Faster early rate of force development in warmer muscle: an in vivo exploration of fascicle dynamics and muscle-tendon mechanical properties

Abstract

While heat exposure has been shown to increase the rate of force development (RFD), the underlying processes remain unknown. This study investigated the effect of heat on gastrocnemius medialis (GM) muscle-tendon properties and interactions. Sixteen participants performed electrically-evoked and voluntary contractions combined with ultrafast ultrasound under thermoneutral (CON: 26°C, core temperature 37.0±0.3°C, muscle temperature 34.0±1.1°C) and passive heat exposure (HOT: 47°C, core temperature 38.4±0.3°C, muscle temperature 37.0±0.8°C) conditions. Maximal voluntary force was unchanged while voluntary activation decreased (-4.6±8.7%, P=0.038) in HOT. Heat exposure increased RFD before 100 ms from contraction onset (+48.2±62.7%; P=0.013), without further changes after 100 ms. GM fascicle dynamics during electrically-evoked and voluntary contractions remained unchanged between conditions. Joint velocity at a given force was higher in HOT (+7.1±6.6%; P=0.004), while the fascicle force-velocity relationship was unchanged. Passive muscle stiffness and active tendon stiffness were lower in HOT than CON (P≤0.030). This study showed that heat-induced increases in early RFD may not be attributed to changes in contractile properties. Late RFD was unaltered, probably explained by decreased soft tissues' stiffness in heat. Investigations are required to explore the possible influence of neural drive and motor unit recruitment in the enhancement of explosive strength elicited by heat exposure.

Maximal fascicle shortening velocity measurements in human medial gastrocnemius muscle in vivo

“…Needless to say, the fascicle shortening velocity was not constant during isokinetic contractions (e.g., Ichinose et al, 2000 ). Unfortunately, it was impossible in this study to calculate the instantaneous maximal fascicle shortening velocity using an ultrafast ultrasonic device (2000 Hz) as in the previous studies (Beaumatin et al, 2017 ; Hauraix et al, 2015 , 2017 ; Mornas et al, 2022 ).…”

Active Heat Acclimation Does Not Alter Muscle–Tendon Unit Properties

“…Soft tissue stiffness, measured through passive GM and active Achilles tendon stiffness, was also unaffected from PRE to POST in the HA and CON groups. Although our recent study reported an acute decrease in passive GM muscle and active Achilles tendon stiffness at the end of a passive acute heat exposure (33), no study has yet reported the effects of active HA on soft tissue stiffness. These findings suggest that changes in tissue stiffness induced by acute heat exposure are transient and do not translate into chronic adaptations after repeated active HA sessions.…”

“…Of note, the effects described in the current study are resulting from testing performed in temperate environment, and it remains unclear how this absence of effects may affect, or not, the acute muscle–tendon unit properties responses to heat exposure during exercise, especially because we have recently demonstrated a modification of some of these properties (i.e., faster early RTD, rightward shift of the joint force–velocity relationship, and reduced soft tissue stiffness after an acute passive heat exposure [33]). An interesting perspective from this work would be to investigate the effects of an active HA on the acute responses of a low-intensity cycling session performed in hot and humid environment.…”

“…The ankle joint torque measured by the ergometer was converted to tendon force (Ft) as Ft = T / m g (30), where m g is the moment arm length of GM at 90° of ankle joint and knee fully extended, which was estimated from the limb length of each participant (31,32). The active stiffness of tendon corresponded to the ratio between the change in force (in N) and the displacement of the aponeurosis (in mm) between 50% and 80% of MVC torque (33).…”

“…Joint velocity, V F , torque, and fascicle force were averaged from 100°to 70°to obtain the joint and fascicle force-velocity relationship (Fig. 1Cvii and Cviii) (33,34). The two trials resulting in the highest mean joint velocity were averaged.…”