A coach’s guide to understanding and measuring stiffness Part 1: Biomechanical concepts

Abstract: The general concept of the stretch and recoil of elastic tissue during ground contact, storing and releasing energy to enhance the propulsive phase of an action is well understood. However different stiffness measures are frequently used incorrectly and interchangably, leading to ineffective monitoring of stiffness changes, limiting the impact of training designed to enhance stiffness. The aim of this narrative review is to discuss how different structures react to ground contacts, how this behaviour can be modeled and how stiffness impacts performance. Hill’s three-compartment model highlights the need for tendon compliance and muscle stiffness in efficient force generation. However, this does not really explain slow stretch shortening cycle (SSC) actions, were both muscle and tendon stretch and recoil. Different models are used to describe a bodies ground impact behaviour, including the spring-mass model which describes centre of mass movement, the torsional spring model which describes leg fuction, with three torsional springs representing the ankle, knee and hip. These models generally link an increase in stiffness to an increase in performance in high intensity action, with vertical stiffness a predictor of high intensity sporting actions independent of sex, age or maturation. Leg stiffness initially increases with running velocity, before remaining constant at high running velocities. When joint function is reviewed ankle stiffness is linked to fast SSC actions, with knee stiffness linked to slow SSC actions. It is concluded that different measures of stiffness should not be used interchangably as different aspects of stiffness impact performance independantly.