The modern version of strength training is arguably the biggest addition to endurance sports. We know it improves economy and resilience, which is why I schedule it for all my athletes.
But there is a hidden cost.
As you get stronger, your physiology changes. Your muscle density changes. If you do not update your endurance training to match these changes, you might actually be reducing your efficiency.
Does the increased muscle mass require more oxygen? Does it create more lactate? Do you need to stop doing Zone 2 and start doing something else? Here is what is actually happening when you mix strength with endurance.
The Reality of the “Penguin Waddle”
Let’s be honest about what this actually feels like. We’ve all been there. It’s Tuesday morning. You hit a heavy leg session yesterday because “strength is good,” right? You lace up your shoes, you head out the door for a recovery run, and suddenly your legs feel like they are made of jelly.
You are doing the Penguin Waddle. You look at your watch, and you are running 30 seconds slower per kilometer, but your heart rate is sky high. You think, “I’m suffering more, I’m moving slower… surely this is making me worse?”
It feels like you have to pick a lane. Are you a gym bro or an endurance athlete? Because right now, waddling down the street with DOMS, you feel like neither. I know I’d rather avoid the gym if I could…
But I want to reassure you: This feeling isn’t you losing fitness. And it’s not just fatigue. It is more of a physiological mismatch. You’ve made massive gains in the gym but those gains need to be adapted to what you actually need them for.
The Myth of the Oxygen Guzzler
Despite how heavy your legs feel, the added weight isn’t “bad” weight. There is a myth that building muscle makes you an “oxygen guzzler”—that you need more oxygen just to move that bulk around.
The science actually says the opposite. When you lift heavy, you convert your malleable Type IIx muscles fibers—the “Dragster” fibers that burn fuel instantly—into Type IIa fibers.
Type IIa are the Hybrid Engine of your muscles. They are powerful enough to sprint up a hill or surge past your mate in a sprint finish, but unlike the dragster engine, they can actually use oxygen. This gives you a Strength Reserve. If you can produce more force with each step without maxing out, you become more economical, not less.
The Plumbing Problem: Capillary Dilution
So, if you have this fancy new Hybrid engine, why does it feel like you’re running with the handbrake on?
The problem isn’t the muscle. The problem is the plumbing.
Simply, when you train strength, your muscles get bigger. This is hypertrophy. But the infrastructure around them remains the same. In this sense: the capillaries.
Bigger muscles on the same network of vessels is a surefire way to build up fatigue quicker. This is termed Capillary Dilution.
The distance oxygen has to travel from your blood to your mitochondria has increased. So, even though you are stronger, your muscles are effectively suffocating during your run. That “heavy” feeling? That’s not just soreness. That is local hypoxia. Your bigger muscles are screaming for air that can’t reach them fast enough.
Why Zone 2 Isn’t Enough
Now, I know what you’re thinking. “Chris, you always tell us to do Zone 2. Zone 2 builds capillaries.”
And yes, for the general population, Zone 2 is king. But here is the catch: When you are trotting along in Zone 2, your body is smart but lazy. It only recruits your Type I (slow twitch) fibers.
It does not recruit those big, new, powerful Type IIa fibers you fought for with a barbell. Your body isn’t going to improve a muscle fiber that you don’t use.
If you are a hybrid athlete—if you lift and run—Zone 2 is not enough to upgrade your muscles. You need to force blood into those high-threshold fibers.
The Solution: High-Intensity Interval Training (HIIT)
You need High-Intensity Interval Training. We aren’t talking about all-out sprinting. We are talking about 4-minute intervals at 90-95% of your Max Heart Rate. This will be threshold and slightly above.
Why this intensity? Because at 90% effort, you have to switch on those Type IIa fibers. You force them to work aerobically. This releases a signal to your body that it needs to create more capillaries.
You only need this once a week. But without it, your strength gains are disconnected from your endurance system.
The Nutritional Trap
Now, there is one more massive reason your legs feel like lead, and nobody talks about this enough. It’s the Nutritional Trap.
When you combine heavy lifting with high intensity cardio, your body becomes super carb hungry. You will be burning through fuel faster than you realize. Most endurance athletes are used to fueling for efficiency, balancing calorie intake with training and only being heavy on carbs in the lead up to a race.
But when you lift heavy, you aren’t just an endurance athlete anymore. You are a hybrid that needs to pay attention to your carb needs.
Runners, you’ve probably felt “Runger.” That bottomless pit feeling where you eat dinner, and 20 minutes later you are staring into the fridge looking for snacks. That is your body screaming for fuel.
Here is the trap: You start lifting to get leaner or stronger, so you keep your calories the same. But muscle is expensive tissue. It costs energy just to exist. And those high intensity intervals I just told you to do? They chew through carbohydrate stores too.
If you don’t increase your intake, specifically Carbohydrates, you end up with chronically depleted glycogen stores. That “heavy leg” feeling? Half the time, it’s not muscle damage. It’s an empty fuel tank.
Stopping the Interference Effect
One final piece of advice: Please, give yourself a break. If you do your high intensity session right after heavy squats, the signal to build muscle and the signal to build endurance essentially cancel each other out. This is called the Interference Effect.
- Keep sessions at least 6 to 9 hours apart. * Ideally do them on different days. Do your heavy lifts on one day, and your hard intervals on a separate day. Your legs might still feel a bit heavy, but at least your body won’t be fighting a war with itself.
So, does lifting weights make you slow? Only if you don’t expand the plumbing and starve the engine.
If you lift heavy to build the power, use high intensity aerobic intervals to build the infrastructure, and eat enough to keep the gains coming, you aren’t just an endurance athlete who lifts. You are turning into a hybrid. You get the strength, you get the speed, and eventually… you might even lose the penguin waddle.
Be efficient.
