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

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 20 N m and 20% of the active torque measured during a maximal contraction, at three ankle angles (30\ua0deg of plantarflexion, 0 and 25\ua0deg 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\ua0deg (-39.4±34.5%) and 25\ua0deg dorsiflexion (-20.6±58.6%) compared with 30\ua0deg plantarflexion, while GM and SOL RMS EMG did not change. As a result, the GM/GL ratio of shear modulus was higher at 0\ua0deg and 25\ua0deg dorsiflexion than at 30\ua0deg 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

THE EFFECT OF THE ERGOMETER DESIGN ON PELVIC TWIST AND LOWERBACK FLEXION IN ELITE ROWERS

Pelvic twist and lower-back flexion are considered as two important risk factors regarding lower-back injuries in rowers training frequently on ergometers. Mobile ergometers could be helpful because their design decreases the inertial loads that the rower has to overcome at the catch. Hence, the purpose of this study was to investigate pelvic twist and lower-back flexion with respect to the ergometer design. These two kinematic parameters were examined on ten elite rowers during one stationary and two mobile ergometer sessions performed at 20 strokes per minutes. The differences related to pelvic twist and lower-back flexion were very small. These findings suggested that further studies should be performed at higher paces and focused on the whole trunk motion and muscle activity to offer an overview of the influence of the ergometer design

APPLIED SESSION: ELASTOGRAPHY FOR MUSCLE BIOMECHANICS

The purpose of this applied session is to demonstrate the potential of shear wave elastography for the study of muscle biomechanics using both real-time demo and recent results, with a special focus on sport applications (stretching, fatigue, pain, damage)

A KINEMATIC MODEL TO PERSONALIZE BOAT SETTINGS IN ORDER TO TARGET A GIVEN RANGE OF MOTION IN SCULLING

The boat settings is a complex topic in rowing as many parts of both boat and oars can be set. The amplitude of the rowing stroke, one main parameter of performance in rowing (Smith & Loschner, 2002), is directly affected by these choices. Surprisingly, the scientific literature is very poor on this subject and boat and oars settings are mainly based on rowers' and coaches' experiences (Nolte, 2011). Most of the time, all crew members have the same settings, while they can display various segment anthropometries and joint flexibilities. Therefore, this study aimed to implement and validate a numeric kinematic model to individualize boat settings in order that scullers can reach given catch and finish angles. For that purpose, it is possible to adjust both inboard of the rigging (boat) and spread (oar) lengths. The inputs of the model should be measured using simple testing that can be performed by coaches and athletes

ELECTROMECHANICAL DELAY AND ITS MECHANISMS ARE NOT IMPAIRED FOLLOWING ECCENTRIC EXERCISE

The aim of the present study was to assess the effect of exercise-induced muscle damage on both electrochemical and mechanical components involved in the electromechanical delay in the gastrocnemius medialis muscle. 15 healthy participants completed 10 sets of 30 maximal eccentric contractions of the plantar flexor muscles at a constant angular velocity of 45°.s-1. Delayed onset muscular soreness, maximal isometric torque, and electromechanical delay were measured before, 1h, and 48h following eccentric exercise. The present study revealed that the time required for both electrochemical and mechanical process involved in electromechanical delay are not impaired by exercise induced muscle damage. This study suggests that the long lasting reduction in force after eccentric exercise cannot be associated to an alteration of the force transmission efficiency

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.Enzo Hollville is funded by the Natural Grass company. We warmly thank Hugo Hauraix for his technical support

Rate of force development relationships to muscle architecture and contractile behavior in the human vastus lateralis

In this study, we tested the hypotheses that (i) rate of force development (RFD) is correlated to muscle architecture and dynamics and that (ii) force–length–velocity properties limit knee extensor RFD. Twenty-one healthy participants were tested using ultrasonography and dynamometry. Vastus lateralis optimal fascicle length, fascicle velocity, change in pennation angle, change in muscle length, architectural gear ratio, and force were measured during rapid fixed-end contractions at 60° knee angle to determine RFD. Isokinetic and isometric tests were used to estimate individual force–length–velocity properties, to evaluate force production relative to maximal potential. Correlation analyses were performed between force and muscle parameters for the first three 50 ms intervals. RFD was not related to optimal fascicle length for any measured time interval, but RFD was positively correlated to fascicle shortening velocity during all intervals (r = 0.49–0.69). Except for the first interval, RFD was also related to trigonometry-based changes in muscle length and pennation angle (r = 0.45–0.63) but not to architectural gear ratio. Participants reached their individual vastus lateralis force–length–velocity potential (i.e. their theoretical maximal force at a given length and shortening velocity) after 62 ± 24 ms. Our results confirm the theoretical importance of fascicle shortening velocity and force–length–velocity properties for rapid force production and suggest a role of fascicle rotation.publishedVersio

ROWING APPLIED SESSION IMPROVING ROWING PERFORMANCE AND MINIMISING INJURY

Competitive rowing has been practiced for millennia as evidenced by Egyptian wall paintings from 2500 BC. Boats from the First Egyptian Dynasty were 25 m long and 2 m wide had 30 rowers. Rowing races were conducted in Ancient Greece as part of festivals and games (Virgil, 19 – 30 BC) and in Rome regattas were organized for entertainment. The complexity of rowing boats probably reached their peak with the Greek Triremes which had 170 rowers in three banks. Today, in the modern day Olympics, there are 14 different events from the single scull to the sweep eight with coxswain. Further, ergometer rowing is an international sport in its own right. Both on-water and ergometer rowing are activities that are enjoyed recreationally and as a form of fitness training. Whatever the motivation for involvement in rowing, an understanding of the mechanisms of performance and injury can deepen appreciation of the sport. Progress will be made towards optimizing performance and minimizing injury when the relevant determinants are known. They can be grouped by their association with the rower, boat, oar and environment