Physiology Deep Dive

Physiology Deep Dive#

A deep dive into physiology, adaptations and training for both athletic performance and general longevity.

Different Physiological Adaptations#

We can broadly classify each adaptation as Neural, Muscular, Connective, Cardiovascular or a mixture.

For each adaptation I will share :

  • Classification
  • What is is
  • What drives it
  • How to train it

Skill / Technique#

Primarily a neural adaptation. Your brain builds and refines motor programmes through repetition. Neural pathways that are frequently used are wrapped in Myelin which speeds up signal transmission. This makes movements smoother and more automatic. This is essentially your neural efficiency at performing certain movements.

Speed#

Primarily a neural adaptation. You learn to increase the rate coding of motor neurons. Rate coding is the frequency at which they fire. As well as learning to reduce antagonist co-contraction, to help opposing muscles stop fighting the movement. Fast-twitch motor units must be recruited, which requires high-intensity intent.

Power#

Power is a mix of muscular and neural. It requires both rapid motor unit recruitment (neural) and fast-twitch fibre development (muscular). Maximal power relies a lot on the “stretch-shortening” cycle, where a muscle stretched and then rapidly contracts (like loading a spring). This means the nervous system must coordinate timing between muscles with millisecond precision.

Strength#

[DRAFT — Claude]

A mix of neural and muscular. Strength is maximal force production in a given movement, and it is far more skill-specific than it first appears — you get strong at a pattern, not in the abstract. The neural side dominates early: recruiting high-threshold motor units, raising rate coding, and improving intermuscular coordination so the right muscles fire in the right order. The muscular side is cross-sectional area, which matters more the longer you train.

How to train it: Heavy loads at 85%+ of your one-rep max, 1-5 reps, 3-6 sets, with long rests of 3-5 minutes so the nervous system recovers between efforts. Frequency beats per-session volume, because practising the pattern often is what builds the neural side. Leave a rep or two in reserve most sessions and save true maximal attempts for testing.

Hypertrophy#

[DRAFT — Claude]

Primarily muscular. Hypertrophy is growth in muscle cross-sectional area, mostly through adding myofibrillar protein within existing fibres rather than adding new ones. Mechanical tension is the primary driver — metabolic stress and muscle damage contribute far less than gym folklore suggests. Training close to failure matters because it forces recruitment of the high-threshold motor units, which is how the largest fibres get stimulated at all.

How to train it: Anything from 5 to 30 reps builds muscle provided the set finishes close enough to failure, so pick a range you can load and recover from. Volume is the main lever: roughly 10-20 hard sets per muscle per week, split across two sessions, with 0-3 reps left in reserve. Training through a full range under load, especially the lengthened position, appears to produce more growth than partial ranges.

Tissue Resilience#

[DRAFT — Claude]

Primarily connective. This is the load your tendons, ligaments, fascia and bone can tolerate. All of them remodel under mechanical strain, but far more slowly than muscle, because collagen turnover is slow and blood supply is poor. Tendons respond to the magnitude and duration of load rather than to rep volume or speed, while bone needs high strain rates and impact.

The practical consequence is the important one: muscle adapts in weeks and tendon in months, so progressing quickly opens a window where your muscles can generate force your tendons cannot yet tolerate. This is why injuries so often appear two or three months into a program that felt like it was going well.

How to train it: Heavy slow resistance and long isometric holds of 30-45 seconds at high intensity, two or three times per week. Leave 24-48 hours between heavy loading of the same tendon, since collagen synthesis passes through a net-negative window before it turns positive. Impact and plyometric work drive the bone adaptation. Mostly this quality is bought with patience — the correct response to tendon pain during a progression is to slow the progression, not to add volume.

Mobility & Flexibility#

These two terms are often used interchangeably, but there are clear definitions in physiology.

  • Flexibility : passive range of motion of a joint (how far it can be stretched)
  • Mobility : active controlled range of motion under load (usable range) You can be flexible but not mobile, and mobility typically matters more than flexibility for health. Flexibility itself is primarily neural (teaching the brain a position is safe) whereas mobility is both neural and physical.

