PhD Fekete - SZIE version - 2.2 - Szent István Egyetem

3.4.2. Aims of the experiment
Materials and Methods
If the magnitude of the patellofemoral forces in the knee joint, or in the ligaments and tendons
connected to the knee joint are to be predicted, the load case (how the center of gravity lines
changes its position horizontally) must be known as well.
The different type of human motor tasks indicates many types of load transmission throughout
the knee joint. The load, derived from the bodyweight (BW), always intersects with the center
of gravity, and during the locomotion is constantly moving.
The path of the center of gravity is mostly investigated experimentally, in two-dimension
[Hasan et al., 1996] or three-dimension [Tesio et al., 2010] as a function of gait cycles.
Gait cycles can be measured as a function of walking speed [Gard et al., 2004], while in
standing case, the path is given as a function of time [Caron et al., 1997].
There are analytical methods to calculate the line of action of the center of gravity (or shortly
the center of gravity line) of the human body by taking all body parts into account
[Hanvan, 1964, Dempster, 1955]. In order to use these methods, 41 anthropometric parameters
have to be measured. On the one hand, multiple parameters make the calculation challenging,
and on the other hand, specifying the accurate position of all body parts during e.g. squatting is
also difficult. Obviously, the describing function of center of gravity depends on the motion
carried out, thus in case of gait, running, squatting, etc. the function is altered.
For the new analytical-kinetcal model, three dimensionless parameters (λ p , λ t , λ f ), two
anatomical angles (β(α), γ(α)) and the dimensionless center of gravity functions (λ 1 (α), λ 3 (α))
must be determined under non-standard squatting.
These constants and functions come from Table 3.1 and Table 3.2, but beside the motivation to
comply the analytical-kinetcal model with the necessary functions they are also meant to prove
the following hypotheses:
1. The horizontal movement of the center of gravity line changes its position during
squatting, in contrary with other assumption [Cohen et al., 2001],
2. The horizontal movement of the center of gravity line can be derived with empirical
function during squatting.
3.4.3. Description of the experimental model
In order to validate these hypotheses and gaining the necessary constants and functions for the
analytical-kinetical model, an experiment has to be carried out.
As a first step, the experiment has to be planned and measurable parameters must be appointed.
Our experimental model creation begins with the following simplifications:
a) The bones are considered as straight lines,
b) The center of gravity line goes through the hip bone, the knee joint and the ankle in case
of standing position (stance) [Szentágothai, 2006],
c) The model is quasi-static, the inertial forces are neglected during the movement,
d) Since the analytical-kinetical model is two-dimensional, only the horizontal component
(y c ) of the center of gravity line is investigated during the movement (Figure 3.7),
e) Only the bodyweight is considered (BW), which points downwards along the z-axis.
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