductors). These aren't glamorous. They're functional. They address the specific force production and stability demands of running, and they can be done in a gym or adapted for a home setup with minimal equipment.

Dose matters more than intensity. Aim for two to three sets of eight to twelve repetitions, with two to three reps in reserve (RIR) at the end of each set, meaning you stop when you could still complete two or three more reps with good form. This is a departure from the maximal-strength protocols used in the research studies, and that's intentional. Percentage-based loading (80% of your one-rep max) requires testing, experience, and confidence under heavy load. RIR is more intuitive and more practical for runners who aren't experienced lifters: if you could do two or three more reps but choose to stop, you're in the right range. The stimulus is enough to drive meaningful adaptation without generating the kind of muscle damage that interferes with your running for days afterward.

That interference question is critical. Strength training should never compete with your key running sessions. Schedule strength on different days from your hard runs, not just separated by a few hours, but on genuinely different days. The goal is that you never arrive at a threshold session or long run with residual soreness from the gym. If you do, the strength work is either too heavy, too voluminous, or too close to the session that matters more. Running is the priority. Strength is supportive.

A note on plyometrics: box jumps, bounding, and depth jumps train the stretch-shortening cycle that makes tendons behave like springs. The research supports their benefit for running economy, particularly at lower speeds. But they carry higher injury risk than other strength modalities and shouldn't be added without a specific reason, whether that's a coach's recommendation, an identified weakness, or preparation for a specific race demand. They're an addition for experienced runners, not a default.

What you don't need: a forty-five-minute circuit, exercises on unstable surfaces, or a programme that treats strength as a separate performance goal. Strength for runners is year-round, periodised around your racing calendar, and always in service of the running.

Periodising strength alongside your running

This is the practical core, and where most runners get it wrong. They either do the same strength routine year-round regardless of where they are in their training block, or they do strength work sporadically and drop it the moment running volume increases. Both approaches leave value on the table.

The principle is the same one we outlined in our article on training load management: one lever at a time. You don't increase running volume and introduce heavy strength work in the same week. You sequence them so that each new stressor is absorbed before the next one is added.

Base phase (12+ weeks out from race day). This is when you have the most capacity for strength work. Two to three sessions per week, progressing gradually over six to eight weeks. Running volume is moderate, intensity is low, and your body can absorb both stressors. This is the phase where you can afford to push the strength work a little harder, adding exercises or increasing load, because you have the recovery bandwidth to handle it.

Build phase (8-12 weeks out). As running intensity increases, strength training shifts from building to maintaining. Drop to one to two sessions per week. Reduce the volume: fewer sets, fewer exercises. The goal is to preserve the adaptations you built in the base phase without creating additional fatigue that competes with your quality running sessions. The scheduling rule is simple: never place strength on the same day as an intense run. Not separated by a few hours. On a genuinely different day. You should never arrive at a threshold session or a long run carrying soreness from the gym.

Taper phase (2-3 weeks out). Reduce strength training to one short session per week or drop it entirely. The adaptations you've built over the preceding months won't disappear in two weeks. What you're protecting now is freshness. As we discussed in a previous article on HRV-guided training, your HRV should be rising during the taper as your body supercompensates. Adding unnecessary strength sessions during this window works against that process.

Post-race. Give yourself at least a week, ideally two, before returning to structured strength work. Your muscles need to repair. Your connective tissue needs to recover. When you do return, start at reduced loads and build back up over two to three weeks rather than jumping straight back to where you left off.

The interference effect (and why it's manageable)

If you've heard of the "interference effect," you might be worried that strength training and endurance training cancel each other out. The concern has a scientific basis: molecular signalling pathways for endurance adaptations (AMPK) and hypertrophy adaptations (mTOR) can compete when activated simultaneously. This was first identified by Hickson in 1980 and has been studied extensively since.

The good news for runners is that the interference effect is largely a problem for athletes trying to maximise muscle size alongside endurance. If your goal is neuromuscular efficiency, not hypertrophy, the interference is minimal. The meta-analyses consistently show that concurrent strength and endurance training improves running economy, VO2max, and time-trial performance without compromising endurance gains. The 2024 Sports Medicine review confirmed this across multiple study designs.

There are practical ways to minimise interference further. The most important is scheduling: keep strength and key running sessions on different days entirely, not just separated by a few hours. If Tuesday is your threshold run, strength goes on Wednesday or Thursday, never on Tuesday morning before or after the run. The key metric is next-run interference: can you show up to your next quality running session without residual soreness from the gym? If the answer is no, something needs to change, whether that's the timing, the volume, or the load.

Why this is hard to manage yourself

The principles above are straightforward. The daily execution, as with most training decisions, is not.

The challenge is that strength training adds a second system of fatigue on top of your running. A heavy squat session on Monday affects your legs on Tuesday, which affects your interval session on Wednesday, which affects your recovery on Thursday. The interactions ripple through the week in ways that aren't always obvious, and the cumulative effect only becomes apparent when something starts to hurt or your running performance dips for reasons you can't quite identify.

This is the interference effect in practice, not the molecular signalling, but the scheduling. When should you lift relative to your hard runs? How much volume can you handle this week given that your HRV has been trending down? Should you keep your maintenance session this week or skip it because the long run took more out of you than expected? Is the fatigue you're feeling in your legs from the squats or from the tempo run, and does the answer change what you do tomorrow?

These are the kinds of multi-signal decisions we described in our article on how AI adjusts your training in real time. An adaptive coaching system doesn't just manage your running load. It manages the interaction between your running and your strength work, adjusting one when the other creates more fatigue than expected, shifting sessions when recovery signals suggest you need more time, and ensuring the strength work supports your running rather than competing with it.

Most static plans treat strength training as a separate track, a PDF of exercises pinned to the fridge alongside your running schedule. But the two systems interact constantly, and managing that interaction well is the difference between strength training that makes you faster and strength training that just makes you tired.

The bottom line

The research is clear: marathon runners who do structured, heavy strength training run more efficiently, hold their form longer under fatigue, and arrive at the later kilometres of a race in better shape than those who only run. The improvement isn't dramatic on any single day. It compounds over months of consistent work and shows up when it matters most, in the final third of a race when everyone around you is slowing down.

The movements are simple. The periodisation is logical. The time commitment is modest. And the evidence base, after decades of study across multiple meta-analyses, is about as strong as it gets in exercise science.

Your legs are your engine. Strength training makes that engine more efficient. And efficiency, over 42.2 kilometres, is everything.

Zepho is adaptive coaching for serious runners. It's live and available now on the App Store.