Muscle-Tendon Unit Length Measurement Using 3D Ultrasound in Passive Conditions: OpenSim Validation and Development of Personalized Models.

This study investigated the validity of using OpenSim to measure muscle-tendon unit (MTU) length of the bi-articular lower limb muscles in several postures (shortened, lengthened, a combination of shortened and lengthened involving both joints, neutral and standing) using 3D freehand ultrasound (US), and to propose new personalized models. MTU length was measured on 14 participants and 6 bi-articular muscles (semimembranosus SM, semitendinosus ST, biceps femoris BF, rectus femoris RF, gastrocnemius medialis GM and gastrocnemius lateralis GL), considering 5 to 6 postures. MTU length was computed using OpenSim with three different models: OS (the generic OpenSim scaled model), OS + INSER (OS with personalized 3D US MTU insertions), OS + INSER + PATH (OS with personalized 3D US MTU insertions and path obtained from one posture). Significant differences in MTU length were found between OS and 3D US models for RF, GM and GL (from - 6.3 to 10.9%). Non-significant effects were reported for the hamstrings, notably for the ST (- 1.5%) and BF (- 1.9%), while the SM just crossed the alpha level (- 3.4%, p = 0.049). The OS + INSER model reduced the magnitude of bias by an average of 4% for RF, GM and GL. The OS + INSER + PATH model showed the smallest biases in length estimates, which made them negligible and non-significant for all the MTU (i.e. ≤ 2.2%). A 3D US pipeline was developed and validated to estimate the MTU length from a limited number of measurements. This opens up new perspectives for personalizing musculoskeletal models using low-cost user-friendly devices.

Consistency of muscle activation signatures across different walking speeds.

BACKGROUND: Using a machine learning algorithm, individuals can be accurately identified from their muscle activation patterns during gait, leading to the concept of individual muscle activation signatures. RESEARCH QUESTION: Are muscle activation signatures robust across different walking speeds? METHODS: We used an open dataset containing electromyographic (EMG) signals from 8 lower limb muscles in 50 asymptomatic adults walking at 5 speeds (extremely slow, very slow, slow, spontaneous, and fast). A machine learning approach classified the EMG profiles based on similar (intra-speed classification) or different (inter-speed classification) walking speeds as training and testing conditions. RESULTS: Intra-speed median classification rates of muscle activation profiles increased with walking speed, from 92 % for extremely slow, to 100 % for self-selected fast walking conditions. Inter-speed median classification rates increased when the speed of the training condition was closer to that of the testing condition. Higher median classification rates were found across slow, spontaneous, and fast walking speed conditions, from 56 % to 96 %, compared with classification rates involving extremely and very slow walking speed conditions, from 6 % to 62 %. SIGNIFICANCE: Our findings reveal that i) muscle activation signatures are detectable for a large range of walking speeds, even those involving different gait strategies (intra-speed median classification rates from 92 % to 100 %), and ii) muscle activation signatures observed during very low walking speeds are not consistent with those observed at higher speeds, suggesting a difference in motor control strategy. Caution should therefore be exercised when assessing gait deviations of a slow walking patient against a normative database obtained at higher speed. Identifying the robustness of individual muscle activation signatures across different movements could help in detecting changes in motor control, otherwise difficult to detect on classical time-varying EMG patterns.

Fatigue in elite fencing: Effects of a simulated competition.

The fatigue induced by fencing remains scarcely investigated. We aimed to investigate both objective (neuromuscular performance fatigability) and subjective (perceived fatigue, effort, and workload) manifestations of fatigue in elite fencers following a five-bout simulated competition. Changes in countermovement jump height, knee extensors maximal isometric torque, rate of torque development, voluntary activation, and contractile response to muscular electrical stimulation were measured in 29 elite fencers [12 epee (6 women), 11 saber (5 women), and 6 foil]. Perceived fatigue and effort were evaluated with visual analog scales, and the perceived workload with the NASA Task Load Index scale. During the competition, maximal torque and rate of torque development decreased by 1.6% (p = 0.017) and 2.4% (p < 0.001) per bout, respectively. Perceived fatigue before each bout increased (12% per bout), with similar values observed at the end of all bouts (bout × period interaction: p < 0.001). Perceived effort increased during the bouts (10% per period, p < 0.001) and during the competition (3% per bout, p = 0.011). Perceived mental demand increased during the competition (2% per bout, p = 0.024). These results suggest that elite fencers needed to increase the allocation of mental rather than physical resources to the task to counterbalance the deleterious effect of fatigue on performance.

