Landing from a jump is a challenging task as the downward movement of the center of mass of the body (CoM) of the participant must be decelerated quickly to allow the subject to return to a standing still position. The energy accumulated during the aerial phase of the jump must indeed be fully dissipated by the lower limbs during landing; the higher the drop jump height, the greater the amount of energy to be dissipated. Depending on the participant and the circumstances, the landing strategy, i.e. the way to dissipate the energy, can be more or less stiff. Generally, a stiff strategy is associated with a higher risk of injury such as an ACL injury (Almonroeder et al., 2020, Laughlin et al., 2011, Southard et al., 2012). To prevent such injuries, basic verbal instructions targeting the knee joint, such as “focus on bending your knees when you land”, are often used in injury prevention programs to soften the landing and reduce constraints, e.g. the peak ground reaction force (Almonroeder et al., 2020). According to others, the role of the ankle should also be considered in prevention programs (Lee et al., 2018, Tait et al., 2022). Therefore, gaining insight into the role of lower limb joints in adjusting the landing strategy can be helpful in reducing constraints and so the risk of injury.

One way to study the landing strategy is to measure the mechanical work done on the CoM to dissipate the energy accumulated during the aerial phase. Although the net mechanical work performed on the CoM is a function of the energy to be dissipated, one can do more or less negative and positive work. According to Zelik & Kuo (2012), the amount of negative and positive work done on the CoM reflects a subjective trade-off between economy of movement and other costs, such as the risk of injury. When landing in a stiff manner, a small amount of negative and positive work is done on the CoM at the expense of the risk of injury. The opposite is true when landing softly: additional negative and positive work is performed on the CoM, decreasing the risk of injury.

Another way is to investigate the mechanical work done at each joint using an inverse dynamics approach (Elftman, 1939, Mauroy et al., 2014). When the amount of energy to be dissipated increases due to an increased jump height, the proximal joint contribution to negative work increases (McNitt-Gray, 1993; S. Zhang et al., 2008; S.-N. Zhang et al., 2000). In contrast, the amount of negative work performed increases at the ankle when landing with instructions to adopt a stiff strategy (Devita & Skelly, 1992; S.-N. Zhang et al., 2000). However, as a part of energy absorbed during negative work is stored in elastic structures and then released, and as energy can also be transferred between the joints by two-joint muscles, determining the energy dissipated by each joint (i.e., net work) and thus the adjustments related to the mechanical demand and to the strategy become challenging (Prilutsky & Zatsiorsky, 1994).

A third way to study the landing strategy is to measure the biomechanical properties using a model. In running and hopping, the stiffness can be estimated by means of a simple spring-mass model with a constant stiffness explaining the behavior of the CoM (Blickhan, 1989, Cavagna et al., 1988, Farley et al., 1991). However, this model is not appropriate when landing because in this case energy has to be dissipated, and a spring does not take into account energy dissipation. Dyhre-Poulsen et al. (1991) suggested that the behavior of the muscles should be modified throughout the task, changing from a spring to a damping-unit. Following this suggestion, a model that first includes a spring to allow energy storage and then another spring associated with a compliant damper to allow energy dissipation was proposed and tested more recently (Gambelli et al., 2015). Using this model, it has been shown that the system’s characteristics were adjusted depending on the circumstances. For example, Schepens et al. (2020) reported that the stiffness during the first part of the landing increased under height-induced threat, leading to an increase in the loading rate, while the characteristics during the second part of landing were not modified. In other circumstances, such as microgravity conditions, Gambelli et al. (2016) reported that the stiffness decreased during both parts of landing together with the decrease of the gravity. These authors also showed in parallel that an overly stiff behavior during the second part of landing resulted in a rebound (Gambelli et al., 2016).

Interestingly the model could be applied to the joints and the relationship between the CoM and joint behavior could then be explored. When hopping, the behavior of the ankle and knee can be characterized by a torsional spring as energy must be stored and released successively (Farley and Morgenroth, 1999, Hobara et al., 2011). During this task, the lower limb stiffness is associated with the stiffness of the ankle and of the knee. When landing from a jump, the relationship between the biomechanical behavior observed at the CoM and that at the lower limb joints remains unknown. Therefore, we used the model of Gambelli et al. (2015) to investigate the landing strategy at the level of the joints, and to identify the role of each joint in conjunction with the behavior at the CoM.

The present study is intended to describe the biomechanics when performing double-leg landings from different heights, i.e., different amounts of energy to be dissipated. Our first aim is to test a biomechanical model to characterize the biomechanical behavior as a function of the mechanical demand. Our second aim is to analyse the relationship between the self-selected landing strategy and the behavior of the joints.