We tested The Body Shelf with surface electromyography and published everything that came back — including the results that were smaller than we hoped.
Why we publish the whole dataset
It is easy to find a flattering number in a study and put it on a banner. We used to do that. The old headline — "50% less strain" — was a male-subject average, described with a word the study never measured. We have retired it.
What follows is the complete picture: what was measured, how, what every task produced, and what this evaluation cannot tell you.
What surface EMG measures — and what it does not
Surface electromyography places electrodes on the skin over a muscle and records the electrical signal it produces when it contracts. Lower signal means that muscle did less electrical work during that movement.
It is not a measurement of load in pounds on the spine, strain in any engineering sense, pain, perceived effort, total physical work, or injury risk. Those are different quantities requiring different studies.
Methodology
| Date | February 2023 |
|---|---|
| Design | Combined objective (sEMG) and subjective (questionnaire) ergonomic evaluation |
| Subjects | 11 total — 8 male, 3 female. Mean age 43 (range 17–58) |
| Loads | Sand-filled boxes, 12.5" × 12.5" × 16", at 25 lb and 50 lb. Male subjects handled both; female subjects handled 25 lb only |
| Tasks | Flat carry · lift-and-twist · lift-and-pivot · stairs without handrail · stairs with handrail |
| Comparison | Each task performed with The Body Shelf and unaided |
| Measurement sites | Back and lower-back musculature, left and right recorded separately |
| Statistical testing | None reported |
| Peer review | None. Authors, laboratory and institution are not named in the document |
Results — reduction in measured back-muscle activity
Male subjects
| Task | 25 lb | 50 lb |
|---|---|---|
| Flat carry | 51% | 36% |
| Lift and twist | 56% | 35% |
| Lift and pivot | 60% | 53% |
| Stairs, no handrail | 43% | 22% |
| Stairs, with handrail | 56% | 2.8% |
Female subjects, 25 lb
| Task | Result |
|---|---|
| Flat carry | 12.5% at back sites; 19% aggregated across measured sites |
| Lift and twist | 21% right lower back; left side showed no reduction |
| Stairs, no handrail | 36% left back |
| Stairs, with handrail | 6.5% left back |
The evaluation summarised male results as approximately 50% at 25 lb and 39% at 50 lb, without publishing the calculation behind those figures. We therefore quote the task-level range — 36–60% for male subjects across 25–50 lb loads — rather than a single number.
Subjective ratings (1–5 scale)
| Measure | With harness | Unaided |
|---|---|---|
| Overall comfort | 4.5 | 3.3 |
| Low back comfort | 4.75 | 2.96 |
| Perceived productivity | 4.6 | 3.1 |
These are self-reported in an unblinded test and are subject to expectation effects. They are still meaningful — comfort determines whether equipment gets worn on hour seven of a shift — but they are opinion, not measurement.
Three things the data actually tells you
Reductions were larger at 25 lb than at 50 lb across every male task. At heavier loads, more of the work falls outside what a passive shelf can absorb.
One-handed carries lose most of the benefit. Stairs with a handrail at 50 lb produced 2.8% — the smallest figure in the dataset. If one hand is on the rail, expect substantially less support.
Results differed by sex. Female subjects saw smaller and less consistent reductions. The evaluation hypothesised that handrail height relative to subject stature may have caused asymmetric loading. That is a hypothesis, not a finding.
Limitations
- Small sample — 11 subjects, only 3 of them female. Directional, not definitive.
- No statistical significance testing. Some portion of these differences may reflect normal trial-to-trial variation.
- Not peer-reviewed; authors, lab and institution unnamed. We are working to commission an independent, properly attributed follow-up.
- Measured muscle activity, not injury. No injury rates were tracked. Nothing here demonstrates injury prevention and we do not claim it.
- Female results were smaller and inconsistent; male-only summary figures would misrepresent the dataset.
- Only two load points tested. The suggested 20–50 lb "optimal range" is an inference, not a measured curve.
- Fit and familiarity materially affected results.
Download the evaluation
Download the February 2023 ergonomic evaluation (PDF, 291 KB)
Related reading
- A plain-English walkthrough of this evaluation
- What an ergonomic carrying harness is and how it works
- How to fit The Body Shelf — fit drives results
The Body Shelf is a passive carrying support tested to 50 lb. It does not replace safe lifting technique or employer-required controls.