[DRAFT — Claude, continuing Dylan’s section]

Static stretching does not primarily lengthen muscle tissue. It works through three mechanisms, in this order:

  1. Neural — the nervous system raises stretch tolerance, allowing more lengthening before the stretch reflex fires. This is why range improves within a single session, and why it decays without maintenance.
  2. Viscoelastic — connective tissue, fascia and tendon gradually remodel.
  3. Sarcomere — muscle adds sarcomeres in series, genuinely lengthening the tissue.

That order is also a timeline: minutes for the neural change, weeks for the viscoelastic, months for the structural.

How to train it: Train under load at long muscle lengths — full-range strength work, eccentrics, and end-range isometrics — rather than relying on passive stretching alone. Passive work will eventually reach the third tier, but loaded work gets there faster and builds the active control that turns flexibility into mobility. Short daily exposures maintain the neural component, which is the one that decays fastest.

Muscular Endurance#

[DRAFT — Claude]

Mixed muscular and cardiovascular, but importantly a local quality rather than a whole-body one. Muscular endurance is the ability to sustain or repeat submaximal contractions without the working muscle failing. It is driven by what happens inside that specific muscle: capillary density, mitochondrial density, the buffering capacity that resists rising acidity, and the transporters that clear lactate and hydrogen ions. Fibres also shift toward more oxidative profiles within the fast-twitch family.

How to train it: Higher reps of 15-30 or more, short rests of 30-60 seconds, and sustained submaximal work such as circuits and loaded carries. Because the adaptation is local it transfers poorly between movements — endurance built in your legs will not show up in your grip. Train it in the specific patterns you need it in, which is why a strong aerobic engine alone does not save you on a movement you have never practised under fatigue.

Anaerobic Capacity#

[DRAFT — Claude]

Primarily muscular and metabolic. This is the total amount of work you can produce through the glycolytic pathway, which dominates efforts lasting roughly 30 seconds to two minutes. It is driven by the activity of glycolytic enzymes, by buffering capacity from bicarbonate and muscle carnosine, and by your tolerance of the acidosis that builds when you work far above what oxygen delivery can support. It is the most unpleasant quality to train and among the fastest to fade.

How to train it: Near-maximal intervals of 30 seconds to two minutes, with work-to-rest ratios between 1:2 and 1:4 so each effort is genuinely hard. Two or three sessions per week is the ceiling for most people, because the recovery cost is high and it interferes with almost everything else on this list. Classic formats are 400-800m repeats, bike sprints, and 30-seconds-on 30-seconds-off blocks.

Aerobic Capacity#

[DRAFT — Claude]

Primarily cardiovascular. Aerobic capacity, usually measured as VO2max, is the maximum rate at which you can take in, transport and use oxygen. The main limiter is central — cardiac output, and specifically stroke volume, the amount of blood the heart moves per beat. The peripheral side contributes too: capillary density and mitochondrial density determine how much of the delivered oxygen the muscle can actually extract.

How to train it: Intervals at or near your maximum sustainable intensity, with work bouts of 3-8 minutes and roughly equal recovery, aiming to accumulate 15-30 minutes at intensity across the session. Four by four minutes is the canonical protocol and remains hard to beat. These sessions sit on top of a large base of easy aerobic volume rather than replacing it, because the base builds the peripheral half of the equation.

Long Duration Endurance#

[DRAFT — Claude]

Primarily cardiovascular, with a large muscular and metabolic contribution. This is the ability to sustain low to moderate intensity for hours, and it is a different quality from aerobic capacity — a high VO2max does not guarantee it. It depends on mitochondrial density, the capacity to oxidise fat and so spare limited glycogen, capillarisation, the efficiency of type I fibres, and on movement economy, which determines how much energy a given pace costs you in the first place. Fuelling and gut tolerance become genuine limiters at the long end.

How to train it: High volume at low intensity, with most weekly time spent easy enough to hold a conversation, plus a progressively longer single session each week. A roughly 80/20 split between easy and hard work is the durable pattern. Practise fuelling in training rather than discovering your gut’s limits on the day, and treat economy as trainable — technique work pays back at every hour of a long effort.

Where to go next#

  • The Systems — the physiology underneath these adaptations, one page per system.
  • Progression — how to actually build each quality over months and years.
  • The Systems

    The physiology underneath the adaptations - one page per system.

  • Progression

    How to actually build each quality over months and years.

The Systems Forward