Effects of Fatigue on Voluntary Electromechanical and Relaxation Electromechanical Delay

The purposes of the present study were to examine: 1) the effects of fatigue on electromechanical delay from the onsets of the electromyographic signal to force production (EMDE-F), the onsets of the electromyographic to mechanomyographic signals (EMDE-M), the onsets of the mechanomyographic signal to force production (EMDM-F), as well as the cessations of the electromyographic to force production (R-EMDE-F), cessation of the electromyographic to mechanomyographic signals (R-EMDE-M), and cessations of the mechanomyographic signal to force production (R-EMDM-F); and 2) the relative contributions from EMDE-M and EMDM-F to EMDE-F as well as R-EMDE-M and R-EMDM-F to R-EMDE-F from the vastus lateralis in non-fatigued and fatigued states. The values EMDE-F, EMDE-M, EMDM-F, R-EMDE-F, R-EMDE-M and R-EMDM-F were calculated during maximal voluntary isometric contractions, before and after 70% 1-repetition maximum leg extensions to failure. There were significant pretest to posttest increases in EMDE-F (73%;p<0.01), EMDE-M (99%;p<0.01), EMDM-F (60%;p<0.01), R-EMDE-F (101%;p<0.01) and R-EMDM-F (368%;p<0.01), but no significant change in R-EMDE-M (25%;p=0.46). Fatigue-induced increase in EMDE-F indicated excitation-contraction coupling failure (EMDE-M) and increases in the compliance of the series elastic component (EMDM-F). Increases in R-EMDE-F were due to increases in relaxation time for the series elastic component (R-EMDM-F), but not changes in the reversal of excitation-contraction coupling (R-EMDE-M).

Active muscle stiffness in the human medial gastrocnemius muscle in vivo

The aims of this study were to 1) directly assess active muscle stiffness according to actual length changes in muscle fibers (fascicles) during short range stretching; and 2) compare actual measured active muscle and tendon stiffness using ultrasonography with the stiffness of active (i.e., muscle) and passive (i.e., tendon) parts in series elastic component of plantar flexors using the alpha method. Twenty-four healthy men volunteered for this study. Active muscle stiffness in the medial gastrocnemius muscle was calculated according to changes in estimated muscle force and fascicle length during fast stretching after submaximal isometric contractions [10, 30, 50, 70, and 90% maximal voluntary contractions (MVC)]. Using the variables measured during this fast stretch experiment, the stiffness of active (i.e., muscle) and passive (i.e., tendon) parts in plantar flexors was assessed using alpha method. Tendon stiffness was determined during isometric plantar flexion by ultrasonography. Active muscle stiffness increased with the exerted torque levels. At 30, 50, 70, and 90% MVC, there were no significant correlations between muscle stiffness using ultrasonography and stiffness of active part (i.e., muscle) by alpha method, although this relationship at 10% MVC was significant ( r = 0.552, P = 0.005). In addition, no correlation was noted in tendon stiffness between the two different methods ( r = 0.226, P = 0.209). The present study demonstrated that ultrasonography could quantified active muscle stiffness in vivo. Furthermore, active muscle stiffness and tendon stiffness using ultrasonography were not related to active (i.e., muscle) or passive (i.e., tendon) stiffness in series elastic component of plantar flexors by alpha method.

Effects of eccentric training on mechanical properties of the plantar flexor muscle-tendon complex

Eccentric training is a mechanical loading classically used in clinical environment to rehabilitate patients with tendinopathies. In this context, eccentric training is supposed to alter tendon mechanical properties but interaction with the other components of the muscle-tendon complex remains unclear. The aim of this study was to determine the specific effects of 14 wk of eccentric training on muscle and tendon mechanical properties assessed in active and passive conditions in vivo. Twenty-four subjects were randomly divided into a trained group ( n = 11) and a control group ( n = 13). Stiffness of the active and passive parts of the series elastic component of plantar flexors were determined using a fast stretch during submaximal isometric contraction, Achilles tendon stiffness and dissipative properties were assessed during isometric plantar flexion, and passive stiffness of gastrocnemii muscles and Achilles tendon were determined using ultrasonography while ankle joint was passively moved. A significant decrease in the active part of the series elastic component stiffness was found ( P < 0.05). In contrast, a significant increase in Achilles tendon stiffness determined under passive conditions was observed ( P < 0.05). No significant change in triceps surae muscles and Achilles tendon geometrical parameters was shown ( P > 0.05). Specific changes in muscle and tendon involved in plantar flexion are mainly due to changes in intrinsic mechanical properties of muscle and tendon tissues. Specific assessment of both Achilles tendon and plantar flexor muscles allowed a better understanding of the functional behavior of the muscle-tendon complex and its adaptation to eccentric training.

Influence of overweight on the active and the passive fraction of the plantar flexors series elastic component in prepubertal children

The influence of overweight, as a precursor to obesity, was analyzed on the elastic properties of the triceps surae. Based on body mass index (BMI), children (9 years ± 4 mo) were classified as control (CON; n = 23; BMI −1SD>Z score<1SD) or overweight (OW; n = 21, BMI 1SD>Z score<3SD) with regard to reference data from the World Health Organization. Musculotendinous (MT) stiffness of the series elastic component (SEC) was determined using quick-release tests to obtain 1) the MT stiffness index from the slope of either linear stiffness-torque (SIMT-Torque) or stiffness-EMG (SIMT-EMG) relationships and 2) passive stiffness from the intercept point with the ordinate. Finally, the SEC active (α0) and passive fractions (Cpassive) were separated as described by Morgan (Am J Physiol, 1977), using alpha-torque (α0-Torque, Cpassive-Torque) or alpha-EMG (α0-EMG, Cpassive-EMG) relationships. No significant differences in SIMT-Torque or α0-Torque were observed between OW and CON. SIMT-EMG or α0-EMG values were significantly different between OW and CON, which indicate an increase in MT stiffness. In all cases, passive stiffness (Kp, Cpassive-torque, Cpassive-EMG) was significantly greater in OW but independent of the activation capacities. These results indicate that a weight-related additional loading of the MT structures in OW children caused the MT system to response accordingly to the functional demand, i.e., higher stiffness of the MT structures due to a concomitant increase in the stiffness of the SEC passive and active fraction. This study also reveals that possible differences in the activation capacities influence the determination of MT stiffness of the SEC active fraction.