Center for Composite Materials - University of Delaware

Research Summary

Design of a Second Spine: A Secondary Parallel Pathway of Transferring Load from the Shoulders to the Pelvis

Authors: Joon-Hyuk Park, Damiano Zanotto, Vineet Vashista, Paul Stegall, Xin Jin and Sunil K. Agrawal


• Carrying a heavy backpack load is a risk factor for back pain, stress fractures, rucksack palsy, metatarsalgia, etc.
• Fatigue associated with disruption of gait/balance from carrying a heavy backpack reduces mission capability and performance of the soldiers.
• Our goal is to prevent musculoskeletal injuries associated with heavy load on back by designing a “Second Spine”, providing secondary load pathway parallel to the spine to transfer load from shoulder to pelvis.


• Provides a secondary pathway to transfer load from the shoulder to the pelvis
• Structurally flexible to accommodate upper body motion
• Accommodates user variability and shoulder gap
• Based on these design criteria, several design iterations were made.

Multi-Segmented Design of “Second Spine”

• Bio-inspiration: Properties similar to human spine
• Cables with nonlinear stiffness elements
• Spatial elasticity due to cable choices and routing
• Active control of tension in cables
• Manual control of base stiffness by a turn-buckle

Cone-shape joint

• Each cone-shaped segment has external cone on one side and internal cone on the other side, which becomes basically a rod when several of them are interconnected.
• The cable is routed from the belt to the top segment and the tension of the cable provides two modalities of the system: flexible and semi-rigid.


• Implementing this cone-shaped joint, modular second spine was fabricated.
• Each column has its cable tensioning mechanism at the bottom, interfacing with the belt and a turn buckle to adjust trunk height as well as air gap on shoulders.
• Multiple adjustment points on shoulder pads allowed better adjustability to fit various trunk sizes.


• Reliability tests using Instron – static compressive (250 lbs.), cyclic compressive (130 lbs.), base excitation (1-3 Hz)
• Results indicate that the device can support 250 lbs. of static load and 100 lbs. of backpack load during walking

Form, fit, and function Experiments

• Form, fit and function tests were conducted on human subjects with two loading condition (15% and 25% BW) and with or without the device
• Results indicate that the device is off-loading the shoulders (40% reduction at 25% BW) and induce less forward trunk inclination of wearer.
• Posture change (trunk tilt) induced by device makes activation of Erector Spinae(ES) more similar to unloaded walking.


This study presents the development of a “Second Spine”, a device capable of providing a secondary load pathway of transferring load from shoulders to pelvis in parallel to the spine.

Evaluation on effectiveness of the device was conducted using Instron device as well as human subject testing.
The device is shown to be effective in reducing load on shoulders and achieving body kinematics similar to unloaded walking while carrying a heavy backpack.


This work is supported by the DARPA and the Army Research Laboratory through the Warrior Web program.

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