The relative effects of repetition, force and posture were studied in
order to investigate how continuous biomechanical measurements can be
combined into a single metric corresponding to subjective discomfort.
A full factorial experiment was conducted involving repetitive wrist f
lexion from a neutral posture to a given angle against a controlled fo
rce. Seven subjects performed the task using two paces (20 and 4 motio
ns/min), two force levels (15 and 45 N) and two angles (15 and 45 degr
ees) for 1 h each. Discomfort was reported on a 10 cm visual analogue
scale anchored between 'no discomfort' and 'very high discomfort'. Rep
eated measures analysis of variance showed that all main effects were
statistically significant (p < 0.05) and no significant interactions w
ere observed. A linear regression model was fitted to the data and use
d for generating frequency weighted digital filters that shape continu
ous recordings of repetitive motions and exertions into an output prop
ortional to relative discomfort. The resulting high-pass digital filte
r had a 22 dB/decade attenuation slope. A simulated industrial task us
ed for validating the model involved repetitively transferring pegs ac
ross a horizontal bar and inserting them into holes against a controll
ed resistance. Angular wrist data were recorded using an electrogoniom
eter and filtered. Six subjects performed the task of the three condit
ions consisting of (1) 15 degrees wrist flexion, 15 N resistance and 6
motions/min, (2) 15 degrees wrist flexion, 45 N resistance and 12 mot
ions/min, and (3) 45 degrees wrist flexion, 45 N resistance and 15 mot
ions/min. Subjective discomfort was reported after performing the task
for 1 h. Pearson correlations between subjective discomfort ratings a
nd the integrated filtered biomechanical data for individual subjects
ranged from 0.90 to 1.00. The pooled correlation across subjects was 0
.67. This approach may be useful for physical stress exposure assessme
nt and for design of tasks involving repetitive motions and exertions.