Muscle synergies are modified with improved task performance in skill learning

Assessing motor skill progression based on smoothness during integration of a new tool among hairdressers

Skill acquisition is traditionally assessed based on productivity measures, such as Movement Time (MT), which reflect task execution speed. However, movement smoothness may provide additional insights into skill progression by assessing improvements in movement execution. This study evaluated the evolution of both smoothness and productivity in hairdressing following a change of tool. Kinematic recordings of 14 professional hairdressers, novice users of Ringless Scissors [RS], were taken in the laboratory over four half-days of initiation and after 4.5 months of use in the hair salon. Three types of cuts were performed with Traditional Scissors [TS], then with RS. Smoothness [Number of Peaks (NoP); Log DimensionLess Jerk (LDLJ); SPectral ARC length (SPARC)] was assessed for an elementary movement sequence, spanning tool transport to the start of cutting. MT, representing the duration of these sequences, was assessed as a measure of productivity. After introduction of the RS, smoothness decreased and MT increased. With practice, both characteristics improved. After training, SPARC showed comparable smoothness between RS and TS for two of three cutting movements, while LDLJ and NoP remained higher with TS. After 4.5 months, smoothness improved further, with LDLJ and NoP approaching TS values. Although MT was significantly reduced, it remained higher with RS than TS after 4.5 months use in the salon. These results suggest that introduction of a new tool affects not only productivity - MT - but also movement quality - smoothness. The results presented highlight the relevance of smoothness measures in occupational contexts involving motor learning.

Neuromuscular Control Strategies in Basketball Shooting: Distance-Dependent Analysis of Muscle Synergies

Basketball victory relies on an athlete's skill to make precise shots at different distances. While extensive research has explored the kinematics and dynamics of different shooting distances, the specific neuromuscular control strategies involved remain elusive. This study aimed to compare the differences in muscle synergies during basketball shooting at different distances, offering insights into neuromuscular control strategies and guiding athletes' training. Ten skilled shooting right-handed male basketball players participated as subjects in this experiment. Electromyographic (EMG) data for full-phase shooting were acquired at short (3.2 m), middle (5.0 m), and long (6.8 m) distances. Non-negative matrix decomposition extracted muscle synergies (motor modules and motor primitives) during shooting. The results of this study show that all three distance shooting can be broken down into three synergies and that there were differences in the synergies between short and long distances, with differences in motor primitive 1 and motor primitive 2 at the phase of 45% - 59% (p < 0.001, t* = 4.418), and 78% - 88% (p < 0.01, t* = 4.579), respectively, and differences in the motor module 3 found in the differences in muscle weights for rectus femoris (RF) (p = 0.001, d = -2.094), and gastrocnemius lateral (GL) (p = 0.001, d = -2.083). Shooting distance doesn't affect the number of muscle synergies in basketball shooting but alters synergy patterns. During long distance shooting training, basketball players should place more emphasis on the timing and synergistic activation of upper and lower limbs, as well as core muscles.

Concerto of movement: how expertise shapes the synergistic control of upper limb muscles in complex motor tasks with varying tempo and dynamics

Objective. This research aims to reveal how the synergistic control of upper limb muscles adapts to varying requirements in complex motor tasks and how expertise shapes the motor modules.Approach. We study the muscle synergies of a complex, highly skilled and flexible task-piano playing-and characterize expertise-related muscle-synergy control that permits the experts to effortlessly execute the same task at different tempo and force levels. Surface EMGs (28 muscles) were recorded from adult novice (N= 10) and expert (N= 10) pianists as they played scales and arpeggios at different tempo-force combinations. Muscle synergies were factorized from EMGs.Main results. We found that experts were able to cover both tempo and dynamic ranges using similar synergy selections and achieved better performance, while novices altered synergy selections more to adapt to the changing tempi and keystroke intensities compared with experts. Both groups relied on fine-tuning the muscle weights within specific synergies to accomplish the different task styles, while the experts could tune the muscles in a greater number of synergies, especially when changing the tempo, and switch tempo over a wider range.Significance. Our study sheds light on the control mechanism underpinning expertise-related motor flexibility in highly skilled motor tasks that require decade-long training. Our results have implications on musical and sports training, as well as motor prosthetic design.

