Antoine Nordez

Thigh Muscle Activities in Elite Rowers During On-Water Rowing

This study analysed the muscle activity levels and patterns of the major thigh muscle activation during training sections at different intensities of on-water rowing. 9 experienced rowers performed 2 imposed-pace sections (B1 and B2) and 2 maximal-speed sections (start, 500 m) of on-water rowing. The knee angle, power output, mean torque and stroke rate were measured using specific instrumentation and were synchronised with surface electromyography signals of 5 superficial quadriceps and hamstring muscles. B1 and B2 sections were not significantly different regarding mechanical parameters and EMG activities, while the start phase induced large differences. The EMG patterns for B1, B2 were similar (cross-correlation coefficients (CC) ranged between 0.972-0.984) and the moderate CC found between both B1 and start (0.605-0.720) and B2 and start (0.629-0.720). Our results suggest that the hamstring muscles have a motor action and contribute to the power production during the leg drive. During an all-out 500 m section, a decrease in power and stroke rate was found (up to 20%). However, EMG patterns were not time shifted for all muscles. During the leg drive, the muscle activity levels of the quadriceps muscles were unchanged, while the activity of the hamstring muscles decreased.

The nervous system does not compensate for an acute change in the balance of passive force between synergist muscles

ABSTRACT It is unclear how muscle activation strategies adapt to differential acute changes in the biomechanical characteristics between synergist muscles. This issue is fundamental to understanding the control of almost every joint in the body. The aim of this human experiment was to determine whether the relative activation of the heads of the triceps surae [gastrocnemius medialis (GM), gastrocnemius lateralis (GL) and soleus (SOL)] compensates for differential changes in passive force between these muscles. Twenty-four participants performed isometric ankle plantarflexion at 20u2005Nu2005m and 20% of the active torque measured during a maximal contraction, at three ankle angles (30 deg of plantarflexion, 0 and 25 deg of dorsiflexion; knee fully extended). Myoelectric activity (electromyography, EMG) provided an index of neural drive. Muscle shear modulus (elastography) provided an index of muscle force. Passive dorsiflexion induced a much larger increase in passive shear modulus for GM (+657.6±257.7%) than for GL (+488.7±257.9%) and SOL (+106.6±93.0%). However, the neural drive during submaximal tasks did not compensate for this change in the balance of the passive force. Instead, when considering the contraction at 20% MVC, GL root mean square (RMS) EMG was reduced at both 0 deg (−39.4±34.5%) and 25 deg dorsiflexion (−20.6±58.6%) compared with 30 deg plantarflexion, while GM and SOL RMS EMG did not change. As a result, the GM/GL ratio of shear modulus was higher at 0 deg and 25 deg dorsiflexion than at 30 deg plantarflexion, indicating that the greater the dorsiflexion angle, the stronger the bias of force to GM compared with GL. The magnitude of this change in force balance varied greatly between participants. Summary: Force sharing between the three heads of the triceps surae during submaximal contractions changes as a function of ankle angle, and the magnitude of this change is individual specific.

Passive stiffness of monoarticular lower leg muscles is influenced by knee joint angle

PurposeWhile several studies demonstrated the occurrence of intermuscular mechanical interactions, the physiological significance of these interactions remains a matter of debate. The purpose of this study was to quantify the localized changes in the shear modulus of the gastrocnemius lateralis (GL), monoarticular dorsi- and plantar-flexor muscles induced by a change in knee angle.MethodParticipants underwent slow passive ankle rotations at the following two knee positions: knee flexed at 90° and knee fully extended. Ultrasound shear wave elastography was used to assess the muscle shear modulus of the GL, soleus [both proximally (SOL-proximal) and distally (SOL distal)], peroneus longus (PERL), and tibialis anterior (TA). This was performed during two experimental sessions (experiment I: nu2009=u200911; experiment II: nu2009=u200910). The shear modulus of each muscle was compared between the two knee positions.ResultsThe shear modulus was significantly higher when the knee was fully extended than when the knee was flexed (Pu2009<u20090.001) for the GL (averaged increase on the whole range of motion: +u20095.8u2009±u20091.3xa0kPa), SOL distal (+u20094.5u2009±u20091.5xa0kPa), PERL (+u20091.1u2009±u20090.7xa0kPa), and TA (+u20091.6u2009±u20091.0xa0kPa). In contrast, a lower SOL-proximal shear modulus (Pu2009<u20090.001, −u20095.9u2009±u20091.0xa0kPa) was observed.ConclusionAs the muscle shear modulus is linearly related to passive muscle force, these results provide evidence of a non-negligible intermuscular mechanical interaction between the human lower leg muscles during passive ankle rotations. The role of these interactions in the production of coordinated movements requires further investigation.

The slack test does not assess maximal shortening velocity of muscle fascicle in human

ABSTRACT 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±10.7u2005cmu2005s−1) compared with the ballistic condition (36.3±8.7u2005cmu2005s−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. Summary: The slack test does not assess the true maximal shortening velocity of muscle fascicles in humans and does not appear appropriate for in vivo measurements.

