Does Lifting Weights Make You Slower at Running? Science Says the Opposite

You have probably heard it at some point. Maybe it was a running club veteran who swore off the gym twenty years ago and has been perfectly miserable ever since. Maybe it was a Reddit thread. Maybe it was your mate Dave, who once read a fitness article in 2014 and has been citing it ever since.

The claim goes something like this: lifting weights makes you slow. All that muscle is dead weight. Runners should stay out of the gym and stick to running.

Here is the problem. Every peer-reviewed study that has looked at this question in a controlled way has found the opposite. Strength training does not slow runners down. Done correctly, it makes them measurably faster, more efficient, and less likely to end up on the physio table. Research shows that adding the right kind of strength work can improve running economy by up to 8%, and cut 5K times by more than 3%, without any meaningful increase in body weight. Let's go through the evidence.

Where Did This Myth Even Come From?

Before dismissing the gym-avoidance crew entirely, it is worth understanding why the belief exists. There are two logic chains behind it, and both are plausible on the surface.

The "Bigger Muscles, More Weight" Fear

The argument goes: muscle weighs more than fat, more muscle means a heavier body, and a heavier body is slower to propel over distance. Therefore, lifting weights makes you slower. This would be true if strength training for runners actually caused significant muscle hypertrophy. The problem is that it essentially never does.

Building meaningful muscle mass requires a very specific combination of high training volumes, progressive hypertrophy-focused programming, sufficient caloric surplus, and time. A runner adding two strength sessions per week while continuing their normal running schedule is not going to turn into a bodybuilder. Research consistently shows that concurrent training (running and strength work combined) produces almost exclusively neural adaptations rather than structural ones. Your muscles get better at firing, recruiting motor units, and generating force. They do not, in any meaningful sense, get bigger (Trowell et al., 2022). In practical terms: virtually every study combining strength training with endurance running has found no significant change in body mass. The fear of bulk is, for the runner, a non-event.

Confusing Moving Slowly in the Gym With Running Slowly

This one is a genuine category error. Yes, when you do a heavy back squat, you move slowly. The conclusion some people draw is that practising slow movements trains your muscles to move slowly. This is not how the nervous system works. Strength gains from resistance training are driven primarily by neuromuscular adaptations: improved motor unit recruitment, increased synchronisation of muscle fibre firing, and a higher rate of force development. The speed of the bar during a lift does not dictate the speed of nervous system adaptation. What matters is the intent to move forcefully, not the actual velocity of movement. Heavy lifts teach your muscles to produce more force per stride. That translates directly to better running mechanics, not slower ones.

What Running Economy Is and Why It Matters More Than You Think

Running economy is a measure of how much oxygen your body uses to sustain a given running pace. The less oxygen you need at a particular speed, the more economical your running is. Think of it like your car's fuel efficiency: the same engine, covering the same distance, using less fuel.

A more economical runner can run at the same pace for less effort, or run faster for the same effort. This is why elite distance runners often have relatively modest VO2 max scores compared to some recreational athletes but can hold 3:00/km pace for two hours. Economy, not raw aerobic capacity, is often the deciding factor in race performance.

Strength training improves running economy primarily by making your muscles stiffer and more reactive. A stiffer leg spring stores and returns more elastic energy with each foot strike. You get more propulsion per step for the same metabolic cost. Even a 1% improvement in running economy translates to roughly a 1% improvement in race performance. Over a 5K, 10K, or half marathon, that compounds significantly. A 5% improvement in economy on a 25-minute 5K would put roughly 75 seconds back in your pocket.

What the Research Actually Shows

Heavy Lifting Makes You a More Efficient Runner

In 2008, Støren and colleagues published what has become one of the landmark studies in this area in Medicine and Science in Sports and Exercise. They took a group of well-trained distance runners and put half of them on an 8-week heavy strength programme: four sets of four repetitions maximum of the half-squat, three times per week, added on top of their normal endurance training. The other half continued their normal running only.

The results were striking. The strength group improved their running economy by 5.0% and increased their time to exhaustion at maximal aerobic speed by 21.3%. The control group showed no meaningful change in either measure. The critical detail: body weight did not change in the strength group. Not by a kilogram. The runners got significantly more efficient without adding a single gram of mass.

Explosive Training Cuts Your 5K Time Directly

Paavolainen and colleagues published an even more striking finding in the Journal of Applied Physiology (1999). They took a group of endurance runners and replaced 32% of their total training volume with explosive strength work: plyometrics, sprints, and jumping exercises. A third of their running was swapped out for gym and track work.

The explosive training group improved their running economy by 8.1% and cut their 5K time by 3.1%. The control group, who kept doing their normal endurance training, showed no improvement. The study also found no change in VO2 max in either group, which tells you something important: the improvement had nothing to do with aerobic capacity and everything to do with how efficiently the muscles were using the oxygen that was already available.

