# SAFETY — Read This First

## The short version

1. Optocoupler/relay isolation between Arduino and stimulator. Always.
2. TENS/FES electrodes go on skin, nowhere near the chest or head.
3. Have a second person present for the first session.
4. 10 minutes first session. Not 30. 10.
5. Serial console `STOP` command kills everything instantly.

## Why electrical isolation matters

The Arduino ground and your body's ground must never be the same conductor during stimulation. The optocoupler board creates an optical barrier — your skin sees stimulator current, which is designed for this. It does NOT see logic ground from a wall-powered computer. A ground loop through your body is not dangerous at mA levels but causes unexpected channel firings. The $9 board prevents this completely.

## Electrode placement for quads

```
RIGHT leg:
  Electrode 1 (motor point): mid-thigh, 4cm below inguinal crease, on rectus femoris
  Electrode 2 (distal):      3cm above patella

LEFT leg:
  Mirror of right.
```

Feel for a visible contraction before attaching to the bike. If you see the leg kick → placement is good. No kick → move electrode 1 up or down 2cm and try again.

## Current settings to start

- Frequency: 35 Hz (smooth contraction, not painful twitch)
- Pulse width: 300 μs
- Current: start at 20mA, increase by 5mA until visible contraction
- Most quads activate at 40–80mA range
- Do not exceed 120mA on surface electrodes

## Sessions

Week 1–2: 10–15 minutes, 3×/week. Let skin adapt. Watch for redness.
Week 3–4: 20 minutes, 4×/week.
Week 5+:  30 minutes, 5×/week. This is the BDNF dose threshold.

## Signs to stop immediately

- Skin irritation under electrodes (reduce current 10mA or reposition)
- Muscle cramps that don't release at crank stop (check stimulator timeout)
- Any chest discomfort (electrodes must not be above the waist on the torso)
- Arduino serial shows "TIMEOUT" — movement has stopped, all channels killed automatically

## What you're actually doing

You are driving the same lumbosacral CPG circuits that fired during 15 years of riding.
The signal pathway is different — external instead of corticospinal.
The destination is the same.
Every pedal stroke is a Hebbian event. Fire together, wire together.
30 sessions is where the BDNF data starts accumulating in the literature.
You already know what 10,000 sessions feels like.
Start at 1.