Biomechanical analysis of movement strategies in human forward trunk bending. I. Modeling

Abstract

Two behavioral goals are achieved simultaneously during forward trunk bending in humans: the bending movement per se and equilibrium maintenance. The objective of the present study was to understand how the two goals are achieved by using a biomechanical model of this task. Since keeping the center of pressure inside the support area is a crucial condition for equilibrium maintenance during the movement, we decided to model an extreme case, called "optimal bending", in which the movement is performed without any center of pressure displacement at all, as if standing on an extremely narrow support. The "optimal bending" is used as a reference in the analysis of experimental data in a companion paper. The study is based on a three-joint (ankle, knee, and hip) model of the human body and is performed in terms of "eigenmovements", i.e., the movements along eigenvectors of the motion equation. They are termed "ankle", "hip", and "knee" eigenmovements according to the dominant joint that provides the largest contribution to the corresponding eigenmovement. The advantage of the eigenmovement approach is the presentation of the coupled system of dynamic equations in the form of three independent motion equations. Each of these equations is equivalent to the motion equation for an inverted pendulum. Optimal bending is constructed as a superposition of two (hip and ankle) eigenmovements. The hip eigenmovement contributes the most to the movement kinematics, whereas the contributions of both eigenmovements into the movement dynamics are comparable. The ankle eigenmovement moves the center of gravity forward and compensates for the backward center of gravity shift that is provoked by trunk bending as a result of dynamic interactions between body segments. An important characteristic of the optimal bending is the timing of the onset of each eigenmovement: the ankle eigenmovement onset precedes that of the hip eigenmovement. Without an earlier onset of the ankle eigenmovement, forward bending on the extremely narrow support results in falling backward. This modeling approach suggests that during trunk bending, two motion units--the hip and ankle eigenmovements--are responsible for the movement and for equilibrium maintenance, respectively.

Multi-muscle synergies in an unusual postural task: quick shear force production

“…The resulting body configurations in these two panels look similar to those resulting from the eigenmovements proposed by Alexandrov and his colleagues (2001), based on decomposition of the equation of motion of a three-link inverted pendulum in two dimensions. In particular, the kinematics induced by activation of M 1 or M 2 +M 3 (see Figure 12A) is similar to the hip eigenmovement (H-eigenmovement) reported by Alexandrov et al (2001), i.e. hip flexion combined with ankle plantarflexion (cf.…”

“…"ankle strategy", Horak and Nashner 1986). Alexandrov et al (2001) showed that, during the same translation of the center of mass (COM; 1 cm in 500 ms), the A-eigenmovement was associated with an almost four times larger COP excursion than the H-eigenmovement (6.5 cm as compared to 1.7 cm). On the other hand, due to the difference in the inertial properties of the body segments (see Alexandrov et al, 2001), the A-eigenmovement is more efficient for slow motions, while the H-eigenmovement is more efficient for fast COM motions (see also Horak and Nashner, 1986;Runge et al, 1999).…”

Quantifying balance control during stance

“…Eigenvectors and eigenvalues were used to describe independent combinations of joint movements, defined by principle component analysis (Kuo et al, 1998;Alexandrov et al, 2001a;Hsu et al, 2007). From the covariance matrix of the ankle and hip angle, the first component may be interpreted as double-inverted pendulum behaviour and quantifies the amount of hip strategy.…”

“…To maintain standing balance in various situations, postural responses must adapt with task, magnitude and type of external disturbance (Alexandrov et al, 2001b;Park et al, 2004;Kim et al, 2012). Studies in the upper and lower extremities showed that the neuromuscular controller also adapted when external force fields were applied (Shadmehr and Mussa-Ivaldi, 1994;Burdet et al, 2001;Noël et al, 2008;van Asseldonk et al, 2009).…”

“…In this specific case, the inputs are the segment angles of the leg and hat, and therefore two independent disturbances must be applied that continuously challenge the balance control system (de Vlugt et al, 2003;Pintelon and Schoukens, 2012;Boonstra et al, 2013). Several studies do not apply disturbances at all (Kuo et al, 1998;Alexandrov et al, 2001b;Speers et al, 2002), or only one disturbance (Park et al, 2004;Alexandrov et al, 2005;Kim et al, 2009Kim et al, , 2012Goodworth and Peterka, 2012), which could lead to incorrect identification of the neuromuscular controller (Boonstra et al, 2013). Only a few studies investigated the multivariate nature of balance control by applying two disturbances (Fujisawa et al, 2005;Kiemel et al, 2011;Boonstra et al, 2013).…”