One-Handed Pruning Tool for Older Gardeners with Arthritis
A community garden group needs a one-handed pruning tool for members aged 65–85, many with arthritis. Existing secateurs require 18–22 N of grip force to cut 8 mm stems; users report hand pain and drop the tool. The new tool must cut green stems up to 12 mm, weigh under 350 g, work in rain with gloves, and retail under NZ$45. Production will use injection-moulded glass-filled nylon handles and a stainless steel blade. Current tools have a 180 mm handle and a 45 mm blade. The group meets twice weekly; members have varied hand sizes, two use wrist braces, and the tool must suit either hand. You have a 3D printer for prototypes and access to five garden members for testing.
1. Propose a mechanism or form change to reduce grip force, and explain the ergonomic reasoning.
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Use a ratchet or geared compound lever rather than a direct single-pivot lever. With a single pivot, force at the blade is roughly handle force × (handle length ÷ blade arm). Lengthening the handle from 180 mm to 220 mm reduces required grip force by about 18%, but a 220 mm handle is already at the limit for a one-handed tool for this group. A ratchet mechanism lets the user make two or three short squeezes, each at a comfortable 8–10 N, to complete one cut. The ratchet holds the blade position between squeezes, so peak force drops even though total work is similar. Ergonomically, this suits arthritic hands because it avoids sustained maximum grip and allows rest between strokes.
Form: use a closed power-grip handle with a 95–105 mm circumference, a soft thermoplastic elastomer overmould, and a 15° forward angle to keep the wrist neutral. Add a large, rounded trigger area rather than a narrow finger groove, and a wrist strap to prevent dropping. The blade should be bypass type with a small hook to hold the stem, reducing the need to squeeze hard to stop slipping. Test the ratchet against a geared cam; the ratchet is simpler to mould and clean.
2. Explain how you would test the prototype with the target users, including what you would measure and how you would decide.
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Recruit the five garden members plus, if possible, two more with wrist braces. Use a within-subject test: each participant cuts 10 stems of 8 mm and 10 of 12 mm, dry and wet (gloves on), with the current tool and two prototypes. Randomise order to reduce learning effects. Measure: peak grip force with a dynamometer (or a force-sensing handle), number of successful clean cuts, time per cut, number of drops, and pain/discomfort on a 0–10 scale immediately and 10 minutes after. Also record subjective preference and any skin marking.
Decision rule: a prototype is viable if median peak force is ≤12 N for 12 mm stems, no increase in drops versus the current tool, and pain ratings at least 2 points lower. If a prototype reduces force but increases time per cut by more than 50%, reject it: the group values speed for repetitive work. Test in the actual garden, not a lab, because wet stems and gloves change friction. Iterate the handle circumference if any participant reports slipping; the 3D printer lets you change the overmould in a day.