Stiffness a coaches guide (Part 2): recommendations for testing and training

Abstract: High levels of vertical, leg and joint stiffness are generally advantageous for short time span performance actions, with stiffness changes sensitive to training. Consequently it is vital to monitor stiffness changes in any training intervention designed to enhance stiffness. Measures of stiffness are often seen as too complex to be carried out as part of a training intervention, but this approach misses key adaptations which drive performance. Part 2 of this narrative review focuses on measurement methods that are accessable to coaches and how subsets of stiffness can be trained. To measure a structures stiffness there is a requirement for the force applied to and the corresponding change in length of a given structure (Hookes law). Vertical stiffness focuses on centre of mass (COM) displacement, rather then compression of the leg-spring seen in leg stiffness measurements. This makes leg stiffness a preferable measure, compared to vertical stiffness, if more horizontal movements are explored. However, leg stiffness will miss torso deviations in vertical actions and should not be used to replace COM deviation seen in vertical stiffness measures. Joint stiffness measures individual joint actions giving valuable insight into how joints impact system stiffness. Measures of stiffness require high technical knowledge and complex equipment, often beyond the scope of coaches. However, practical monitoring of stiffness can be relatively easly accumplished by tracking temporal and performance outcomes interactions reliably via readly available high sampling frequency phone apps. Enhancing stiffness has been achieved with isometric, eccentric, isotonic and plyometric training, frequently linked to higher intensity interventions, whether acute or chronic stiffness increases are required. Interventions must maximise force output without increasing ground contact or contraction time, while it is recommended to sequence structural, neural and coordinative-based objectives in any training intervention.