Impact biomechanics
When people get hit or hit things the transient forces involved are normally high and produce mechanical shocks when compared to active muscularly directed human motion. In these situations standard biomechanical methods may not be applicable. In biomechanical calculations of forces the human body is normally considered to be a series of rigid links connected by simple rotational joints, however people are not made of rigid links and there can be times when this assumption is valid and times when it is not.
Consider the following list of activities that can occur within the domain of human movement and when rigid body modeling is applicable and when it is not.
- Passive low acceleration swing motions of an end segment when only proximal joints are active, such as the hand when only rotation about the shoulder joint is occurring, have almost no measurable intra-segment motion.
- Active low acceleration motions such as reaching or quite breathing have muscle shape changes and segment volume changes associated with them. Here some intra-segmental motion can be seen but it is not great.
- Active high acceleration motions involving large muscle groups and power transfer across segments, such as baseball pitching, have large intra-segmental motions associated with them.
- Controlled impacts such as landing from a drop or running show intra-segmental motion of up to 70 mm relative to the underlying bone
- The most extreme case of intra-segmental soft tissue motion is in uncontrolled impacts. Being hit unexpectedly in the abdomen will give anything but rigid body motion of the torso.
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