ARUP; ISBN: 978-0-9562121-5-3 - CMBBE 2012 - Cardiff University

ehavior [5]. From the viewpoint of disorder’s incidence, labor loss, and cost, workrelated
LBP is a very important occupational health area. In the U.S.A., LBP claims are
the most common category of workers’ compensation losses, accounting for 15% to
25% of all claims and up to 40% of costs [6, 7]. In Korea, work-related LBP was
reported about 15% of all work-related disorders for which workers’ compensation is
claimed, since the mid-1990s [8]. Work-related musculoskeletal disorders (WMSDs)
are a type of functional disorder caused by external factors such as occupational
repetitive movements, overexertion, awkward postures, and vibration. According to
worker’s compensation statistics in Korea, the occurrence of WMSDs increased from
124 cases in 1998 to 4,532 in 2003 and then gradually decreased from 4,112 cases in
2004 to 2,901 in 2005. In 2006, the figure increased again to 6,233 cases; however, this
was largely due to the inclusion of accidental low back pain as a type of WMSDs [9].
When a person lifts a load, the mass of his/her upper body and the load to be lifted
induce torques at various joints of the body. When we stand, the lower back is
functioning to support the weight of the upper body. Movements in the lumbar spine,
including flexion and extension, are governed by a complex neuromuscular system
involving both active and passive components, where muscles are active components
and vertebral bones, intervertebral disks, ligaments, tendons, and fascia are passive
components [10, 11]. Stooped, restricted, kneeling, and other awkward postures adopted
during manual materials handling have frequently been associated with LBP onset [12].
There is a lot of research that shows that sustained stooped postures and poor movement
patterns cause stresses and strains that may have something to do with getting LBP.
The discs at levels lower lumbar discs L4/L5 and lumbosacral joint (L5/S1) are the sites
that appear to be most associated with clinical problems and the development of spinal
diseases. Furthermore, elucidating how the posture or loading mode influences the
biomechanical behavior at such level is also interest to researchers [2].
For biomechanical studies at the spine the loads are an essential factor. The spine is
stabilized by muscle forces. Thus, the muscle forces are an important factor for the
spinal loading [13]. Muscle forces have been estimated form some standard exercises
like standing, flexion and extension of the upper body, and lifting a load [14, 15].
Difference approaches have been used in these studies. Some groups used mathematical
models with and without EMG measurements. There is evidence to suggest that EMG
differences exist between patients with back pain and healthy subjects during dynamics
flexion tasks performed at peak flexion [16]. To this extent, several studies have
examined the apparent myoelectric silencing of the low back extensor musculature
during a standing to full trunk flexion maneuver or the flexion-relaxation phenomenon
(FRP). The FRP is modulated by a number of factors including the magnitude of
applied load, loading rate, and patient clinical status.
For the last several decades, despite the growing advancement in mechanization and
automation, MMH has attracted great interest from researchers in many disciplines,
since huge amount of work and financial losses, and human suffering caused by LBP
and injuries. Consequently, it is a major concern to researchers and organizations to
predict, control, and prevent such injuries [17, 18]. One approach to LBP prevention
involves the study of the forces acting on the spine and related muscles during MMH.
The main assumption underlying this approach is that the risk of injury to the low back
associated with MMH is causally related to spinal loading [19]. This study focused to
biomechanically analysis younger Korean male and females lumbar muscle forces and