The impact of fatigue on lifting kinematics with-in a middle-aged population

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University of New Brunswick
Lifting tasks have been highly associated with musculoskeletal disorders (MSDs) in the workplace (Burdorf & Sorock, 1997). Although much research has been put forth, injury mechanisms are still not fully understood. The purpose of this study was to investigate how muscle fatigue modified kinematic and neuromuscular lifting patterns during prolonged symmetrical lifting within a 40-60 year old population. Twenty-five volunteers (11 male and 14 females) performed a 75 minute sagittal lift from floor to shoulder height at a rate of 6 lifts/minute. To measure lifting kinematics and to measure neuromuscular activity of targeted muscles, electromagnetic motion tracking sensors and electrodes to record electromyography (EMG) were attached to participants, respectively. Statistical analysis was conducted using ANOVA for kinematic variables and cross correlation for relative phase (RP) waveforms. Results indicate that fatigue did occur after the 75 minute symmetrical lifting task. Perceived exertion significantly increased in both males and females (p < .001). Male maximum voluntary contractions (MVC) decreased for the shoulder by 24% and 31% for the back, respectively. Female MVC decreased for the back by 31% and only 13% for the shoulder. Also, the MPF & RMS quadrant indicated that the majority of participants experienced fatigue in the right upper trapezius and erector spinae (TS). Kinematic data showed significant pre/post changes over time. During lifting, females significantly increased angular velocity of the right shoulder rotation (p<.030) and right elbow flexion (p<.018). Males significantly increased ROM of trunk lateral bend (p<.041) and decrease elbow flexion (p<.018). RP waveforms showed significant pre/post coordination changes over time. There was no Correlation coefficient difference for males and female during lifting and lowering (p<.001) for all four relative phase waveforms. Lag time was significantly different for males and female during lifting and lowering for all four relative phase waveforms. Females exhibited earlier pattern of coordination in Forearm to Upper Arm relative phase which may predispose them to a higher risk of sustaining an upper extremity injury.