Fitness is a measure of the body’s work capacity.

Three indicators to evaluate fitness: 
Strength. Endurance. Skill

Three levers for improving fitness:
Stress. Recovery. Food.


Strength can be divided into three characteristics: capacity, alignment and mobility

Capacity is the measure usually associated with strength and is usually associated soley with muscles, ‘how much can you bench?’. A more nuanced approach I learned from Coach Christopher Sommers is to recognize there are three components to strength: bones, tendons and muscles and they all have different stress and recovery timelines.

Alignment is positioning. Bones are held in position by tension in muscles and tendons. Proper alignment maximizes power output and requires balanced tension. Posture is the most visible indicator of alignment.

Mobility is range of motion. To optimize fitness, mobility requires a balance between flexibility and tension. Hypermobility reduces work capacity and increases the possibility of injury. Limited range of motion reduces work capacity and increases the possibility of injury.


Endurance is determined by the bodies ability to store, release and burn fuel. For simplicity, fuel can be divided into three categories: sugar, fat and blends

Anaerobic (absence of oxygen) endurance is fueled by glycogen (sugar) stored in the muscle. Effort at levels above the heart’s ability to supply blood with fresh oxygen are generally limited to two minutes or less. When anaerobic endurance thresholds are exceeded, muscles begin to shut down from acidosis (build up of hydrogen ions).

Aerobic (with oxygen) endurance is fueled by fat. The average body can be fueled by fat for well over a month (45 – 60+ days as a rough estimate).

Most exercise is fueled by a blend of sugar and fat with the proportion determined by the amount of oxygen available. For the average person at 65% VO2 max, a 50/50 blend of sugar and fat are burned with the sugar provided by triglycerides stored in tissue and glycogen stored in the liver. ‘The Wall’ in marathons or ‘bonking’ is primarily a result of depleting glycogen stores in the body. In a fasted state (ketosis), the body burns a blend of fat and ketones manufactured from fat by the liver to conserve sugar.

Metabolic chemistry is more complicated and involves other means of producing ATP (fuel), but characterizing the duration of effort (< 10 sec < 2:00 min < 4 hours) and the amount of effort (% of max heart rate) is a good way to develop a training and fueling strategy.


Skills consists of cues and direction of applied force. Skill is the demonstration of coordination. Coordination consists of muscles fired by cues from the senses. Said a different way, coordination is the right muscles fired by the right cues to generate force in the right direction. 

Stress & Recovery For Fitness

Fitness is different than health. At elite levels of athletics, health is often compromised to increase fitness. For the military and first responders, optimum fitness is often a higher necessity for survival than optimum health (longevity).
Although the goals are different, the same stress & recovery loads (physical, mental and emotional) are useful for prioritizing training.
The type, quantity and duration of load and recovery are adjusted to make specific impacts on the strength, endurance and skill indicators listed above.

The indicators listed above provide higher fidelity on where work capacity (fitness) can be improved. 

Food For Fitness

Like stress and recovery, food can also be used as a lever for improving fitness but the intent is different.

The quantity and sources of sugar and fat in the diet will directly effect body composition and the ability of the body to store, release and burn fuel.

While ingredients from plants and animals can easily provide all the required nutrients for an average person, high work outputs may require concentrated supplements with the associated risk of toxicity.