The Meta-Analysis Confirms It Is Not a Fluke

Balsalobre-Fernandez and colleagues (2016) published a systematic review and meta-analysis in the Journal of Strength and Conditioning Research specifically examining the effect of strength training on running economy in highly trained runners. Their conclusion, across controlled trials: a large, beneficial effect. The recommended dose from the review was two to three sessions per week for 8 to 12 weeks.

A more recent review by Trowell and colleagues (2022), published in Sports Medicine Open, looked at 22 studies comparing heavy resistance training versus plyometric training for running economy and time-trial performance. Both types of training improved running economy. Heavy resistance training produced the larger effect. Neither caused meaningful increases in body mass. Not one of 22 studies found that strength training made runners slower.

Does This Apply to Normal Runners, Not Just Elites?

The short answer is yes, and if anything the gains are proportionally similar or larger in recreational runners, because their neuromuscular systems have more headroom to adapt. The mechanisms driving the improvement (motor unit recruitment, rate of force development, tendon stiffness) operate at all training levels. The study populations vary, but the direction of the effect does not.

So What Actually Makes Runners Slower?

Since strength training is clearly not the culprit, it is worth naming what actually does slow runners down, because the gym gets blamed for problems it is not causing. The things that genuinely impair running performance are: accumulated injury (which strength training actually reduces by reinforcing tendons, ligaments, and supporting musculature), excessive fatigue from too much total training load, and poor programming that layers heavy lifting on top of an already maxed-out mileage base without adjusting for recovery.

The One Way Strength Training Can Slow You Down

If you are already running 70 kilometres per week at the edge of your recovery capacity and you add three heavy gym sessions on top without reducing anything else, you will be fatigued. Your running will suffer. But here is the thing: the problem is not the lifting. The problem is the total load exceeding what you can absorb and recover from. When you add strength sessions, adjust your easy running volume slightly downward to account for the extra recovery demand. Runners who fail to do this and then conclude that the gym is slowing them down are drawing the wrong lesson from the right experience.

How to Start Lifting Without Ruining Your Running

The Minimum Effective Dose

Two strength sessions per week is the threshold supported by research for meaningful running economy improvements. Three sessions per week is slightly more effective and aligns with the HSE's updated 2024 physical activity guidelines for Ireland. Each session should be 40 to 55 minutes of focused work. This is not bodybuilding. It is targeted neuromuscular training.

The Best Exercises for Runners

Focus on compound lower-body movements that develop the specific strength patterns running demands. The essentials are: barbell back squats or goblet squats (for quad and glute strength), Romanian deadlifts (for posterior chain and hamstring resilience), Bulgarian split squats (single-leg strength and hip stability), calf raises (for Achilles tendon stiffness and propulsion), and hip thrusts (for glute activation and drive). Once you have built a base of strength over six to eight weeks, add plyometric work: box jumps, bounding, single-leg hops. These develop the reactive stiffness that directly improves running economy.

How to Fit It Into Your Running Week

The scheduling rule that makes or breaks this: do not lift the day before a hard running session or a long run. The day after a hard session is fine. On easy-run days, lifting in the same session (run first, lift after) works well and keeps your total number of training days manageable. The non-negotiable is protecting 24 to 48 hours of recovery before your key running efforts.

A practical four-day structure for a runner adding strength work: Monday (easy run plus gym session), Tuesday (rest or easy active recovery), Wednesday (hard run, intervals, or tempo), Thursday (gym session), Friday (rest), Saturday (long run or parkrun), Sunday (easy recovery run). This keeps lifting and hard running well separated while hitting the two-session-per-week minimum.

What to Expect in the First 8 to 12 Weeks

Be warned: the first three to four weeks will probably feel slightly off. Your legs will be carrying some additional fatigue from the unfamiliar loading stimulus and your easy runs may feel a fraction harder than normal. This is normal and temporary. The neuromuscular adaptations you are after typically become measurable between weeks six and twelve, which is exactly when most people give up and conclude that lifting does not work for them. Stay with it past the initial adjustment period. The research says the gains are there.

The Bottom Line

The myth that lifting weights makes runners slower is not just wrong. It is backwards. Every properly controlled study on this question finds the same thing: strength training improves running economy, reduces injury risk, and in most cases directly improves race times. The reason the myth persists is that it sounds logical, it gets repeated in running communities with confidence, and most of the people repeating it have never actually read the studies they are paraphrasing. Now you have read them.

The practical takeaway: add two to three gym sessions per week, focus on compound lower-body work, protect your recovery around your hard running sessions, and give it twelve weeks before you judge the results. The science is not ambiguous on this one.

Sources: Støren, O. et al. (2008). Medicine and Science in Sports and Exercise, 40(6), 1087-1092; Paavolainen, L. et al. (1999). Journal of Applied Physiology, 86(5), 1527-1533; Balsalobre-Fernandez, C. et al. (2016). Journal of Strength and Conditioning Research, 30(8), 2361-2368; Trowell, D. et al. (2022). Sports Medicine Open, 8(1), 128.