In 2009, the operating height of approximately one fourth of
underground coal mines in the U.S. restricted mine workers to
kneeling, crawling, and/or stooping posture to perform work MSHA
2009]. The large number of knee injuries to these workers is likely
attributed to exposure to musculoskeletal disorder risk factors
(prolonged kneeling, crawling, and twisting on one's knees).
Therefore, the National Institute for Occupational Safety and
Health has investigated three different biomechanical parameters
(muscle activity of the knee flexors and extensors, pressure at the
knee, and the net forces and moments at the knee) as subjects
assumed postures common to low-seam mining, both with and without
kneepads. The postures evaluated included: (1) kneeling near full
flexion; (2) kneeling near 90 of knee flexion; (3) kneeling on one
knee; and (4) squatting. The pressure and the net forces and
moments at the knee were evaluated as subjects statically assumed
these postures. However, negligible muscle activity existed for
these static postures. Therefore, muscle activity of the knee
flexors and extensors was evaluated for each posture while subjects
performed a lateral lift that is common to low-seam mining where
they lifted a 25-lb block from their right side, transferred it
across their body, and placed it on the ground on their left side.
The results indicated that, relative to the stresses posed by other
kneeling postures, some postures had may have more detrimental
effects than others. Considering the potential impact of the three
biomechanical parameters, several key recommendations were made
regarding when it may be most appropriate to use specific postures.
Additional recommendations were also made regarding the design of
kneepads. This study investigated three biomechanical parameters
associated with knee loading that are potentially related to knee
injury risk. In order to determine the effects that commonly used
postures in low-seam mining and wearing or not wearing kneepads
have on these parameters, three objectives were met for three
kneepad conditions (no kneepad, articulated kneepad, and
nonarticulated kneepad, both of which were reported by distributors
to be very commonly used): 1. Examine the electromyographic (EMG)
responses of knee flexors and extensors during a lateral lifting
task in kneeling and squatting postures associated with low-seam
mining (Note: Static trials were excluded from this analysis
because only minimal activity was observed in these trials). It was
hypothesized that changes in posture would result in altered
muscular demands required to accomplish the lifting task, and would
be reflected by changes in the magnitudes and patterns of EMG
activity of the knee flexors and extensors. 2. For static postures
associated with low-seam mining, determine the pressures applied to
the landmarks of the knee identified during pilot testing as being
responsible for transmitting the majority of load to the knee
(patella, patellar tendon, and tibial tubercle). It was
hypothesized that the pressure and pressure distribution at the
knee would be significantly affected by wearing a kneepad and by
the simulated posture. It was further hypothesized that the type of
kneepad worn would significantly affect the pressure and pressure
distribution at the knee. 3. For postures associated with low-seam
mining, investigate the net externally applied forces and moments
at the knee and resulting joint kinematics. It was hypothesized
that significant differences will be detected in the loading
profiles between kneeling and squatting, as well as between the
low-flexion (kneeling on one knee and kneeling near 90 flexion) and
higher-flexion (squatting and kneeling near full flexion) postures.
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