Chapter 6

Bones, joints, and movement

Where your mechanics is most useful — and most dangerous

Human Biology for the Medical Maker

Why this chapter exists

What transfers directly

Joints are pivots; muscles apply forces
Tendons are tension members
A free-body diagram of a forearm is a real one

What breaks the simple model

Centres of rotation move
Muscle force depends on length and speed
Moment arms change through the motion
Comfortable range is not maximum range

Bone is a living composite

Collagen matrix (tough, tensile) plus hydroxyapatite mineral (stiff, brittle)
Cortical bone: dense shell carrying bending and torsion
Trabecular bone: a lattice aligned with habitual load paths
Cells continuously build, resorb, and sense the structure

Correct in a bridge, wrong in a body

Bone responds to how it is loaded. A stiffer implant carries more load — so the bone carries less, and weakens.

Stress shielding, the practical consequence of remodelling

Mechanical properties, for intuition only

Material	Elastic modulus	Note
Trabecular bone	0.01 to 3 GPa	Varies over two orders of magnitude
Tendon	1 to 2 GPa	Non-linear; compliant toe region
PLA, printed / acetal	2 to 4 GPa	Printed parts weakest between layers
Cortical bone	15 to 20 GPa	Anisotropic
Aluminium 6061	about 69 GPa	About 4 times cortical bone
Ti-6Al-4V	about 110 GPa	Closest common metal to bone
Stainless 316L	about 200 GPa	About 10 times cortical bone

Bone is anisotropic — and so is almost everything you make.

The skeleton you actually need

Do not memorise 206 bones
Axial: skull, vertebral column, thoracic cage
Appendicular: girdles and limbs
Learn your project's region in detail

The upper limb

Clavicle: the only bony strut to the trunk
Scapula glides on the thorax — much of "shoulder" movement
Radius is lateral, thumb side, and rotates
Ulna is medial; its olecranon is the point of the elbow
Eight carpals; the thumb's saddle joint gives opposition

The elbow is not one joint

Humeroulnar: the main hinge for flexion and extension
Humeroradial: also takes part in flexion and extension
Proximal radioulnar: rotation for pronation and supination
All three share one capsule

"This model omits forearm rotation" takes four seconds to say and prevents a wrong mental model.

Anatomy of a synovial joint

Articular cartilage: smooth, low friction, compressible
Capsule and synovial membrane secreting fluid
Plus ligaments, muscles, bursae, menisci, sensory nerves
Friction coefficient roughly 0.003 to 0.02

What a metal hinge silently claims does not exist

Cartilage, capsule, and synovial fluid
Ligamentous constraint
Active muscular stabilisation
Sensory feedback
A centre of rotation that moves through the range

Each omission is a claim your model makes unless you say otherwise.

The six synovial joint types

Plane: sliding — intercarpal
Hinge: one axis — elbow, finger joints
Pivot: rotation — radioulnar, head shake
Condyloid and saddle: two axes — knuckle, thumb base
Ball-and-socket: three axes — shoulder, hip

Mobility is bought with stability — the shoulder versus the hip.

Describing motion precisely

Flexion decreases the joint angle; extension increases it
Abduction away from the midline; adduction toward it
Supination: palm up — carrying soup
Pronation: palm down
Opposition: thumb pad to fingertip

Active, passive, and functional range

Term	Meaning
Active range (AROM)	What the person achieves with their own muscles
Passive range (PROM)	What the joint permits when moved by an external force
Functional range	What is used comfortably and repeatedly for a real task

Being able to move a model through an angle tells you nothing about a person.

Typical adult ranges — orientation only

Joint and motion	Approximate range	Caveat
Elbow flexion	0 to 145 degrees	Many daily tasks use about 30 to 130
Forearm pronation and supination	80 to 90 degrees each	Measured with elbow flexed
Wrist flexion / extension	70 to 80 / 60 to 70 degrees	Tasks use far less
Shoulder flexion	0 to 180 degrees	Includes scapular movement
Knee flexion	0 to 135 degrees	—

Design for less than the person's maximum — and have a therapist measure them.

