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

Although heat exposure has been shown to increase the skeletal 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 subjects performed electrically evoked and voluntary contractions combined with ultrafast ultrasound under thermoneutral [control (CON): 25.8 ± 1.8°C, core temperature 37.0 ± 0.3°C, muscle temperature 34.0 ± 1.1°C] and passive heat exposure [hot (HOT): 47.4 ± 1.8°C, core temperature 38.4 ± 0.3°C, muscle temperature 37.0 ± 0.8°C] conditions. Maximal voluntary force changes did not reach statistical significance (-5.0 ± 11.3%, P = 0.052) whereas voluntary activation significantly decreased (-4.6 ± 8.7%, P = 0.038) in HOT. Heat exposure significantly increased voluntary 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) but the fascicle force-velocity relationship remained 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 voluntary RFD may not be attributed to changes in contractile properties. Late voluntary RFD was unaltered, possibly due to decreased soft tissues' stiffness in heat. Further investigations are required to explore the influence of neural drive and motor unit recruitment in the enhancement of explosive strength elicited by heat exposure.

The slack test does not assess maximal shortening velocity of muscle fascicles in humans.

The application of a series of extremely high accelerative motor-driven quick releases while muscles contract isometrically (i.e. slack test) has been proposed to assess unloaded velocity in human muscle. This study aimed to measure gastrocnemius medialis fascicle shortening velocity (VF) and tendinous tissue shortening velocity during motor-driven quick releases performed at various activation levels to assess the applicability of the slack test in humans. Gastrocnemius medialis peak VF and joint velocity recorded from 25 participants using high frame rate ultrasound during quick releases (at activation levels from 0% to 60% of maximal voluntary isometric torque) and during fast contractions without external load (ballistic condition) were compared. Unloaded joint velocity calculated using the slack test method increased whereas VF decreased with muscle activation level (P≤0.03). Passive and low-level quick releases elicited higher VF values (≥41.8±10.7 cm s-1) compared with the ballistic condition (36.3±8.7 cm s-1), while quick releases applied at 60% of maximal voluntary isometric torque produced the lowest VF These findings suggest that initial fascicle length, complex fascicle-tendon interactions, unloading reflex and motor-driven movement pattern strongly influence and limit the shortening velocity achieved during the slack test. Furthermore, VF elicited by quick releases is likely to reflect substantial contributions of passive processes. Therefore, the slack test is not appropriate to assess maximal muscle shortening velocity in vivo.

Mechanical determinants of forward skating sprint inferred from off- and on-ice force-velocity evaluations in elite female ice hockey players.

This study aimed to investigate the correlations between players' mechanical capacities determined during off- and on-ice tests. Whole body force-velocity relationships were assessed in elite female ice hockey players (n = 17) during jumping [squat jump (SJ)], running (5 and 30 m) and skating (5 and 40 m) sprint tasks. Mechanical capacities estimates include relative maximal theoretical force (F0rel), velocity (V0), power (Pmaxrel), slope of the linear relationship between force relative to body mass and velocity (SFVrel), maximal horizontal component of the ground reaction force to the corresponding resultant force (RFmax) and minimal rate of decrease of this ratio (DRF). On-ice mechanical capacities (F0rel, Pmaxrel, RFmax and DRF) largely-to-very largely correlated with 40-m skating split time (r ranging from 0.82 for DRF to -0.91 for Pmaxrel; p < 0.001). Performance variables (SJ height, 30-m running and 40-m forward skating split time) and Pmaxrel demonstrated the largest associations between jumping, running and skating tasks (r ranging from -0.81 for 30-m sprint running time to 0.92 for SJ height; p < 0.001). Small (V0, SFVrel, DRF and force-velocity deficit) to very large (Pmaxrel) correlations (r ranging from 0.58 to 0.72; p < 0.05) were obtained between mechanical variables inferred from off- and on-ice force-velocity tests. The capacity to generate high amounts of horizontal power and effective horizontal force during the first steps on the ice is paramount for forward skating sprint performance. Mechanical capacities determined during forward skating sprint could be considered in ice hockey testing to identify fitness and/or technical/training requirements.

