Stiffness: What is it?

Usually when you hear the term “stiffness” in strength training you conjure up negative associations, for example tight muscles, poor posture, and restricted movement. Another type of stiffness can occur when you encounter something like this (see pic) but that kind of stiffness is not appropriate for this blog.

In the world of sprint and jump biomechanics, “stiffness” and “compliance” refer to the amount of deformation of an object (ie: muscles, tendons, fascia, etc.) in relation to the amount of force acting on the object. Stiff materials are hard to deform while compliant materials are easy to deform. An athlete can be both flexible and able to demonstrate considerable stiffness. A deflated basketball is compliant because it deforms considerably upon landing, whereas a golf ball is stiff because it does not deform much upon landing and is more “springy.” Elite athletes display more stiffness than novice athletes as their joints tend to move less when they come into contact with the ground when jumping and sprinting.

Increased stiffness is usually a good thing. Athletes adjust their level of stiffness depending on the task and surface. Here are some of the effects of increased stiffness (usually the effects are positive however as seen in point number five stiffness can be detrimental):

1. Increase cadence or bouncing frequency
2. Decrease ground contact time
3. Decrease ROM in legs
4. Possibly increase peak force and RFD
5. Possibly reduce impulse

Stiffness contains structural and contractile components, which means that tissues can adapt to become more stiff and muscles can contract faster and harder to create more stiffness.

In biomechanics the two most common types of stiffness measured are leg stiffness and vertical stiffness. Leg stiffness can be calculated mathematically by dividing the ground reaction force (GRF) by the change in movement of the leg range of motion (ROM). Vertical stiffness can be calculated by dividing the GRF by the change in vertical movement of the center of mass (COM). Leg stiffness and vertical stiffness are equal during vertical hopping tests (pure axial vectors), but not equal in cyclical actions such as sprinting (anteroposterior vector component). The reactive strength index (RSI) is calculated by dividing the height of a jump by the ground contact time. The height of the jump is proportional to the flight time. For this reason, RSI and leg spring stiffness are closely associated. See the pics below for an illustration regarding stiffness.