Prediction of shoulder flexion endurance from personal factors

With the primary purpose of improving current data bases for ergonomic guid elines, isometric shoulder flexion endurance was investigated in a healthy population of 20 males and 20 females (age: 20-55 years, height: 1.53-1.90 m, weight 48-106 kg). The subjects were encouraged to hold the straight dom inant arm in a horizontal position in front of the body until exhaustion. T his task corresponded to between 10% and 21% of the maximal shoulder flexio n strength (%MVC). Regression analysis showed that endurance time (T-lim) w as significantly related to shoulder torque expressed relative to maximal c apacity (%MVC), but not to absolute torque, gender or age. The resulting mo del was: T-lim = 40.609exp( - 0.097 [%MVC]), T-lim measured in minutes. Sho ulder flexion endurance was less than predicted by most previous models bas ed on other muscle synergies and load levels. Only 30% of the total varianc e in T-lim between individuals was explained by the model, and it is sugges ted that additional physiologic factors may account for, at the most, anoth er 20%. Thus, about half of the total variance may be attributed to individ ual differences in psychological Factors, e.g. pain tolerance, motivation a nd mood. The results demonstrate that pre-employment screening procedures f or endurance capacity, based on readily accessible personal factors, have a low efficiency. Guidelines for occupational shoulder loads in the investig ated range (10-20%MVC) should be constructed with consideration to the spec ific endurance properties of shoulder muscles, and they should recognize th e large variability between individuals in endurance capacity, for instance by using a percentile approach as exemplified in the paper.