Hyperthermia reduces electromechanical delay via accelerated electrochemical processes.

The present study aimed to determine the effect of hyperthermia on both electrochemical and mechanical components of the electromechanical delay (EMD), using very-high-frame-rate ultrasound. Electrically evoked peak twitch force, EMD, electrochemical (Dm; i.e., delay between stimulation and muscle fascicle motion), and mechanical (Tm; i.e., delay between fascicle motion and force production onset) components of EMD were assessed in 16 participants. Assessments were conducted in a control ambient environment (CON; 26°C, 34% relative humidity) and in a hot ambient environment (HOT; 46-50°C, 18% relative humidity, after ∼127 min of heat exposure). Following heat exposure, gastrocnemius medialis temperature was 37.0 ± 0.6°C in HOT vs. 34.0 ± 0.8°C in CON (P < 0.001). EMD was shorter (9.4 ± 0.8 ms) in HOT than in CON (10.8 ± 0.6 ms, P < 0.001). Electrochemical processes were shorter in HOT than in CON (4.0 ± 0.8 ms vs. 5.5 ± 0.9 ms, respectively, P < 0.001), whereas mechanical processes were unchanged (P = 0.622). These results demonstrate that hyperthermia reduces electromechanical delay via accelerated electrochemical processes, whereas force transmission along the active and passive parts of the series elastic component is not affected following heat exposure. The present study demonstrates that heat exposure accelerates muscle contraction thanks to faster electrochemical processes. Further investigations during voluntary contractions would contribute to better understand how these findings translate into motor performance.NEW & NOTEWORTHY Hyperthermia (targeted core temperature: 38.5°C) reduces the time between gastrocnemius medialis stimulation and the onset of plantar flexor force production in vivo. This reduction in electromechanical delay is concomitant to an earlier motion of muscle fascicle compared with thermoneutral environment. However, hyperthermia has no impact on the duration of force transmission along aponeurosis and tendon, thereby reflecting different effects of heat exposure on contractile and elastic properties of the muscle-tendon unit.

Influence of joint angle on muscle fascicle dynamics and rate of torque development during isometric explosive contractions.

This study investigated how joint angle influences fascicle shortening dynamics of gastrocnemius medialis (GM) during explosive contractions and the resulting impact on rate of torque development (RTD). Sixteen participants performed six sets of five maximal explosive voluntary isometric plantar flexions at -20°, -10°, 0° (neutral position), 10°, 20°, and 30° of ankle angle and five no-load ballistic plantar flexions. RTD assessed over all time windows (from 0 to 200 ms) was significantly lower in extreme plantar flexed (≥20°) and dorsiflexed (-20°) positions compared with -10, 0° (475 ± 105 N·m·s-1), and 10°. At these neutral positions, RTD was maximal and muscle fascicles mainly operated over the plateau of the force-length relationship. At 0°, fascicle shortening velocity peaked at 9.26 ± 2.85 cm/s (i.e., 28.2% of maximal shortening velocity measured during no-load ballistic condition). At 112 ms after RTD onset, fascicle force reached 208 ± 78 N (i.e., 85.6% of the theoretical maximum force at the corresponding shortening velocity) and was thereafter comprised within the 95% confidence interval of the force-velocity curve. This clearly indicates that muscle force reached the maximal force that accounts for the fascicle shortening velocity. These findings suggest that the dynamic behavior of muscle fascicles, and the associated fascicle shortening velocity, may influence the rapid force-generating capacity mainly from 100 ms of RTD onset. The present study provides important information for better understanding of the determinants of human muscle performance during explosive tasks.NEW & NOTEWORTHY Ankle angle influences the operating muscle fascicle lengths of gastrocnemius medialis and the rate of torque development during explosive isometric plantar flexions. The rate of torque development peaks in neutral angles where muscle fascicles shorten over the plateau of the force-length relationship. When fascicles operate over the plateau of the force-length relationship (neutral ankle positions), the force-velocity properties represent a limiting factor for the rapid force-generating capacity from 100 ms after the onset of explosive contractions.