Validity and Reliability of 3-D Ultrasound Imaging to Measure Hamstring Muscle and Tendon Volumes.

OBJECTIVE: The validity and reliability of 3-D ultrasound (US) in estimation of muscle and tendon volume was assessed in a very limited number of muscles that can be easily immersed. The objective of the present study was to assess the validity and reliability of muscle volume measurements for all hamstring muscle heads and gracilis (GR), as well as tendon volume for the semitendinosus (ST) and GR using freehand 3-D US. METHODS: Three-dimensional US acquisitions were performed for 13 participants in two distinct sessions on separate days, in addition to one session dedicated to magnetic resonance imaging (MRI). Volumes of ST, semimembranosus (SM), biceps femoris short (BFsh) and long (BFlh) heads, and GR muscles and from the tendon from semitendinosus (STtd) and gracilis (GRtd) were collected. RESULTS: The bias and the 95% confidence intervals of 3-D US compared with MRI ranged from -1.9 mL (-0.8%) to 1.2 mL (1.0%) for muscle volume and from 0.01 mL (0.2%) to -0.03 mL (-2.6%) for tendon volume. For muscle volume assessed using 3-D US, intraclass correlation coefficients (ICCs) ranged from 0.98 (GR) to 1.00, and coefficients of variation (CV) from 1.1% (SM) to 3.4% (BFsh). For tendon volume, ICCs were 0.99, and CVs between 3.2% (STtd) and 3.4% (GRtd). CONCLUSION: Three-dimensional US can provide a valid and reliable inter-day measurement of hamstrings and GR for both muscle and tendon volumes. In the future, this technique could be used as an outcome for strengthening interventions and potentially in clinical environments.

Correlation networks of spinal motor neurons that innervate lower limb muscles during a multi-joint isometric task.

Movements are reportedly controlled through the combination of synergies that generate specific motor outputs by imposing an activation pattern on a group of muscles. To date, the smallest unit of analysis of these synergies has been the muscle through the measurement of its activation. However, the muscle is not the lowest neural level of movement control. In this human study (n = 10), we used a purely data-driven method grounded on graph theory to extract networks of motor neurons based on their correlated activity during an isometric multi-joint task. Specifically, high-density surface electromyography recordings from six lower limb muscles were decomposed into motor neurons spiking activity. We analysed these activities by identifying their common low-frequency components, from which networks of correlated activity to the motor neurons were derived and interpreted as networks of common synaptic inputs. The vast majority of the identified motor neurons shared common inputs with other motor neuron(s). In addition, groups of motor neurons were partly decoupled from their innervated muscle, such that motor neurons innervating the same muscle did not necessarily receive common inputs. Conversely, some motor neurons from different muscles-including distant muscles-received common inputs. The study supports the theory that movements are produced through the control of small numbers of groups of motor neurons via common inputs and that there is a partial mismatch between these groups of motor neurons and muscle anatomy. We provide a new neural framework for a deeper understanding of the structure of common inputs to motor neurons. KEY POINTS: A central and unresolved question is how spinal motor neurons are controlled to generate movement. We decoded the spiking activities of dozens of spinal motor neurons innervating six muscles during a multi-joint task, and we used a purely data-driven method grounded on graph theory to extract networks of motor neurons based on their correlated activity (considered as common input). The vast majority of the identified motor neurons shared common inputs with other motor neuron(s). Groups of motor neurons were partly decoupled from their innervated muscle, such that motor neurons innervating the same muscle did not necessarily receive common inputs. Conversely, some motor neurons from different muscles, including distant muscles, received common inputs. The study supports the theory that movement is produced through the control of groups of motor neurons via common inputs and that there is a partial mismatch between these groups of motor neurons and muscle anatomy.

