Movement starts in the brain
Muscles don't contract on their own. Every movement is initiated by the nervous system. Strength gains in the first 4-6 weeks of training are almost entirely neural — your muscles haven't gotten bigger yet, your brain has just gotten better at using them.
Central nervous system (CNS)
Brain + spinal cord. Origin of voluntary movement commands. The motor cortex sends signals down the spinal cord, out through nerves, to muscles.
Fatiguing the CNS (heavy max-effort lifting, intense neural demand) takes longer to recover from than peripheral muscle fatigue. This is why a max-effort 1RM day shouldn't be followed by another max day for 48-72+ hours.
Peripheral nervous system (PNS)
Everything outside CNS. Includes:
- Motor nerves (signals OUT to muscles)
- Sensory nerves (signals IN from skin, muscles, joints)
- Autonomic system (heart rate, digestion — not under voluntary control)
Motor units
A motor unit = one motor neuron + every muscle fiber it innervates.
- Small motor units: few fibers, slow-twitch (Type I), recruited first for light loads
- Large motor units: many fibers, fast-twitch (Type II), recruited last for heavy loads
This is why training to failure with light weight CAN build strength and hypertrophy — eventually the small units fatigue and the large ones get recruited. But it's less time-efficient than just lifting heavy.
Proprioception
Your sense of where your body is in space. Comes from sensors in:
- Muscle spindles (detect stretch)
- Golgi tendon organs (detect tension)
- Joint capsules
- Skin
- Inner ear (balance)
Practical implications
- A novice's first 6 weeks of "strength gains" are 80%+ neural
- Max-effort lifts hit the CNS hard; recover 48-72h
- Balance/proprioception training is non-negotiable for older adults
- Skill rehearsal (perfect-form light reps) trains the neural pattern for heavy work later