Prediction of shoulder flexion endurance from personal factors

Abstract With the primary purpose of improving current data bases for ergonomic guidelines, 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 dominant arm in a horizontal position in front of the body until exhaustion. This task corresponded to between 10% and 21% of the maximal shoulder flexion strength (%MVC). Regression analysis showed that endurance time ( T lim ) was significantly related to shoulder torque expressed relative to maximal capacity (%MVC), but not to absolute torque, gender or age. The resulting model was: T lim =40.609exp(−0.097[%MVC]), T lim measured in minutes. Shoulder flexion endurance was less than predicted by most previous models based on other muscle synergies and load levels. Only 30% of the total variance in T lim between individuals was explained by the model, and it is suggested that additional physiologic factors may account for, at the most, another 20%. Thus, about half of the total variance may be attributed to individual differences in psychological factors, e.g. pain tolerance, motivation and mood. The results demonstrate that pre-employment screening procedures for endurance capacity, based on readily accessible personal factors, have a low efficiency. Guidelines for occupational shoulder loads in the investigated range (10–20%MVC) should be constructed with consideration to the specific endurance properties of shoulder muscles, and they should recognize the large variability between individuals in endurance capacity, for instance by using a percentile approach as exemplified in the paper. Relevance to industry Isometric endurance has been used as the basis of several guidelines addressing industrial work tasks with increased risk for disorders or fatigue in the shoulders and neck. The paper presents extensive data on shoulder flexion endurance and discusses whether it is predictable from the personal factors gender, age, anthropometry and maximal strength. The results may guide the practitioner considering pre-employment selection of workers, and they may help in improving future ergonomic guidelines.