Muscle: how force is actually produced

The motor is the muscle; the tendon transmits

From whole muscle to filaments

Muscle, fascicle, fibre, myofibril, sarcomere, actin and myosin — calcium triggers, ATP powers

The fact a maker needs most

The motor is the muscle. The tendon is a tension member that transmits the force. They are not the same component.

The single most common misconception a simplified joint model creates

Types of contraction

Type	What happens	Example
Isometric	Force, no change in length, no movement	Holding a bag still
Concentric	Muscle shortens while producing force	Lifting the bag
Eccentric	Muscle lengthens while producing force	Lowering it slowly; going downstairs

Holding still is work. Sustained isometric holding is strongly fatiguing.

Muscle is not a linear actuator

Muscles work in teams

Role	Function	Elbow flexion example
Agonist	Produces the movement	Biceps, brachialis
Antagonist	Opposes and controls it	Triceps
Synergist	Assists and refines	Brachioradialis
Stabiliser	Holds the proximal segment steady	Scapular muscles

Co-contraction stiffens a joint for precision — and costs energy.

Engineering bridge: levers and statics

Torque, moment arms, and why internal forces are large

Torque and the moment arm

Torque = force times perpendicular distance to the line of action
Load: 2 N at 0.20 m gives 0.40 N·m
Cable at 0.04 m must pull 10 N
Halve the moment arm: 20 N

The moment arm is not the distance along the beam.

Including the limb's own weight

about 64 N

muscle force to hold a 2 N object with the forearm level

The forearm's own weight contributes nearly three times the torque of the object held.

Lever classes: the body chose speed and range

Class	Arrangement	Body example
First	Fulcrum between effort and load	Nodding the head
Second	Load in the middle — force multiplier	Standing on tiptoe
Third	Effort in the middle — speed and range	Biceps flexing the elbow — the commonest

"A terrible mechanical arrangement" is a correctly identified trade-off with its purpose missed.

Grip, strength figures, and percentiles

Power, hook, precision, key, tripod, and spherical grips
Maximum is not sustainable — design for as little force as possible
Your users may be nowhere near the population figure
Percentiles are per dimension: there is no 95th percentile person
Design for a range; adjustability usually beats one optimised size

Devices, software, and training technology

Clinical devices

Joint replacements; fracture fixation
Orthoses, prostheses, mobility aids
Assistive equipment — the classic maker-and-therapist collaboration
Exoskeletons; dynamometers and goniometers

Software and training

OpenSim musculoskeletal modelling; pose estimation
Orthotic and prosthetic CAD, scan to fabrication
Articulated models, fracture trainers, arthroscopy simulators
Instrumented movement and strength rigs

The line to watch is load bearing

The moment an object supports weight, restricts a joint, or applies force to a limb, it has become a prescribed clinical device.

"Could you make it a bit stronger so they can lean on it?" is a change of project.

Worked example: moving the cord attachment

Move the attachment from 40 to 20 mm: force roughly doubles
Less cord travel for the same sweep — force traded for range
The model feels strongest at a different angle
Unlabelled, it teaches "the cord is the muscle"
Label: input force = muscle action; cord = tendon; pivot = simplified joint

Workshop exercise and exit criteria

Build the two-strip model; predict, then pull
Vary the attachment and record force and sweep
Calculate cord force for a 2 N load, then add segment weight
List at least eight anatomical features the model omits
Write a functional brief for a grip aid — without choosing a material

What this chapter does not establish

You cannot assess range of motion
You cannot specify forces for a person — order of magnitude only
You cannot design load-bearing or motion-restricting devices unsupervised
Strength and range tables are not design values
Your model is one honestly labelled relationship, not an anatomy reference

Next — Nerves, sensation, and accessible design.