Civil Engineering Reference
In-Depth Information
11.2.6.2 Muscle and Tendon Strain
Muscle appears to be the structure that has the lowest tolerance in the musculoskeletal system. The ulti-
mate strength of a muscle has been estimated at 32 MPa (Hoy et al., 1990). In general, it is believed that
the muscle will rupture prior to the tendon in a healthy tendon (Nordin and Frankel, 1989), since tendon
stress has been estimated at between 60 and 100 MPa (Nordin and Frankel, 1989; Hoy et al., 1990).
Hence, as indicated in Table 11.1, there is a safety margin between the muscle failure point and the
failure point of the tendon of about twofold (Nordin and Frankel, 1989) to threefold (Hoy et al., 1990).
11.2.6.3 Ligament and Bone Tolerance
Ligaments and bone tolerances within the musculoskeletal system have also been estimated. Ultimate
ligament stress has been estimated at approximately 20 MPa. The ultimate stress of bone varies depend-
ing upon the direction of loading. Bone tolerance can range from as low as 51 MPa in transverse tension
to over 190 MPa in longitudinal compression. Table 11.1 also indicates the ultimate stress of bone loaded
in different loading conditions.
A strong temporal component to ligament recovery appears to exist. Solomonow has found that liga-
ments require long periods of time to regain structural integrity and compensatory muscle activities are
recruited (Solomonow et al., 1998, 1999, 2000, 2002; Stubbs et al., 1998; Gedalia et al., 1999; Wang et al.,
2000; Solomonow, 2004). Recovery time has been found to be several fold the loading duration and can
easily exceed the typical work-rest cycles observed in industry.
11.2.6.4 Disc
Endplate and Vertebrae Tolerance
The mechanism of cumulative trauma in the disc is thought to be related to repeated trauma to the ver-
tebral endplate. The endplate is a very thin (about 1 mm thick) structure that facilitates nutrient flow to
the disc fibers (anulus fibrosis). Repeated microfracture of this vertebral endplate is thought to impair the
nutrient flow to the disc fibers and thereby lead to atrophy and degeneration of the fiber. It is believed
that if one can determine the level at which the endplate experiences a microfracture, one can then mini-
mize the effects of cumulative trauma and disc degeneration within the spine. Several studies of disc end-
plate tolerance have been performed. Figure 11.10 shows the levels of endplate compressive loading
tolerance that have been used to establish safe lifting situations at the worksite (NIOSH, 1981). This
figure shows the compressive force mean (column value) as well as the compression force distribution
(thin line and normal distribution curve) that would result in vertebral endplate failure (microfracture).
This figure indicates that for those under 40 years of age endplate microfracture damage begins to occur
at about 3432 N, of compressive load on the spine. If the compressive load is increased to 6375 N,
approximately 50% of those exposed to the load will experience vertebral endplate microfracture. When
/
TABLE 11.1 Tissue Tolerance of the Musculoskeletal System
Estimated Ultimate Stress (s
u
)
(MPa)
Structure
Muscle
32-60
Ligament
20
Tendon
60-100
Bone longitudinal loading
Tension
133
Compression
193
Shear
68
Bone transverse loading
Tension
51
Compression
133
Source: Adapted from Ozkaya and Nordin, Fundamentals of Biomecha-
nics, Equilibrium, Motion and Deformation, Van Nostrand Reinhold,
New York, 1991. With permission.
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