Axillary Crutch Biomechanics

1. Basic Design and Fitting

• Height: The axillary pad should sit one hand-width (approximately 5 cm) below the axilla - not pressed into it.
• Grip bar: Adjusted so the elbows are mildly flexed (15-30°) when the hands bear weight.
• Weight bearing point: The patient must be instructed to bear pressure against the sides of the thorax, NOT in the axilla itself - compressing the brachial plexus and radial nerve in the axilla causes crutch palsy (see Section 5 below).

2. Force Transmission and Ground Reaction Forces

• Vertical GRF during axillary crutch walking is 21-25% higher than normal gait - the upper limbs absorb forces the lower extremities normally handle.
• Swing-through gait generates peak crutch forces exceeding 100% of body weight (BW) - the highest of all gait patterns, requiring significant arm strength.
• Three-point gait generates forces up to ~70% BW per crutch during the stance phase.
• Two-point and four-point gaits keep forces below 50% BW per crutch, but are slower.

3. Crutch Gait Patterns

Three-Point Gait (most common)

• Both crutches advance simultaneously with the injured leg kept off the ground.
• The well leg then swings to ("swing-to") or past ("swing-through") the crutches.
• Used for non-weight-bearing situations.
• Fastest crutch gait but demands the most arm strength and balance.
• Stance phase starts at ~25-35% of the gait cycle.

Two-Point Gait

• One crutch advances simultaneously with the opposite extremity, then the other pair.
• Mimics the reciprocal arm-swing of normal walking.
• Used when the patient can bear partial weight on the injured limb.
• Forces remain below 50% BW; slower forward progression.

Four-Point Gait

• Sequence: one crutch → opposite leg → other crutch → remaining leg (all separately).
• Maximum stability - at least three points of contact at all times.
• Slowest pattern; used for patients with bilateral lower limb weakness who need partial weight-bearing.
• Forces remain below 50% BW; stance phase only ~15% of cycle.

Swing-To Gait

• Both crutches advance, then both legs swing up to meet them.
• Used in spinal cord injury or complete non-weight bearing on both legs.

Swing-Through Gait

• Both crutches advance, then both legs swing past the crutches.
• Fastest ambulatory speed; forces exceed 100% BW - greatest upper extremity demand.

4. Stair Climbing Biomechanics

• Ascending stairs: The intact (well) leg advances first to the next step, then the crutches and injured leg follow.
• Descending stairs: The crutches are lowered to the lower step first, then the injured leg, then the well leg.

5. Neurovascular Complications - Crutch Palsy

• Radial nerve is the most commonly injured (it courses through the axilla and then the spiral groove of the humerus).
• Results in: weakness of all extensors of the wrist, hand, and fingers below the shoulder; wrist drop; weakness of forearm supination; loss of the triceps reflex; and sensory loss in the radial nerve distribution.
• The triceps may be spared if the compression is distal enough (below where the triceps branches leave the radial nerve).
• The axillary nerve and brachial plexus can also be compressed, causing deltoid weakness and broader upper limb deficits.

6. Energy Expenditure

• The upper limb muscles (triceps, shoulder depressors, wrist extensors) are not designed for sustained weight-bearing.
• Increased GRF (21-25% higher than normal) must be absorbed by the upper extremity.
• Three-point and swing-through patterns have the highest energy cost.
• Low back loads also increase with axillary crutch use compared to normal walking, as the trunk compensates for the altered gait mechanics.

7. Comparison with Other Assistive Devices