Why Your Summer Runs Feel Terrible: Heat, Heart Rate, and Knowing When to Stop

Slower pace, higher heart rate, and a run that feels twice as hard as it should. Most people read that as lost fitness. It usually isn't. Here is what heat actually does to your cardiovascular system, what the research really says about muscle cramps, how long adaptation takes, and when the honest move is to take the run indoors.

I went out for a solo run through the Edmonton River Valley this week — Hawrelak down to Whitemud and back. 7.64 kilometres in 52:19, in the full heat of the day.

Anyone who knows me knows I love the heat. I am usually the one who is perfectly happy while everyone else is complaining. But loving the heat and performing in it turn out to be two entirely different things. I walked more of that run than I planned to, and by the back half I was on the verge of cramping.

I am training for HYROX. I know what my aerobic base feels like. And that run did not feel like a fitness problem — it felt like a conditions problem. So I want to walk through the physiology, because if your summer runs have been quietly demoralising you, there is a good chance you have been blaming the wrong thing.

What heat is actually doing to your heart rate

When you run in the heat, your body is doing two jobs at once: moving you forward, and keeping you from overheating. Those two jobs compete for the same resource — blood.

As your core temperature climbs, your body sends blood toward the skin so heat can escape. That is blood which is temporarily unavailable to your working muscles. At the same time, sweating gradually reduces your plasma volume, which means less blood returning to the heart with each beat.

The result is that your stroke volume — the amount of blood your heart pushes out per beat — falls. Your body's demand for oxygen has not changed, so to keep cardiac output constant, the only lever left is rate. Your heart beats faster to deliver the same oxygen.

This is called cardiovascular drift (or cardiac drift), and it is well documented: it commonly adds 10 to 20 beats per minute over 30 to 60 minutes of steady effort, at an unchanged pace. Crucially, it happens without any corresponding rise in your actual effort or oxygen consumption.

Which means your pace is lying to you

Here is the practical consequence. If you train by pace in summer, you will consistently conclude that you are getting slower. If you train by heart rate, you will conclude that you are getting unfit, because every zone now sits at a slower pace than it did in May.

Neither is true. Both numbers are correct — they are just reporting on the weather as much as on you.

This matters more than it sounds, because most people respond to a bad-looking run by trying harder next time. In the heat, trying harder is precisely the wrong adjustment. You end up pushing deeper into fatigue for a session that was never going to produce the adaptation you wanted anyway.

The cramp conversation is more interesting than you have been told

Now, the cramping. I was close to it on that run, and the story most of us grew up with is simple: cramps mean you are dehydrated or short on electrolytes. Drink more, add salt, problem solved.

The current evidence is considerably less tidy than that.

Research increasingly points to neuromuscular fatigue and altered spinal reflex control as the primary driver of exercise-associated muscle cramps. The proposed mechanism is that repeated muscle overload produces local fatigue, which increases excitatory signalling from the muscle spindles while decreasing inhibitory signalling from the Golgi tendon organs. The muscle, in effect, loses part of its "stop" signal.

The observation that tipped the field is a simple one. Exercise-associated cramps show up locally, in the specific muscles doing the work. If the cause were genuinely systemic — whole-body dehydration, whole-body electrolyte depletion — you would expect it to affect muscles everywhere, not just your calves at kilometre six. Several prospective cohort studies have failed to support the dehydration and electrolyte hypotheses, which are largely built on anecdotal clinical observation and small case series.

I want to be careful here, because this is a live area of research and the picture is genuinely complex. Fluid and sodium still matter enormously for performance and for safety in the heat — that part is not in dispute, and you should absolutely still be drinking and salting appropriately in an Edmonton August. But if you have been diligently hydrating and cramping anyway, and quietly wondering what is wrong with you: the answer may simply be that the muscle was fatigued beyond what it could control. Which is a different problem with a different solution.

You do adapt — but it takes about two weeks

The encouraging part: heat adaptation is real, and it is faster than most people expect.

Meaningful changes begin within the first few days of consistent heat exposure and become substantial across roughly 7 to 14 days. In the first week or so you get plasma volume expansion and a lower heart rate at any given workload. Across the fuller two-week window you develop earlier and more effective sweating, lower core and skin temperature at a given effort, and reduced sodium losses in sweat.

Two caveats worth knowing. First, adaptation decays once heat exposure stops — it is not banked permanently. Second, and this is the specifically Edmonton problem: our summer is not consistent enough to reliably drive it. A stretch of 28-degree days followed by a week of 16 degrees does not give your body the uninterrupted stimulus that a genuinely hot climate does. We get the cost of training in heat without much of the adaptation.

So here is what I am actually doing

My running goes back to the treadmill for now — unless I am out early in the morning, or the day is genuinely cooler.

I want to be clear that this is not a retreat. HYROX asks for eight one-kilometre segments interspersed with heavy functional work, so the run side of my training has to keep growing. The purpose of most of my aerobic running is to accumulate time in a specific heart-rate zone. If the heat makes that zone impossible to hold — if I am walking half of it and flirting with cramps — then the session is no longer building an aerobic base. It is just accumulating fatigue and risk.

The treadmill lets me hold the paces and the zones the plan actually calls for. I will move back outside when Edmonton gives me a reasonable morning. And come fall, I will be out there constantly.

What to do with your own summer running

Run early

The single highest-value change. Early morning is the coolest part of the day, and it comes with the bonus of morning daylight, which anchors your circadian rhythm and improves your sleep that night. Two benefits, one decision.

Judge yourself on effort, not pace

In summer, hold the effort constant and let the pace be whatever it is. If you train by heart rate, accept that your zones will sit at slower paces for a couple of months. Compare summer runs to other summer runs, not to your spring times.

Shorten before you slow down badly

A shorter run held at the right intensity is worth more than a long one that degrades into walking. If it is 30 degrees, do 5 kilometres properly rather than 10 kilometres badly.

Hydrate with sodium, not just water

Even though sodium may not be the primary cramp mechanism, replacing what you lose in heavy sweat still matters for performance, for blood volume, and for how you feel afterward. Water alone, in large volumes, is not the answer.

Take the indoor option without guilt

A treadmill session that hits the prescribed zone beats an outdoor session that does not. The goal is the physiological stimulus, not the postcard.

The bigger principle

What I keep coming back to is this: the plan is a tool, not a test of character. Conditions change — heat, illness, sleep, stress, life. The skill worth developing is not the ability to grind through everything regardless. It is the ability to read what is happening and adjust so the training still does its job.

If your summer runs have felt inexplicably terrible, it is very probably not you. Heat drives your heart rate up at any given pace, and cramping is a signal worth respecting rather than pushing through. Run early, run shorter, or run indoors — but train in the conditions you can actually train well in.

Your fitness is still there. It is just waiting for a cooler morning.