Cardio = Concentrated Muscle Damage

One of the reasons endurance exercise is more damaging than other forms of exercise, is because intense oxidation and acidity occurs in the same muscles, with no time to recover.  With this ‘over-load’ in specific areas causing damage, as opposed to stressing a single muscle (triceps) or group (pecs and triceps), for a short period of time (30sec), followed by ample rest (90sec).

“Excessive: Training with insufficient rest and variety of training.” Urhausen et al., Sports Medicine

Diaphragm muscle, for example, is continuously stressed throughout an aerobic session, meaning free radical and lactate accumulation is consistently produced for the entire 45, 90, or 120min bout. Exposing the only skeletal muscle dubbed “essential for life” to a consistent dose of excessive stress.


Arguably, this is why we see:

Twice the Upper Respiratory Tract Infections (URTI) in runners completing 96km/week compared to those completing 32km/week

Nearly 5 times the rate of respiratory infection in well-trained swimmers compared to amateurs.

And similarly, a higher prevalence of asthma in endurance athletes.  With one study finding 23/42 elite runners with asthma and 31/42 with asthma-like symptoms, and another one from Finland testing 103 athletes (average age of 23) finding16 with asthma (and 24 with allergies).

Although, it’s likely also related to the compromised immune function seen in marathoners and aerobic athletes.  Because of the excessive inflammation, oxidative damage, cortisol, and lactate we’ve discussed in the past.


The irony being, that respiratory (or lung) muscle does not improve with exercise – since the lungs are already ‘overbuilt.’ Suggesting that there’s no positive adaptation for the lungs to maintain homeostasis. And implying that steady-state cardio exposes us to a lot of respiratory stress and fatigue, for not a lot of respiratory reward.

In contrast to an activity like walking, which actually reduces URTIs when performed 5 days a week for 45 minutes:

Stay Lean!
Coach Mike


Chronic Cardio = Elevated Cortisol

Chronic Cardio = Excess Free Radicals

Working Too Hard For The Runners High