On the impact force analysis of two-leg landing with a flexed knee

Abstract

This article looks into the effects of the initial knee flexion angle at the contact time on the peak of the impulsive lower limb forces during landing, and how these effects are related to muscular activities. The impact dynamics of drop landing is studied via a musculoskeletal model with eight Hill-type lower-limb muscles. A method is proposed for the representation of two landing strategies: landing with high and low joint stiffness. Then, in each landing strategy, the effect of the initial knee flexion angle on the peak ground reaction force (GRF), the peak knee ligaments force and the peak tibiofemoral contact force is investigated by considering different initial contact postures. It is observed that while landing with a flexed knee decreases the peak GRF in both landing strategies, it decreases the peak tibiofemoral and knee ligaments forces only in landing with low joint stiffness. Specifically, by increasing the initial knee flexion from 0° to 70°, the peak tibiofemoral and knee ligaments forces decrease monotonically by 54% and 82%, in landing with low joint stiffness. For high joint stiffness, however, as the initial knee flexion increases from 10° to 70°, the peak tibiofemoral force is seen to increase monotonically by 42% and the peak knee ligaments force is seen to have a non-monotonic behavior, first decreasing by 42%, and then, increasing by 250%. These results can be considered in training landing strategies to reduce the risk of knee injury.

Keywords:

• Drop landing
• impact ground reaction force
• tibiofemoral contact force
• knee ligaments force
• knee flexion
• muscle activation level

Disclosure statement

No potential conflict of interest was reported by the authors.

Notes

1 This simplifying assumption imposes some constraints on the movement of the segments associated with the knee joint which are explained in (Delp 1991).

2 While this is not hard to check by expanding the equations, it can also be understood from the physical intuition that the muscle forces can only change the relative angles between two links, thus corresponding only to three degrees of freedom: $q_4,$ $q_5$ and $q_6.$