The Anticipation of Gravity in Human Ballistic Movement

Abstract

Stretch-shortening type actions are characterized by lengthening of the pre-activated muscle-tendon unit (MTU) in the eccentric phase immediately followed by muscle shortening. Under 1 g, pre-activity before and muscle activity after ground contact, scale muscle stiffness, which is crucial for the recoil properties of the MTU in the subsequent push-off. This study aimed to examine the neuro-mechanical coupling of the stretch-shortening cycle in response to gravity levels ranging from 0.1 to 2 g. During parabolic flights, 17 subjects performed drop jumps while electromyography (EMG) of the lower limb muscles was combined with ultrasound images of the gastrocnemius medialis, 2D kinematics and kinetics to depict changes in energy management and performance. Neuro-mechanical coupling in 1 g was characterized by high magnitudes of pre-activity and eccentric muscle activity allowing an isometric muscle behavior during ground contact. EMG during pre-activity and the concentric phase systematically increased from 0.1 to 1 g. Below 1 g the EMG in the eccentric phase was diminished, leading to muscle lengthening and reduced MTU stretches. Kinetic energy at take-off and performance were decreased compared to 1 g. Above 1 g, reduced EMG in the eccentric phase was accompanied by large MTU and muscle stretch, increased joint flexion amplitudes, energy loss and reduced performance. The energy outcome function established by linear mixed model reveals that the central nervous system regulates the extensor muscles phase- and load-specifically. In conclusion, neuro-mechanical coupling appears to be optimized in 1 g. Below 1 g, the energy outcome is compromised by reduced muscle stiffness. Above 1 g, loading progressively induces muscle lengthening, thus facilitating energy dissipation.

Muscle in Variable Gravity: “I Do Not Know Where I Am, But I Know What to Do”

“…as reported in Waldvogel et al (2021)], so that the ratio GM/ TA EMG did not show significant changes (Table 2); the only exceptions were 1 g (+18 folds, p = 0.031) and 1.25-1.5 g (−55.2%, p = 0.005). A trend for a reduction in GM/TA EMG ratio was also observed in 1-1.25 g (−41.3%, p = 0.068).…”

“…Seventeen healthy participants (7 females and 10 males, height 175 ± 9 cm, body mass 72 ± 12 kg, age 31 ± 5 years) were recruited for the study ( Waldvogel et al, 2021 ). Among them, two males were excluded from the analysis reported in this study due to low ultrasound data quality.…”

“…The parabola ends with a new “steady-flight” phase. Jumps were performed during the hyper-gravity (green) and the transition phases A and B (blue); reference jumps were performed during 1 g. Adapted from Waldvogel et al (2021) .…”

“…The parabola ends with a new "steady-flight" phase. Jumps were performed during the hyper-gravity (green) and the transition phases A and B (blue); reference jumps were performed during 1 g. Adapted from Waldvogel et al (2021). active and crucial for ankle joint stiffness and power generation when bouncing in different gravity levels (Ritzmann et al, 2016).…”

“…However, most of the available observations on sarcomere and muscle fascicle behavior were made in condition of constant gravity. Thus, in order to understand how sarcomere and muscle fascicle length are regulated in variable gravity conditions, we performed experiments in a parabolic flight, involving variable gravity levels, ranging from about zero-g to about double the Earth’s gravity (1 g; Waldvogel et al, 2021 ).…”