Violaine Cahouet

Interactive visualization of muscle activity during limb movements: towards enhanced anatomy learning

We propose a framework to investigate a new way to learn musculoskeletal anatomical kinetics using interactive motion capture and visualization. It can be used to facilitate the learning of anatomy by medicine and sports students, and for the general public to discover human anatomy in action. We illustrate our approach using the example of knee flexion and extension by visualizing the knee muscle activation prediction with agonist and antagonist co-contraction. Muscle activation data for specified movements is first measured during a preliminary phase. The user is then tracked in real time, and its motion is analyzed to recognize the motion being performed. This is used to efficiently evaluate muscle activation by interpolating the activation data stored in tables. The visual feedback consists of a user-specific 3D avatar created by deforming a reference model and animated using the tracking. Muscle activation is visualized using colored lines of action or 3D meshes. This work was made possible by the collaboration of three complementary labs specialized in computer-aided medical intervention, computer graphics and biomechanics.

Musculo-tendinous stiffness of head-neck segment in the sagittal plane: An optimization approach for modeling the cervical spine as a single-joint system

Despite the multi-linked architecture of the cervical spine, all previous studies that have made estimations of mechanical properties of the neck have considered the head-neck segment as a rigid link, with a fixed center of rotation at C7. The aim of this study was to consider the head-neck segment as a changeable geometry system for locating the resultant center of rotation and for calculating the musculo-tendinous stiffness by the quick-release method. Head kinematics during quick-releases was analyzed by recording the trajectory of surface markers. With an optimization procedure, the position of the resultant center of rotation of the head-neck segment was estimated. Thereafter, the angular displacement and acceleration of the head, together with the isometric torque developed by the cervical muscles were used to calculate the segments stiffness. The results showed a consistent center of rotation and a significant increase of the musculo-tendinous stiffness with increasing torque.

Effects of plantar flexor muscle fatigue induced by electromyostimulation on postural coordination

The aim of the present study was to investigate the influence of a modification of an intrinsic capacity (plantar flexor strength) on the implementation of in-phase and anti-phase mode of coordination. Analysis of hip and ankle relative phases during fore-aft tracking task was done before and after an electromyostimulation fatigue protocol on the soleus muscles. Results showed participants used exclusively in-phase and anti-phase modes of coordination, with a sudden switch from one to the other with target frequency increase. Regarding tracking tasks, fatigue induces a decrease of performance for lower frequencies, and a significant decrease of switch frequency (-0.08 Hz) for each subject. In conclusion, changes in mode of coordination implementation suggest that the in-phase mode implementation is highly linked to the strength production capacity at the ankle joint.

Influence of Cervical Muscle Fatigue on Musculo-Tendinous Stiffness of the Head-Neck Segment during Cervical Flexion.

Aim The aim of this study is to determine if the fatigue of cervical muscles has a significant influence on the head-neck segment musculo-tendinous stiffness. Methods Ten men (aged 21.2 ± 1.9 years) performed four quick-release trials of flexion at 30 and 50% MVC before and after the induction of muscular fatigue on cervical flexors. Electromyographic activity was recorded on the sternocleidomastoids (SCM) and spinal erectors (SE), bilaterally. Musculo-tendinous stiffness was calculated through the quick-release method adapted to the head-neck segment. Results We noticed a significant linear increase of the head-neck segment musculo-tendinous stiffness with the increase of exertion level both before (P < 0.0001) and after the fatigue procedure (P < 0.0001). However, this linear relationship was not different before and after the fatigue procedure. EMG analysis revealed a significant increase of the root mean square for the right SCM (P = 0.0002), the left SCM (P < 0.0001), the right SE (P < 0.0001), and the left SE (P < 0.0001) and a significant decrease of the median power frequency only for the right (P = 0.0006) and the left (P = 0.0003) SCM with muscular fatigue. Discussion We did not find significant changes in the head-neck segment musculo-tendinous stiffness with fatigue of cervical muscles. We found a significant increase in EMG activity in the SCM and the SE after the induction of fatigue of the SCM. Our findings suggest that with fatigue of cervical flexors, neck muscle activity is modulated in order to maintain the musculo-tendinous stiffness at a steady state.

EMG-based estimation of muscular efforts exerted during human movements

Methode EMG-assistee pour l’estimation des efforts musculaires developpes au cours du mouvement humainLa quantification des efforts musculaires peut etre d’une importance considerable dans les differents domaines de la biomecanique. Cependant, l’estimation des moments musculaires agoniste et antagoniste ou de la force developpee par chaque muscle au cours d’un mouvement necessite de resoudre le probleme de la redondance musculaire. Cet article fait la synthese des differentes etapes du developpement d’une methode « EMG-assistee » permettant de quantifier les moments et les tensions musculaires lors de contractions isometriques ou de tâches dynamiques, en presence ou non de fatigue. En associant de maniere appropriee l’optimisation numerique et l’utilisation de donnees electromyographiques, la methode proposee ameliore la qualite de l’estimation de ces efforts musculaires. Cette approche pourra trouver de nombreuses applications, particulierement en biomecanique du sport.

In vivo Neck Musculo-Tendinous Stiffness in Response to Quick-Releases

This study was designed to assess musculo-tendinous stiffness of the head-neck segment in response to quick-release in flexion and extension. Musculo-tendinous stiffness of the neck was calculated using analysis of head kinematics with optimization procedure during the first 30 ms after the acceleration peak following the release at different exertion levels (20 % to 70 % MVC). Results showed a linear relationship between torque and musculo-tendinsous stiffness for flexion (R = 0.82, P < 0.5) and extension (R = 0.69, P < 0.05). Resultas shows also an increase of moment of inertia with torque. Correlation between stiffness and torque indicates that quick-release perturbations applied to the head associated with optimization procedure to assess head kinematics provide an attractive approach to investigate head-neck musculo-tendinous stiffness. In addition, changes noticed on head-neck moment of inertia shows that mechanical properties evaluated in this study could be altered by modifications of geometry of the corporal segment induced by exertion level.

Force sharing among fingers after tendon transfers (Tsuge's procedure): a case study

During the application of fingertip forces with simultaneous flexion of the four fingers, namely index (I), middle (M), ring (R) and little (L) fingers, a stable force sharing among fingers is adopted. Several studies have hypothesised that this stable force sharing is established to minimise secondary moments at wrist (induced by the main flexion moments). This principle labelled ‘minimisation of secondary moments’ is presented in the literature as a principle used by the central nervous system to solve musculoskeletal redundancy (Li et al. 1998). This principle has been proved with fatigue (Danion et al. 2001) and different solicited degree of freedom and finger postures (Vigouroux et al. 2008). We further illustrate this principle through the case of a patient who benefited from a tendon transfer surgery after he lost his right hand and finger extension functions. The Tsuge (2008) procedure was used. It corresponds to the transfer of the Pronator Teres to the Extensor Carpi Radialis Brevis and Longus, the Flexor Carpi Radialis to the Extensor Digitorum Communis, the Palmaris Longus to the Extensor Pollicis Brevis and the 4th Flexor Digitorum Superficialis to the Extensor Pollicis Longus. We report on the progressive force sharing pattern for flexion forces before and following the tendon transfer through this preliminary study.