# How Do Humans Recognize the Motion Arousal of Non-Humanoid Robots?
## 3 Theory and Hypothesis
### Classification and Research Gaps of NHRs  
Based on the theory of physical motion constraints [45], robots can be divided into two types; see Figure 1. This classification is often used in 3D animation: by guiding actions through only two constraints, the movements of all living beings can be restored. At the same time, NHRs that utilize the same constraints exhibit similar motion characteristics and commonalities.  
Type one is primarily guided by IK (Inverse Kinematics) constraints to display movement. The application of inverse kinematics is mainly used in robotic arms or limb endpoints. The movement of this type of NHR is object-oriented, with the components at the mechanical endpoints taking the lead in movement. For instance, human arms are primarily led by the hands at their ends, moving towards objects they intend to touch as targets. Robotic arms operate similarly. For such NHRs, their movement structure always connects with the supporting structure, like a base, but their movement does not affect the supporting parts. This is analogous to the way the human body remains still and unaffected while the arm moves. Therefore, robotic arms and similar non-humanoid robots are relatively free and flexible in movement, with the ability to start and stop randomly.  
Type two is guided by FK (forward kinematics) constraints and mainly applied to the main structural parts of robots. The movement of this type of robot is interconnected with, and influences, the surrounding structures. For instance, the human body's movement adheres to the principles of forward kinematics, where the movement of the body necessarily impacts the movement of the limbs and head. In this type of NHR, when overall movement is required, the main structure of the robot acts as the decision-maker. Taking a spider-like robot as an example, for it to move as a whole, all its legs must follow the movement decisions made by the body, which serves as the central decision-maker.  
Correspondingly, when this type of machinery needs to move as a whole, special situations arise due to its structure. For example, the robot must consider the weight and other influences of its connected structures. It cannot move as freely and flexibly as a robotic arm guided by IK constraints. Generally, a movement is composed of a starting point and an endpoint. However, for NHRs dominated by FK constraints, a special case can occur: a single movement may require a series of continuous actions with coexisting opposite directions. For instance, in a holistic jumping movement, a spider-like robot does not have components that can support it to stop at any height. After jumping, it is subject to the influence of weight and gravity and needs to fall to the ground for support.  
Figure 1: Classification of NHR by kinematics constraints.  
At this time, their movement amplitude is not simply composed of a \"starting point\" and an \"end point\", but consists of two sections of movement: the \"jump segment\" and the \"falling segment\", which include three nodes: \"starting point-midpoint-end point\". There has been no discussion about human perception of this type of movement. In this case, is the concept of amplitude still applicable in terms of its impact on arousal?  
Therefore, the first key point explored in this study is the following: For NHRs guided by FK constraints, when the robot's action expression is influenced by factors such as weight and gravity, can amplitude also be considered as an element expressing arousal?  
The second key point primarily investigates whether NHRs with both types of constraints guiding their movement can also express arousal through amplitude. NHRs that are simultaneously dominated by both IK and FK constraints have a main structure that controls the overall movement and can also perform independent movements of their limbs, representing a complex type that combines both characteristics.