Human Body Models: BOSS-Models - Preventive Biomechanics - page 191

Table 5.2 Volunteer-data

Acronym M1 M2 M3 M4 F1 F2 F3 F4

Age 35 40 26 34 42 20 31 48

Mass (kg) 81 68 67 75 65 70 55 75

Fat layer (mm) 30.91 24.42 24.95 28.37 42.49 45.86 28.73 44.94

Muscle layer (mm) 57.84 53.82 50.35 61.16 44.31 52.08 42.43 43.94

was based on the hypothesis of unique indenter force transmission through both
tissue layers, cf. Fig. 5.17 b. Thus, combining the indentation force with the
displacement information of both tissue layers obtained from the MR-images,
a force-displacement relation for the skin/fat and the muscle layer was postulated.
Due to the indenter head geometry, a deformation was achieved which could be
clearly reconstructed and evaluated from the MR-images. The actual thickness of
the skin/fat layer (l i,F ) and the muscle layer (l i,M ) at each deformation step was
measured along the indentation axis, Fig. 5.17 a. With respect to the undeformed
initial lengths of each layer (skin/fat: l 0,F = 32 mm, muscle: l 0,M = 58 mm), the
actual displacement of the skin/fat layer and the muscle layer could be derived.
Assigning the measured indenter force to the corresponding tissue displacement
along the indenter axis, (long-term elasticity) force-displacement data as depicted
in Fig. 5.18 a for gluteal skin/fat and muscle tissue were obtained. The muscle
tissue exhibits a stiffer material response (in the sense of a steeper tangent line and
larger modulus of elasticity, at the origin). In contrast to muscle tissue, which
showed a moderate increase in force values at higher compression, fat tissue
tended to block, exhibiting a large force increase with a small displacement change
(from approx. 12 mm displacement in Fig. 5.18 a). The blocking phenomena may
be interpreted similarly to that of soft polymeric foam materials. The material
structure: the (incompressible) fat droplets (adipocytes) may be changed in shape
at low force (low bending stiffness of the cell walls) up to the point where the cell
wall structure starts bearing tensile force (strong increase in force). In addition, the
blocking effect can be observed in Fig. 5.16 , where at larger indenter displace-
ments, the fat layer thickness along the indenter axis shows almost no change. This
is equivalent to an almost rigid material behaviour (nearly vertical force-
displacement fat curve in Fig. 5.18 a). Muscle tissue, in contrast, shows gradual
force progression, with stiffening at comparably larger strain.

5.2.2.4 Biological Variability

Comparing the separated force-displacement curves for human gluteal fat and
muscle, Fig. 5.18 a, with the separated force-displacement data from male vol-
unteers (M1-M4) and female volunteers (F1-F4), Fig. 5.18 b, shows good quali-
tative agreement. The curves depicted in Fig. 5.18 b show that fat from males
exhibits a stiffer initial response (larger modulus of elasticity at the origin)