Muscle Group Assembly — Full Arm Build

Overview

A muscle group is a bundle of multiple fascicles working together to actuate one joint movement (e.g., biceps for elbow flexion). All fascicles in a group share a mechanical termination but have independent or grouped electrical control.

This document covers the full arm + forearm assembly with 2 control variants for comparison.

Arm Muscle Groups

Total: 4 muscle groups, 12 fascicles

Mechanical Assembly

Bundling Multiple Fascicles

Each muscle group contains N fascicles that:

1. Stay separate along their length — individual PET sleeves maintain independence
2. Interweave cores at the LOAD END — mechanical merging for shared pull point
3. Share one ferrule — single termination point for the entire bundle

SIDE VIEW (muscle group):
                    
Fascicle 1 ──┐
Fascicle 2 ──┼──► [INTERWEAVE ZONE] ──► [SHARED

## Updated Multi-Tendon Architecture (May 2026)

The muscle group architecture has evolved from a single bundled actuator into a layered biomechanical system.

Each muscle group now contains:

1. Active fascicles
- TCP contraction elements
- Nylon + nichrome + Kevlar/Aramid

2. Elastic return tendons
- 0.15 mm TPU elastic cords
- Sparse helical Kevlar wrap
- Passive return force during cooling

3. Rigid limiter tendons
- Pure Kevlar/Aramid structural tendon
- Prevents over-contraction and creep
- Acts as a failsafe load path

4. Side ligaments
- Woven aramid ligament panels
- Joint stabilization
- Hyperextension prevention

## Updated Material Specifications

| Component | Specification |
|---|---|
| Nylon fiber | 0.1 mm monofilament |
| Nichrome wire | 0.08 mm |
| Kevlar/Aramid thread | 40s/2 sewing thread |
| Polyester thread | 0.1 mm stabilization thread |
| TPU elastic tendon | 0.15 mm |
| PET sleeve | 2-4 mm depending on fascicle scale |

## Fascicle Scaling Revision

The previous 6 mm PET sleeve architecture is now considered oversized for single fascicles.

New scaling:
- 2 mm PET → compact fascicle
- 3 mm PET → medium fascicle
- 4 mm PET → multi-fascicle trunk

This reduces:
- internal fiber migration
- uneven heating
- contraction inconsistency
- hotspot formation

## Mechanical Load Separation

Critical rule:

Nichrome must NEVER carry structural load.

Load path:

Kevlar/Aramid + Nylon → Ferrule → Bone Anchor

Electrical leads should remain mechanically isolated from tension.

## Bone Attachment Revision

The system now favors biomechanical-style potting and mechanical capture instead of surface gluing.

Preferred methods:
- ferrules
- trapped knots
- embedded anchor cavities
- pins
- epoxy potting with geometric locking

Avoid relying on smooth adhesive joints alone.

## Cooling Bottleneck Note

Mechanical architecture is now significantly more mature than thermal architecture.

The major remaining limitation is cooling speed and thermal cycling.

Future upgrades may require:
- airflow channels
- aluminum heat spreaders
- staggered activation timing
- phase-separated muscle groups
- localized forced cooling

[SHARED FERRULE] ──► LOAD
Fascicle 3 ──┘

FRONT VIEW (bundle cross-section):
┌─────┬─────┬─────┐
│ F1  │ F2  │ F3  │  ← 3 separate fascicle tubes
│  ○  │  ○  │  ○  │  ← cross-section of cores inside
└─────┴─────┴─────┘
     ↓         ↓
  (interweave at load end)
     └────┬────┘
       FERRULE

Ferrule Application

1. Interweave the nylon+kevlar cores at the load end (overlap ~20-30mm)
2. Slide ferrule (0.8/1.8 round aluminum) over the interweaved cores
3. Crimp with pliers — firm, even pressure