Musculoskeletal simulations to examine the effects of accentuated eccentric loading (AEL) on jump height

Abstract

Background: During counter movement jumps, adding weight in the eccentric phase and then suddenly releasing this weight during the concentric phase, known as accentuated eccentric loading (AEL), has been suggested to immediately improve jumping performance. The level of evidence for the positive effects of AEL remains weak, with conflicting evidence over the effectiveness in enhancing performance. Therefore, we proposed to theoretically explore the influence of implementing AEL during constrained vertical jumping using computer modelling and simulation and examined whether the proposed mechanism of enhanced power, increased elastic energy storage and return, could enhance work and power.

Methods: We used a simplified model, consisting of a ball-shaped body (head, arm, and trunk), two lower limb segments (thigh and shank), and four muscles, to simulate the mechanisms of AEL. We adjusted the key activation parameters of the muscles to influence the performance outcome of the model. Numerical optimization was applied to search the optimal solution for the model. We implemented AEL and non-AEL conditions in the model to compare the simulated data between conditions.

Results: Our model predicted that the optimal jumping performance was achieved when the model utilized the whole joint range. However, there was no difference in jumping performance in AEL and non-AEL conditions because the model began its push-off at the similar state (posture, fiber length, fiber velocity, fiber force, tendon length, and the same activation level). Therefore, the optimal solution predicted by the model was primarily driven by intrinsic muscle dynamics (force-length-velocity relationship), and this coupled with the similar model state at the start of the push-off, resulting in similar push-off performance across all conditions. There was also no evidence of additional tendon-loading effect in AEL conditions compared to non-AEL condition.

Discussion: Our simplified simulations did not show improved jump performance with AEL, contrasting with experimental studies. The reduced model demonstrates that increased energy storage from the additional mass alone is not sufficient to induce increased performance and that other factors like differences in activation strategies or movement paths are more likely to contribute to enhanced performance.

Keywords: Accentuated eccentric loading; Biomechanics; Computer simulation; Exercise training; Musculoskeletal modelling; OpenSim; Sports performance; Sports science.

Figure 1. Four muscles model to perform two-dimensional forward dynamic simulation.

Figure 2. Example of one combination of “ excitation onset” and “slope of excitation.”

Figure 3. Vertical ground reaction force during the non-AEL condition using the original (continuous simulation) and split approaches.

The graph shows the results from the pilot simulation using a single muscle (knee extensor) model. The x-axis shows the time (second) from the start of the descent to take-off, and the y-axis shows the vertical ground reaction force (N). The blue line represents the original approach (continuous simulation), and the red line represents the push-off phase from the split approach.

Figure 4. Muscle work and power during push-off phase across normal, 15% AEL, and 30% AEL conditions.

The percentage values above bars denotes the relative change from the corresponding normal condition. Positive sign represents increased work and power, and negative sign represents decreased work and power.

Figure 5. Force, velocity, and power profile representing the whole system dynamics in both models across normal, 15% AEL, and 30% AEL conditions.

The graph in the left column shows the VGRF for the entire jumping motion until take-off. The color-filled markers represents the time when the model achieved the lowest posture. Three different conditions in each model were time-normalized from the lowest posture to take-off. The three graphs in the right column show the time-normalized VGRF, COM vertical velocity, and COM vertical power during the push-off phase.

Figure 6. VAS activation, MTU force, tendon length, fiber length, and fiber velocity across normal, 15% AEL, and 30% AEL conditions.

The main graphs show the data for the entire jumping motion until take-off. The color-filled markers represent the time at the beginning of upward motion for each condition. The dashed blue lines represent the time when the tendon achieved its maximal length in the normal condition. The blue arrows indicate the change in length in the normal condition. Three different conditions were time-normalized from the beginning of upward motion to take-off (i.e., push-off phase), as shown in the smaller inset plots.