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Background When landing from a jump or a drop, muscles contract before touchdown to anticipate imminent collision with the ground, soften ground contact and allow to return to a stable standing position without stepping or rebounding. Research question This study assesses the effect of the unsteadiness of the environment on the motor control of landing. The ‘unsteady environment’ was induced by asking participants to perform drop landings inside an aircraft that underwent trajectories parallel to Earth’s surface. The participants also performed the same task in a ‘steady environment’ in our laboratory. Methods Ground reaction forces, lower limb joints’ movements and the activity of lower limb muscles were recorded. The stability of the landing was assessed by the vertical and anterior-posterior stability indexes, center of pressure measures and by the coefficient of variation of kinetic and kinematic parameters. Results On one hand, participants slowdown their joint movements and reduce the knee joint excursion during landing, probably to avoid excessive movements that may induce imbalance. On the other hand, the stability of the landing is reduced while the variability of the movement is increased, illustrating a less stable and less consistent landing. In addition, whatever the environment, landing parameters associated with increased stiffness (i.e., increased impact forces and decreased joint range of motion) are correlated with decreased landing stability. Significance Overall, landings in the‘unsteady environment’ appear to be more cautious but less stable and less finely tuned. Since the stability of the landing is not directly influenced by the steadiness of the environment, this more cautious behavior could be, at least in part, related to the fear/apprehension induced by sudden acceleration variations of the frame of the aircraft.

We explored the tibiofemoral contact forces and the relative contributions of muscles and external loads to those contact forces during various gait tasks. Second, we assessed the relationships between external gait measures and contact forces. A calibrated electromyography-driven neuromusculoskeletal model estimated the tibiofemoral contact forces during walking (1.44 ± 0.22 m s−1), running (4.38 ± 0.42 m s−1) and sidestepping (3.58 ± 0.50 m s−1) in healthy adults (n = 60, 27.3 ± 5.4 years, 1.75 ± 0.11 m, and 69.8 ± 14.0 kg). Contact forces increased from walking (∼1–2.8 BW) to running (∼3–8 BW), sidestepping had largest maximum total (8.47 ± 1.57 BW) and lateral contact forces (4.3 ± 1.05 BW), while running had largest maximum medial contact forces (5.1 ± 0.95 BW). Relative muscle contributions increased across gait tasks (up to 80–90% of medial contact forces), and peaked during running for lateral contact forces (∼90%). Knee adduction moment (KAM) had weak relationships with tibiofemoral contact forces (all R2 < 0.36) and the relationships were gait task-specific. Step-wise regression of multiple external gait measures strengthened relationships (0.20 < Radj2 < 0.78), but were variable across gait tasks. Step-wise regression equations from a particular gait task (e.g. walking) produced large errors when applied to a different gait task (e.g. running or sidestepping). Muscles well stabilized the knee, increasing their role in stabilization from walking to running to sidestepping. KAM was a poor predictor of medial contact force and load distributions. Step-wise regression models results suggest the relationships between external gait measures and contact forces cannot be generalized across tasks. Neuromusculoskeletal modelling may be required to examine tibiofemoral contact forces and role of muscle in knee stabilization across gait tasks.

A B S T R A C T Background and purpose: The ability to ascend and descend a flight of stairs is considered as one of the best predictors of free-living activity and is correlated with domestic extrinsic activity in hemiparetic patients. However, the relationship between timed-stair performance and clinical and biomechanical parameters has never been studied this population. The aim of this study was to determine if performance on the Timed Up and Down Stairs (TUDS) test was related to clinical variables (maximal gait speed, strength and spasticity) and to biomechanical gait parameters (spatio-temporal, kinematic and kinetic gait parameters) in hemiparetic patients. Methods: Sixty hemiparetic patients performed the TUDS test, underwent 3D gait-analysis and a clinical assessment. Pearson’s correlations and two stepwise multiple linear regression analyses were carried out to identify the parameters which were the most highly correlated with TUDS test performance among the clinical variables and gait parameters on the paretic side. Results: Maximal walking speed on the 10-m walk test and strength of the ankle dorsiflexors were the clinical variables that were the most related to TUDS test performance (63% of variance explained). The percentage of single support phase on the paretic side was the biomechanical gait parameter which was the most related to TUDS test performance (58% of variance explained). Conclusion: The results of this study identified three parameters which predicted the performance to ascend and descend a flight of stairs as fast as possible in hemiparetic patients. Rehabilitation programs which aim to improve stair performance and independence in daily life activities should focus on these three parameters.