Common synaptic input between motor units from the lateral and medial posterior soleus compartments does not differ from that within each compartment.

The human soleus muscle is anatomically divided into four separate anatomical compartments. The functional role of this compartmentalization remains unclear. Here, we tested the hypothesis that the common synaptic input to motor units between the medial and lateral posterior compartments is less than within each compartment. Fourteen male participants performed three different heel-raise tasks that were considered to place a different mechanical demand on the medial and lateral soleus compartments. High-density electromyography (EMG) signals from the medial and lateral soleus compartments and the medial gastrocnemius of the right leg were decomposed into individual motor unit spike trains. The coherence between cumulative spike trains of the motor units was estimated. The coherence analysis was also repeated for motor units that were matched across all three tasks. Furthermore, we calculated the ratio of significant correlations between the spike trains of pairs of motor units. We observed that the coherence between motor units of the two soleus compartments was similar as the coherence between motor units within each compartment, regardless of the task. The correlation analysis performed on pairs of motor units confirmed these results. We conclude that the level of common synaptic input between the motor units innervating the medial and lateral posterior soleus compartment is not different than the common synaptic input between motor units innervating each of these compartments, which contrasts with findings from previous studies on finger muscles. This suggests that there is no independent neural control for the individual posterior soleus compartments.NEW & NOTEWORTHY The human soleus muscle is anatomically subdivided into four compartments. The functional role for this compartmentalization remains unknown. Here, we showed that, contrary to previous findings in finger muscles, the common synaptic input between motor units innervating the medial and lateral posterior soleus compartment was similar as that between motor units within the individual compartments. We suggest that the contradictory findings with other compartmentalized muscles may be explained by differences in muscle-tendon anatomy and function.

External rotation of the foot position during plantarflexion increases non-uniform motions of the Achilles tendon.

The medial (GM) and lateral gastrocnemius (GL) muscles enroll to different subparts of the Achilles tendon to form their respective subtendons. The relative gastrocnemii activations during submaximal plantarflexion contraction depend on the position of the foot in the horizontal plane: with toes-in, GL activation increases and GM activation decreases, compared to toes-out. The aim of the current study was to investigate whether horizontal foot position during submaximal isometric plantarflexion contraction differently affects the subtendons within the Achilles tendon in terms of their (i) length at rest, and (ii) elongations and distal motions. Twenty healthy subjects (12 females/8 males) participated in the study. Three-dimensional ultrasound images were taken to capture subtendon lengths at rest and during isometric contraction. Ultrasound images were recorded at the distal end of Achilles tendon (sagittal plane) during ramped contractions and analyzed using a speckle tracking algorithm. All tasks were conducted twice, ones with toes-in and ones with toes-out. At rest, subtendons were shorter with toes-out compared to toes-in. During contraction, the GM subtendon lengthened more in toes-out, compared to the GL, and vice versa (all p <.01). The relative motions within the Achilles tendon (middle minus top layers displacements) were smaller in toes-in compared to toes-out (p =.05) for higher contraction intensity. Our results demonstrated that the horizontal foot position during plantarflexion contraction impacts Achilles tendon motions. Such findings may be relevant in a clinical context, for example in pathologies affecting Achilles tendon motions such as Achilles tendinopathy.

Detection of pronator muscle overactivity in children with unilateral spastic cerebral palsy: Development of a semi-automatic method using EMG data.

