Date Published: November 16, 2018
Publisher: Public Library of Science
Author(s): Cannan Yi, Kai Way Li, Fan Tang, Huali Zuo, Liang Ma, Hong Hu, Dominic Micklewright.
Truck pulling is one of the common manual materials handling tasks which contribute to musculoskeletal disorders. The maximum endurance time (MET) for two-handed truck pulling tasks has been rarely discussed in the literature. The objectives of this study were to explore the development of muscular fatigue when performing two-handed pulling task and to establish models to predict the MET. A simulated pallet truck pulling experiment was conducted. Sixteen healthy adults including eight females and eight males participated. The participants pulled a handle simulating that of a pallet truck using two hands until they could not pull any longer under two postures. The forces applied for females and males were 139.65 N and 170.03 N, respectively. The maximum voluntary contractions (MVC) of the pulling strength both before and after the simulated pull were measured. After each trial, both the MET and subjective ratings of muscular fatigue on body segments were recorded. The results showed that posture significantly affected MVC of pull both before and after the trial. It was found that foot/shank of the front leg had higher subjective ratings of muscular fatigue than the other body segments. The MET equations employing both power and logarithmic functions were developed to predict the MET of the two-handed pulling tasks. Predictive models established in this study may be used to assess the MET for two-handed pulling tasks.
Manual Material handlings (MMH) are common at workplaces. They contribute to the occurrence of work-related musculoskeletal disorders (MSDs) . Carts, trolleys, and pallet trucks are commonly used materials handling aids . These aids are either pushed or pulled manually. A survey  conducted in automotive supply sectors showed that approximately 10% of all working processes involved pushing or pulling. Forty one percent of the materials handled in one transport were between 200 kg and 1000 kg. In the USA, 20% of all industrial back injuries were associated with pulling or pushing tasks . Repetitive force exertion, overloading, long time exposure, and unnatural posture when performing the MMH tasks have been recognized as the main causes of the MSDs .
A simulated pallet truck pulling experiment was conducted in the laboratory of the Hunan Institute of Technology in China. This study was approved by an Ethical Review Committee of the Institute. The temperature and humidity during the experiment were 17.35°C (SD = 4.02) and 87.45% (SD = 11.68), respectively.
To verify MET models, comparison between proposed models and existing models are usually adopted [26, 27, 35]. In our two-handed pulling task, the participants terminated their trials mainly due to pains on feet/shanks and hands in their two-handed pulling tasks based on the results of the CR-10 (Table 2). In the static one-handed pallet truck pulling task , however, the participants stopped their pulling task mainly due to pains on hand/wrist and elbow. All the hand models and elbow models in the literature [20, 28–32] underestimated the MET for one-handed pulling task in a previous study . The predicted MET estimated by the joint-based models  and back/hip models [28, 30] provided better fit than those of the general and upper limb models in the one-handed pulls in the literature . Normally, two-handed pulling is adopted when the workers feel hard to pull using one hand. People could pull more easily at the same load and sustain a longer time when pulling with two hands. The hand and elbow models [20, 28–32] may not fit the two-handed pulling tasks since they were established using one-handed pulling data.
A simulated two-handed pulling experiment was performed. We found that posture significantly affected MVCbefore and MVCafter but was insignificant to MET. Feet/shanks were the body segments most likely to suffer muscular fatigue for the two-handed pulling, followed by hands and other body segments. MET models were obtained for females and males, respectively. The MET models are beneficial for job design and work-rest scheduling for workers where static two-handed pulling tasks are commonly performed.