Why Do Runners Cramp? Fatigue, Hydration, and Strength Training

Muscle cramps can change a race quickly.

One minute your pace feels controlled. A few miles later, your calf, hamstring, or quad locks up and the goal shifts from racing well to simply getting to the finish.

For years, the most common explanation was simple: you must be dehydrated or low on electrolytes.

Hydration and electrolytes absolutely matter during endurance events. But current research suggests that exercise-associated muscle cramping is more complicated.

For many runners, fatigue and the ability of the muscles and nervous system to keep producing and controlling force may be a major part of the story.

That changes how we should think about both preparation and training.

What Happened When Researchers Studied Marathon Runners Who Cramped?

One study followed marathon runners and compared those who developed cramps with those who did not.

The researchers looked at body mass changes, urine concentration, sodium, potassium, muscle damage markers, race pacing, and training history.

The runners who cramped were not more dehydrated.

Their sodium and potassium levels were also not meaningfully lower.

What stood out was fatigue.

Runners who cramped showed greater markers of muscle damage after the race. They also slowed down more during the later stages of the marathon.

That does not prove that fatigue is the only cause of cramping. Exercise-associated muscle cramps appear to be multifactorial, and different athletes may cramp for different reasons.

But it does challenge the idea that every cramp can be explained by fluid or electrolyte loss.

The runners who cramped appeared to reach a point where their legs were having a harder time continuing to do the work.

What Is Neuromuscular Fatigue?

Your muscles do not contract on their own.

The nervous system constantly sends signals telling muscles when to contract, how much force to produce, and when to relax.

As a muscle becomes increasingly fatigued, that control can become less efficient.

One leading explanation for exercise-associated cramping is that fatigue changes the balance between signals promoting muscle contraction and signals helping regulate relaxation.

Eventually, the muscle may enter a sustained involuntary contraction.

That is the cramp.

The simplest way to understand it is this:

The demand eventually became greater than the neuromuscular system's available capacity to keep producing and controlling force normally.

For a marathon runner, that makes intuitive sense.

A marathon asks the same muscles to absorb and produce force thousands of times. Even if cardiovascular fitness is excellent, the legs still need enough muscular capacity to tolerate the repeated workload.

So Do Hydration and Electrolytes Still Matter?

Yes.

The takeaway should not be that hydration or electrolytes are irrelevant.

A marathon already creates a tremendous amount of fatigue. Poor fueling, inadequate fluid intake, excessive fluid loss, heat, or inappropriate electrolyte replacement can make that challenge harder.

Good hydration and fueling help prevent unnecessary contributors to fatigue.

Think of them as part of creating the best possible environment for your body to perform.

But they cannot replace physical preparation.

Someone can execute a reasonable hydration strategy and still cramp if the muscular demands of the race eventually exceed what their legs are prepared to tolerate.

That is why the conversation should move beyond:

“What electrolyte supplement should I take?”

and toward:

“How do I prepare my body to handle more work before fatigue becomes the limiting factor?”

Running Is Still the Most Specific Preparation for Running

Long runs matter.

Mileage matters.

Race pace work matters.

If you are preparing for 26.2 miles, your body needs progressive exposure to the specific demands of running.

Running is still the most specific preparation for running.

Your training gradually develops cardiovascular fitness, movement efficiency, tissue tolerance, and familiarity with prolonged impact.

But running does not have to be the only way we build capacity.

Strength Training Helps Raise the Ceiling

Strength training gives runners another way to increase what their muscles are capable of producing and tolerating.

Research has shown that appropriately programmed strength training can improve running economy and running performance in distance runners. More recent work also suggests strength training may help runners preserve running economy and performance as fatigue accumulates during prolonged running.

That does not mean strength training has been proven to prevent muscle cramps.

It has not.

But the connection is worth understanding.

If fatigue contributes to cramping, then increasing muscular strength and work capacity gives the runner a larger physical reserve before reaching their limit.

Imagine one runner needs to use a large percentage of their available calf strength with every stride.

Another runner is significantly stronger.

The absolute demand of running may be similar, but the stronger runner may be working farther below their maximum capacity.