Neuromechanical coupling within the human triceps surae and its consequence on individual force sharing strategies

ABSTRACT Little is known about the factors that influence the coordination of synergist muscles that act across the same joint, even during single-joint isometric tasks. The overall aim of this study was to determine the nature of the relationship between the distribution of activation and the distribution of force-generating capacity among the three heads of the triceps surae [soleus (SOL), gastrocnemius medialis (GM) and gastrocnemius lateralis (GL)]. Twenty volunteers performed isometric plantarflexions, during which the activation of GM, GL and SOL was estimated using electromyography (EMG). Functional muscle physiological cross-sectional area (PCSA) was estimated using imaging techniques and was considered as an index of muscle force-generating capacity. The distribution of activation and PCSA among the three muscles varied greatly between participants. A significant positive correlation between the distribution of activation and the distribution of PCSA was observed when considering the two bi-articular muscles at intensities ≤50% of the maximal contraction (0.51<r<0.62). Specifically, the greater the PCSA of GM compared with GL, the stronger bias of activation to the GM. There was no significant correlation between monoarticular and biarticular muscles. A higher contribution of GM activation compared with GL activation was associated with lower triceps surae activation (–0.66<r<–0.42) and metabolic cost (–0.74<r<–0.52) for intensities ≥30% of the maximal contraction. Considered together, an imbalance of force between the three heads was observed, the magnitude of which varied greatly between participants. The origin and consequences of these individual force-sharing strategies remain to be determined. Summary: The greater the force-generating capacity of the medial compared with the lateral gastrocnemius, the stronger the bias of activation to the medial gastrocnemius. This coupling leads to a force imbalance between synergist muscles, the magnitude of which varies greatly between individuals.

Interactions between fascicles and tendinous tissues in gastrocnemius medialis and vastus lateralis during drop landing

Animal tendons have been shown to act as shock absorbers to protect muscle fascicles from exercise‐induced damage during landing tasks. Meanwhile, the contribution of tendinous tissues to damping activities such as landing has been less explored in humans. The aim of this study was to analyze in vivo fascicle‐tendon interactions during drop landing to better understand their role in energy dissipation. Ultrafast ultrasound images of the gastrocnemius medialis (GM) and vastus lateralis (VL), lower limb electromyographic activity, 2‐D kinematics, and ground reaction forces were collected from twelve participants during single‐ and double‐leg drop landings from various heights. For both muscles, length changes were higher in tendinous tissues than in fascicles, demonstrating their key role in protecting fascicles from rapid active lengthening. Increasing landing height increased lengthening and peak lengthening velocity of VL fascicle and GM architectural gear ratio, whereas GM fascicle displayed similar length and velocity patterns. Single‐leg landing lengthens the tendinous tissues of GM and, to a greater degree, VL muscles, without affecting the fascicles. These findings demonstrate the adjustment in fascicle‐tendon interactions to withstand mechanical demand through the tendon buffer action and fascicle rotation. The higher VL fascicle contribution to negative work as the drop height increases would suggest muscle‐specific damping responses during drop landing. This can originate from the distal‐to‐proximal sequence of joint kinetics, from differences in muscle and tendon functions (one‐ and two‐joint muscles), architectural and morphological properties (eg, tendon stiffness), as well as from the muscle activity of the GM and VL muscles.

In vivo assessment of the levator ani muscles using shear wave elastography: a feasibility study in women

Introduction and hypothesisWe hypothesized that shear wave elastography (SWE) technology might be useful for assessing the elastic properties of the pelvic floor in women. Our primary objective was to evaluate the feasibility of assessing the levator ani muscles using SWE in women. Our secondary aim was to investigate the changes in their elastic properties from rest to Valsalva maneuver.MethodsDuring this prospective feasibility study in nonpregnant female volunteers, we collected data on participant age, body mass index (BMI), parity, and time since the delivery. The levator ani muscles of each participant were assessed using SWE technology at rest and during a Valsalva maneuver by measuring the shear modulus (in kilopascals). We then assessed the changes in the shear modulus at rest and during the Valsalva maneuver using a Wilcoxon test.ResultsTwelve parous women participated in this study. The mean time since the last delivery was 14xa0months, the mean age was 31xa0years, and mean BMI was 28xa0kg.m−2. All the assessments performed at rest were successfully completed, but we encountered two failures during the Valsalva maneuver. The mean shear modulus increased by a factor of more than 2 from rest to the Valsalva maneuver for both the right (16.0 vs 35.4xa0kPa) and left side (17.1 vs 37.6xa0kPa).ConclusionsAn assessment of the elastic properties of the levator ani muscles is feasible for nonpregnant women. The reproducibility of the technique and its application in pregnant women and women with pelvic floor disorders must be investigated.