- Students of physics define acceleration as the rate of change of velocity (or speed) with respect to time. Expressed mathematically, acceleration is the time derivative of speed or velocity.
- In turn, the time derivative of acceleration is “jerk”. For further time derivatives after jerk, the words are, “snap”, “crackle” and “pop” for each successive derivative.
- Force, another familiar concept of physics, is measured in units of mass times acceleration. Unlike velocity and acceleration, however, time derivatives for force have never been defined.
- Now, biologists and biomedical engineers are proposing to define a new term, “yank”, for changes in force over time. Their ideas are published in the Journal of Experimental Biology.
- The objective is to quantify something that our muscles and nerves can feel and respond to. Scientists who study sports often use the term “rate of force development”, a measure of explosive strength.
- Also, scientists who study gait and balance in animals and humans often analyse how quickly forces on the body change. “Yank” could be useful in understanding spasticity, a common muscle impairment in multiple sclerosis, spinal cord injury, stroke and cerebral palsy.