That is the idea behind raising the ceiling.

What Should Runners Actually Do?

A well-designed marathon program should not chase one isolated solution.

It should address the entire demand.

That means building mileage gradually, completing appropriately dosed long runs, practicing race fueling, managing hydration and electrolytes, developing muscular strength, and allowing enough recovery to adapt to the work.

Strength training should also look like training.

For many runners, that means progressively loading the calves, quads, hamstrings, hips, and trunk rather than relying only on light band exercises or mobility drills.

The exact program depends on training history, injury history, current mileage, race goals, and where the runner's limitations actually are.

The Bigger Lesson About Cramping

Cramping is frustrating because it often feels unpredictable.

But the solution may be broader than finding the right sports drink.

Hydrate well. Fuel well. Prepare specifically. Then build enough physical capacity to tolerate the work.

That is also how we approach running physical therapy at Zero Point One in NoMad, NYC.

Pain, recurring injuries, late-race breakdown, and performance limitations often look different on the surface. The common question underneath them is frequently the same:

Does your body currently have enough capacity for what you are asking it to do?

Our goal is to understand the problem, rebuild the foundation, and then continue raising the ceiling so you can handle more of the running you want to do.

If you are dealing with recurring running pain, repeated cramping, or difficulty maintaining your training as mileage increases, a performance-focused physical therapy assessment can help identify where your current capacity may be falling short of the demands of your program.

FAQs

What causes muscle cramps while running?

Running cramps appear to be multifactorial, but neuromuscular fatigue may be an important contributor, especially during long or intense events. As fatigue accumulates, the muscles and nervous system can have a harder time continuing to produce and regulate force normally, which may increase the likelihood of cramping.

Are running cramps caused by dehydration or low electrolytes?

Not always. Research in marathon and endurance runners has found that athletes who cramped were not necessarily more dehydrated or more electrolyte depleted than those who did not cramp. Hydration, fueling, and electrolytes still matter for endurance performance, but they may be only one part of the cramping picture.

Why do runners often cramp late in a marathon?

Late in a marathon, muscular fatigue is much higher than it was early in the race. If the demands of the race begin exceeding what the runner's muscles and nervous system are prepared to tolerate, force production and muscular control may start to decline. This may help explain why cramping frequently appears later in endurance events rather than during the opening miles.

Can strength training help prevent running cramps?

Strength training has not been proven to prevent all exercise-associated muscle cramps. However, strength training can improve muscular strength, running economy, and physical capacity. If fatigue contributes to cramping, stronger and better-conditioned muscles may be able to tolerate more work before fatigue becomes a major limitation.

What muscles should runners strengthen?

A running strength program should generally develop the calves, quadriceps, hamstrings, hips, and trunk while also exposing the body to progressively greater amounts of force. The exact exercises matter less than whether the program progressively builds the qualities required by the individual runner and their training goals.

Can physical therapy help if I keep cramping while running?

A running physical therapist can evaluate strength, muscular endurance, training load, race preparation, recovery, and other factors that may contribute to late-run fatigue. At Zero Point One Physical Therapy in NoMad, NYC, our goal is not simply to address the cramp itself, but to determine whether there is a gap between the demands of your running and your current physical capacity.

References

Martínez-Navarro I, et al. Muscle Cramping in the Marathon: Dehydration and Electrolyte Depletion vs. Muscle Damage. Journal of Strength and Conditioning Research. 2022.

Nelson NL, Churilla JR. A Narrative Review of Exercise-Associated Muscle Cramps: Factors That Contribute to Neuromuscular Fatigue and Management Implications. Muscle & Nerve. 2016.

Llanos-Lagos C, et al. The Effect of Strength Training Methods on Middle-Distance and Long-Distance Runners' Athletic Performance: A Systematic Review With Meta-Analysis. Sports Medicine. 2024.

Balsalobre-Fernández C, et al. Effect of Strength Training Programs in Middle- and Long-Distance Runners' Economy at Different Running Speeds: A Systematic Review With Meta-Analysis. Sports Medicine. 2024.