A SINGLE METRIC FOR QUANTIFYING BIOMECHANICAL STRESS IN REPETITIVE MOTIONS AND EXERTIONS

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.