BACKGROUND: The pronator teres and pronator quadratus muscles are frequently injected with neuromuscular blocking agents to improve supination in children with spastic cerebral palsy and limited active elbow supination. However, determining by simple clinical examination whether these muscles are overactive during active movement is difficult. OBJECTIVE: This study aimed to develop a semi-automatic method to detect pronator muscle overactivity by using surface electromyography (EMG) during active supination movements in children with cerebral palsy. METHODS: In total, 25 children with unilateral spastic cerebral palsy (10 males; mean [SD] age 10.6 [3.0] years) and 12 typically developing children (7 males; mean age 11.0 [3.0] years) performed pronation-supination movements at 0.50Hz. Kinematic parameters and surface EMG signals were recorded for both pronator muscles. Three experts visually assessed muscle overactivity in the EMG signals of the children with cerebral palsy, in comparison with the reference group. The reliability and discrimination ability of the visual assessments were analysed. Overactivity detection thresholds for the semi-automatic method were adjusted by using the visual assessment by the EMG experts. The positive and negative predictive values of the semi-automatic detection method were calculated. RESULTS: Intra-rater reliability of visual assessment by EMG experts was excellent and inter-rater reliability was moderate. For the 25 children with unilateral spastic cerebral palsy, EMG experts could discriminate different profiles of pronator overactivity during active supination: no pronator overactivity, one overactive pronator, or overactivity of both pronators. The positive and negative predictive values were 96% and 91%, respectively, for this semi-automatic detection method. CONCLUSIONS: Detection of pronator overactivity by using surface EMG provides an important complement to the clinical examination. This method can be used clinically, with the condition that clinicians be aware of surface EMG limitations. We believe use of this method can increase the accuracy of treatment for muscle overactivity, resulting in improved motor function and no worsening of paresis.

Patterns of upper limb muscle activation in children with unilateral spastic cerebral palsy: Variability and detection of deviations.

BACKGROUND: The aim of this study was two-fold: (1) to quantify the variability of upper limb electromyographic patterns during elbow movements in typically developing children and children with unilateral spastic cerebral palsy, and to compare different amplitude normalization methods; (2) to develop a method using this variability to detect (a) deviations in the patterns of a child with unilateral spastic cerebral palsy from the average patterns of typically developing children, and (b) changes after treatment to reduce muscle activation. METHODS: Twelve typically developing children ([6.7-15.9yo]; mean 11.0 SD 3.0yo) and six children with unilateral spastic cerebral palsy ([7.9-17.4yo]; mean 12.4 SD 4.0yo) attended two sessions during which they performed elbow extension-flexion and pronation-supination movements. Surface electromyography of the biceps, triceps, brachioradialis, pronator teres, pronator quadratus, and brachialis muscles was recorded. The Likelihood method was used to estimate the inter-trial, inter-session, and inter-subject variability of the electromyography patterns for each time point in the movement cycle. Deviations in muscle patterns from the patterns of typically developing children and changes following treatment were evaluated in a case study of a child with cerebral palsy. FINDINGS: Normalization of electromyographic amplitude by the mean peak yielded the lowest variability. The variability data were then used in the case study. This method detected higher levels of activation in specific muscles compared with typically developing children, and a reduction in muscle activation after botulinum toxin A injections. INTERPRETATION: Upper limb surface electromyography pattern analysis can be used for clinical applications in children with cerebral palsy.

Classification of upper limb disability levels of children with spastic unilateral cerebral palsy using K-means algorithm.

Treatment for cerebral palsy depends upon the severity of the child's condition and requires knowledge about upper limb disability. The aim of this study was to develop a systematic quantitative classification method of the upper limb disability levels for children with spastic unilateral cerebral palsy based on upper limb movements and muscle activation. Thirteen children with spastic unilateral cerebral palsy and six typically developing children participated in this study. Patients were matched on age and manual ability classification system levels I to III. Twenty-three kinematic and electromyographic variables were collected from two tasks. Discriminative analysis and K-means clustering algorithm were applied using 23 kinematic and EMG variables of each participant. Among the 23 kinematic and electromyographic variables, only two variables containing the most relevant information for the prediction of the four levels of severity of spastic unilateral cerebral palsy, which are fixed by manual ability classification system, were identified by discriminant analysis: (1) the Falconer index (CAI E ) which represents the ratio of biceps to triceps brachii activity during extension and (2) the maximal angle extension (θ Extension,max). A good correlation (Kendall Rank correlation coefficient = -0.53, p = 0.01) was found between levels fixed by manual ability classification system and the obtained classes. These findings suggest that the cost and effort needed to assess and characterize the disability level of a child can be further reduced.