Stiffness is a variable that has been mainly used in human and animal locomotion analyses. Such research has been conducted using both during cyclic (e.g., walking, running, or hopping) and single (e.g., vertical jumps) movements. Stiffness is a quantitative measure of the elastic properties that determine the ability to accumulate potential elastic energy in compliant tissues.

Despite the clarity of the above phenomenon, the practice of sports training reveals some questions regarding the role of potential elastic energy as a key factor responsible for determining performance. The reason for this uncertainty is the lack of concrete recommendations that would allow for the determination of whether the aforementioned theory can be used to improve the speed-strength abilities of an athlete’s motion system. The speculations concerning a desirable value of stiffness that is the most advantageous for the accumulation of potential elastic energy and most favours reaching maximal sport performance have been partially examined. However, no studies have provided unequivocal evidence for the presence of a desired value of stiffness. Moreover, the risk of limb injury may be associated with changes in leg stiffness. There is currently no consensus on which values of stiffness are more likely to increase the risk of injuries to soft tissues and joints. Excessive stiffness may increase bone injury, whereas insufficient stiffness may result in joint instability and soft tissue injury. Moreover, the conceptual and methodological confusion surrounding stiffness makes it difficult to organize the knowledge and compare the results obtained in past research.

This Special Issue therefore aims to collect original research articles that discuss the relationships of stiffness (joint, limb, or vertical) with kinematics and kinetic variables of the human motion system, as well as muscle activity and physiological factors. Especially welcome are studies focusing on the application of stiffness in sports practice and injury prevention. In addition, the Special Issue will also emphasize the use of experimental protocols and calculation methods to estimate stiffness. Review articles which describe the current state of knowledge are also welcome.

Potential topics include but are not limited to the following:

• Application of stiffness in sports training
• Contribution of stiffness in sport performance
• Stiffness as a factor in injury risk
• Accumulation and contribution of potential elastic energy
• Description, verification, and comparison of stiffness calculation methods (e.g., based on force-displacement curve)
• Technologies enabling stiffness measurement and injury prevention (such as vibrations)