# Bio-Inspired Model-Based Design and Control of Bipedal Robot
## 2 Analysis of the Problem Area  
### Dynamic Side of the Problem--Human Walking  
Human biomechanics deals with the analysis of gait as part of the movements of human beings. In order to analyze human gait, the human walking was divided into the unique atomic periodic movements called step phases (Figure 1). These phases are further divided into two primary step phases, which are made up of eight secondary step phases, referred to as BAC 1-8 (Basic Action Concept). A set of two primary or eight secondary phases constitutes a gait cycle. The cycle can be measured from the start of any phase back to the start of the same phase of the same leg. A cycle is defined by two basic parameters: the cycle time and the spatial measures of the step. Based on the parameters of the cycle, the symmetry, variability and quality of the step are further determined. It is obvious that the step quality parameters are variable for each individual [5,6]. Two primary phases of the step are identified as the stance and the swing. During the stance phase, the tracked leg is in contact with the ground at all times. In the swing phase, the tracked leg is in the air all the time. In secondary phases, it is possible to observe three basic events [5]. **The weight transfer** makes up 12% of the total gait cycle. This event can be observed in phases BAC 1 (3%) to BAC 2 (9%). During this event, the initial contact of the heel with the ground takes place and damping also occurs. Damping factors are in ascending order of total contribution: bending at the knee, rolling over the heel and elasticity of the skin, muscles and tendons in the leg. During weight transfer, there is an initial increase in the moment of force M acting on the person's center of gravity [5,6]. **The support of one leg** makes up 38% of the total gait cycle. This event can be observed in phases BAC 3 (19%) to BAC 4 (19%). During this action, there is a movement  
Figure 1: Visual representation of the gait cycle [7]. of the upper part of the body and an overall increase in stability. At the end of the BAC 3 phase, the system (human body) acquires the greatest stability during the walking cycle. At the end of BAC 4, the person's center of gravity is so deviated that it is not possible to finish the step stably. In general, single-limb stance as an event is best defined by ankle rotation [5]. **The swing** makes up 50% of the total gait cycle. This event can be observed at BAC 5 (12%), BAC 6 (13%), BAC 7 (12%) and BAC 8 (13%) phases. Three sub-events occur during this event. * Lifting the heel off the ground while keeping the toe in contact with the ground. This action is included in the BAC 5 phase. It forms the transition between the one-limbed stance and the swing itself [5]. * The movement (swaying) of the leg in front of the person's gravity center is described in the events of BAC 6 and BAC 7. The distance that the leg travels from the beginning of the BAC 6 phase to the end of BAC 7 is defined as the step length [5]. * Preparing for impact. During the BAC 8 phase, the center of gravity acquires the greatest moment of force (while reducing the acceleration) and the system (human being) prepares to absorb the impact [5,6]. Based on detailed standardized video footage of real human walking cycle considering previous movement analysis, data were experimentally collected from the figure's walking cycle, which describes the movement of the hip, knee and ankle in space and time. A virtual measurement unit for distance \(j\) was involved in this video footage. During one phase, three measurements were taken, in which the position of the joint was monitored on the x and y axes (\(\alpha\)-plane--side view of the figure) and in the z and y axes (\(\beta\)-plane--front view of the figure). The beginning of the measurement is in phase BAC 3, because the measurement also dealt with starting from the rest position, when the joints are located on one vertical axis. From the graphical representation of the data in Figures 2 and 3, the critical region and the damping region were identified.