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

Abstract

Purpose: Fascicle and sarcomere lengths are important predictors of muscle mechanical performance. However, their regulation during stretch-shortening cycle (SSC) activities in usual and challenging conditions is poorly understood. In this study, we aimed to investigate muscle fascicle and sarcomere behavior during drop jumps (a common SSC activity) in conditions of variable gravity.Methods: Fifteen volunteers performed repeated drop jumps in 1 g, hypo-gravity (0 to 1 g), and hyper-gravity (1 to 2 g) during a parabolic flight. Gastrocnemius medialis (GM) electromyographic activity and fascicle length (Lf) were measured at drop-off, ground contact (GC), minimum ankle joint angle (MAJ), and push-off. GM sarcomere number was estimated by dividing Lf, measured by ultrasound at rest, by published data on GM sarcomere length, and measured in vivo at the same joint angle. Changes in sarcomere length were estimated by dividing GM Lf in each jump phase by sarcomere number calculated individually. The sarcomere force-generating capacity in each jump phase was estimated from the sarcomere length-tension relationship previously reported in the literature.Results: The results showed that, regardless of the gravity level, GM sarcomeres operated in the ascending portion of their length-tension relationship in all the jump phases. Interestingly, although in hypo-gravity and hyper-gravity during the braking phase (GC-MAJ) GM fascicles and sarcomeres experienced a stretch (as opposed to the quasi-isometric behavior in 1 g), at MAJ they reached similar lengths as in 1 g, allowing sarcomeres to develop about the 70% of their maximum force.Conclusion: The observed fascicle behavior during drop jumping seems useful for anchoring the tendon, enabling storage of elastic energy and its release in the subsequent push-off phase for effectively re-bouncing in all gravity levels, suggesting that an innate neuromuscular wisdom enables to perform SSC movements also in challenging conditions.

Energy transfer in reactive movements as a function of individual stretch load

“…Consequently, due to insufficient muscle contraction, cross-bridges possibly are detached beyond their SRES ( Ishikawa et al, 2005 ) resulting in lower force generating capacity evidenced by the theoretical sarcomere contextualization. Similarly, during hyper-gravity-induced overload it was shown that the estimated sarcomere operating range shifts towards the ascending limb ( Monti et al, 2021 ). However, it has been shown that forces similar to those at 1 g can be generated after the deceleration phase ( Monti et al, 2021 ), suggesting that mechanically detached cross-bridges may reattach more rapidly during the take-off phase ( Ishikawa et al, 2005 ).…”

“…Similarly, during hyper-gravity-induced overload it was shown that the estimated sarcomere operating range shifts towards the ascending limb ( Monti et al, 2021 ). However, it has been shown that forces similar to those at 1 g can be generated after the deceleration phase ( Monti et al, 2021 ), suggesting that mechanically detached cross-bridges may reattach more rapidly during the take-off phase ( Ishikawa et al, 2005 ). Therefore, the data implies that lower pre-activity combined with reduced reflex associated muscle activity ( Helm et al, 2020 ) potentially impairs additional muscle generated contractile force (SRES and RFE effect) due to titin folding ( Eckels et al, 2018 ).…”

“…For the figures the averaged fascicle trajectories per subject and condition were normalized to the time interval from GC ranging until COM min . The sarcomere length was theoretically contextualized by the procedures reported by Monti et al (2021) . In detail sarcomere number was calculated by dividing the individual fascicle length while standing in an upright position by the reported sarcomere resting length of 3.09 μm measured in vivo in the same position ( Sanchez et al, 2015 ).…”

“…We acknowledge that this study has some limitations. First, the sarcomere length was contextualized based on fascicle data recorded by ultrasound ( Monti et al, 2021 ). As existing technology to directly measure sarcomere length in vivo is invasive ( Llewellyn et al, 2008 ; Sanchez et al, 2015 ; Lichtwark et al, 2018 ) and is not yet applicable to dynamic movements, the theoretically derived contextualisation was the only way to get an detailed insight into the force-length relationship on the sarcomere level, however the results only represents average sarcomere behaviour, therefore allowing only limited conclusion.…”