Aerobic Training and Fatigue: Why Endurance Events Tax Your CNS More Than You Think

When most people think about central nervous system (CNS) fatigue, they often associate it with high-intensity strength training or explosive efforts like sprinting. After all, heavy deadlifts or max-effort squats leave you feeling drained, shaky, and mentally foggy. However, what many overlook is the profound impact aerobic training, particularly endurance events, can have on CNS fatigue—and how it often exceeds what you’d experience from traditional strength training.

In this blog, we’ll explore the unique ways aerobic training taxes your CNS, compare it to strength-based fatigue, and discuss why this often gets underestimated.

The CNS: A Quick Refresher

Your CNS, consisting of your brain and spinal cord, is the control center for everything your body does. It:

- Processes signals: Coordinates motor output and sensory input.

- Recruits motor units: Activates muscle fibers to produce force.

- Manages fatigue: Balances energy demands and recovery.

While CNS fatigue is a complex phenomenon, it essentially involves reduced efficiency in how your brain communicates with your muscles, affecting both performance and perception of effort.

This means that if your CNS is fatigued, everything will feel harder but high motor unit recruitment will be reduced. Leaving you in a state where you’re training hard but at low intensity, essentially making your strenght training unproductive.

Why Aerobic Work Is More Draining Than It Seems

Duration Magnifies CNS Fatigue

Aerobic training, particularly in endurance sports like running, cycling, or swimming, often lasts hours compared to the brief but intense efforts of strength training.

- The CNS must sustain prolonged motor unit recruitment to maintain repetitive movements, which places continuous demand on neuromuscular signaling.

- Over time, the brain’s ability to send clear, efficient signals declines, leading to motor coordination issues, reduced force output, and slower reaction times.

In contrast, strength training sessions are typically shorter (30–60 minutes) and include long rest periods that allow partial recovery between sets.

Muscular damage and recovery

I can see how one may think that aerobic may lead to less muscle damage then strength training, after all runners, rowers and swimmer are not the biggest athletes out there right?

But that’s not because of a lack of muscle damage, it has to do with a few things but mainly, mechanical tension increase through high motor unit recruitment increasing the amount of muscle fibre recruitment, especially type 2 which are more incline to muscle growth.

As you can see it’s not just about muscular damage it’s about which kind of muscular damage (which kind of training).

Let’s crunch some numbers to illustrate the forces involved in long-form aerobic training (e.g., running) vs. strength training (e.g., 5x5 at 1.5x body weight).

Scenario 1: Running for 1 Hour

Key Factors:

- Weight of person: 80 kg

- Ground Reaction Force (GRF): Approximately 2.5x body weight per stride during running (varies based on speed, stride length, and biomechanics).

- Steps per minute: Average is 160–180 steps per minute, so we’ll use 170 steps/min.

- Duration: 60 minutes.

Total Force:

- Force per step = 2.5 × 80 kg × 9.8 m/s² = 1960 N (Newtons).

- Steps in an hour = 170 steps/min × 60 min = 10,200 steps.

- Total force dealt with over the session = 1960 N × 10,200 steps = 19,992,000 N (nearly 20 million Newtons).

This represents an immense cumulative force on the musculoskeletal system, with significant eccentric loading during each stride.

Scenario 2: Strength Training (5x5 deadlift at 1.5 Body Weight)

Key Factors:

- Weight of person: 80 kg

- Lift weight: 1.5 × body weight = 1.5 × 80 kg = 120 kg.

- Total reps: 5 sets × 5 reps = 25 reps.

- Force per lift = (80 kg body weight + 120 kg barbell) × 9.8 m/s² = 1960 N.

Total Force:

- Force per rep = 1960 N.

- Total force dealt with over 25 reps = 1960 N × 25 = 49,000 N.

This is significantly less total force compared to running for an hour. I know what you’re going to say I’ve compared a 60 minute event to a 25-30min event but you could still fill up that same session with 4x4 bench and weighted pull ups and still not deal with the same amount of force.

Even though CNS fatigue and force dealt with per reps or sets is higher, you can see how repeated contraction through time no matter how small can creates huge amount of muscle damage. It is no wonder why tendinitis, stress fractures, and muscle strain are far more common in endurance athletes due to repetitive loading over time.

This muscle damage will increase CNS fatigue even more in attempt to recover from the session.

Mental Effort and Focus

Often overlooked endurance events require constant mental engagement to maintain pacing, navigate terrain, and manage discomfort.

- This sustained focus taxes the prefrontal cortex, leading to mental fatigue that compounds physical fatigue.

- Over long durations, this mental strain can impair decision-making, coordination, and even motivation, hallmarks of CNS fatigue.

Strength training, while mentally challenging, involves shorter bursts of focus, often followed by moments of mental and physical recovery.

active recovery and zone 2

This is bringing us now to why I do not believe in active recovery session anymore. In my opinion people often get this very wrong. There’s not such things as active recovery, there’s only workload management. Why would you exercise instead of resting, actually recovering?

The idea is that by increasing blood flow to a certain body part, you can increase oxygen and nutrients delivery to that part and thus increase recovery. But here’s the thing, to initiate high blood flow to any given area you need repeatable muscle contraction (basically work) so you’re basically working out to recover? See the paradox?

The only way active recovery could work is if the session is very light and short, ie, 20min bike ride at very easy pace, some movement or a walk lol

There’s no such thing as active recovery - if your session is not designed to create any adaptation then what are you doing? The best thing you can do to rest and recover from training is eat well, sleep well, manage stress and perhaps most importantly don’t exercise and actually rest.

What does this means

If you’re a CrossFit / Hybrid athlete, this means you need to program in such way that allows for recovery between session. You need to eat and sleep plenty to allow for adaptation to happens as quickly as possible post session. If you’re looking for a easy session mid-week probably focusing on skill work will be more productive than going easy on strength or conditioning.

If you’re not sure whether your training is going in the right direction, click here and fill up the training + lifestyle form and we can discuss all of this together over a free consult :)