A Study of the Effects of Motor Experience on Neuromuscular Control Strategies During Sprint Starts

Much of the current research on sprint start has attempted to analyze the biomechanical characteristics of elite athletes to provide guidance on the training of sprint technique, with less attention paid to the effects of motor experience gained from long-term training on neuromuscular control characteristics. The present study attempted to investigate the effect of motor experience on the modular organization of the neuromuscular system during starting, based on he clarification of the characteristics of muscle synergies during starting. It was found that exercise experience did not promote an increase in the number of synergies but rather a more focused timing of the activation of each synergy, allowing athletes to quickly complete the postural transition from crouching to running during the starting.

Influence of Pedal Interface During Pedaling With the Upper Versus Lower Limbs: A Pilot Analysis of Torque Performance and Muscle Synergies

Pedaling is a physical exercise practiced with either the upper or the lower limbs. Muscle coordination during these exercises has been previously studied using electromyography and synergy analysis, and three to four synergies have been identified for the lower and upper limbs. The question of synergy adaptabilities has not been investigated during pedaling with the upper limbs, and the impact of various modalities is yet not known. This study investigates the effect of pedal type (either clipped/gripped or flat) on the torque performance and the synergy in both upper and lower limbs. Torques applied by six participants while pedaling at 30% of their maximal power have been recorded for both upper and lower limbs. Electromyographic data of 11 muscles on the upper limbs and 11 muscles on the lower limbs have been recorded and synergies extracted and compared between pedal types. Results showed that the torques were not modified by the pedal types for the lower limbs while a deep adaptation is observable for the upper limbs. Participants indeed used the additional holding possibility by pulling the pedals on top of the pushing action. Synergies were accordingly modified for upper limbs while they remain stable for the lower limbs. In both limbs, the synergies showed a good reproducibility even if larger variabilities were observed for the upper limbs. This pilot study highlights the adaptability of muscle synergies according to the condition of movement execution, especially observed for the upper limbs, and can bring some new insights for the rehabilitation exercises.

An analysis of the effect of motor experience on muscle synergy in the badminton jump smash

The jump smash is badminton's most aggressive technical manoeuvre, which is often the key to winning a match. This paper aims to explore the neuromuscular control strategies of advanced and beginner players when jumping smash in different ways. Collecting sEMG and kinematic data from 18 subjects with different motor experiences when jumping smash. Nonnegative Matrix Factorization and K-Means clustering were used to extract muscle synergies and exclude irrelevant combined synergies. Uncontrolled manifold analysis was then used to explore the association between synergies and shoulder stability. In addition, motor output at the spinal cord level was assessed by mapping sEMG to each spinal cord segment. The study found that advanced subjects could respond to different jump smash styles by adjusting the coordinated activation strategies of the upper-limb and postural muscles. Long-term training can induce a rapid decrease in the degree of co-variation of the synergies before contact with a shuttlecock to better cope with an upcoming collision. It is recommended that beginners should focus more on training the coordination of upper-limb muscles and postural muscles.

The effect of motor experience on knee stability and inter-joint coordination when cutting at different angles

BACKGROUND

Most studies on cutting have focused on the biomechanics of the knee and lower-limb muscle activation characteristics, with less consideration given to the influence of motor experience on control strategies at the joint level. This study aimed to investigate the differences in knee stability and inter-joint coordination between high- and low-level athletes when cutting at different angles.

METHODS

A Vicon motion capture system and a Kistler force table were used to obtain kinematic and ground reaction force data during cutting. Joint dynamic stiffness and vector coding were used to assess knee stability and inter-joint coordination. Uncontrolled manifold analysis was used to clarify whether there was synergy among lower-limb joints to maintain postural stability during cutting.

RESULTS

During the load acceptance phase, skilled subjects had the smallest joint stiffness at 90° compared with novice subjects (P < 0.05). Compared with novice subjects, skilled subjects had smaller knee-hip ellipse areas at 90° and 135° (P < 0.05), but larger knee-ankle ellipse areas at 135° (P < 0.05). The synergy index in load acceptance was significantly higher (P < 0.05) for skilled subjects at 90° and 135°.

CONCLUSIONS

Advanced subjects can adjust joint control strategies to adapt to the demands of large-angle cutting on the change of direction. Advanced subjects can reduce knee stability for greater flexibility during cutting compared with novice subjects. By increasing the degree of synergy among the lower-limb joints, advanced athletes can maintain high postural stability.

Rectified Latent Variable Model-Based EMG Factorization of Inhibitory Muscle Synergy Components Related to Aging, Expertise and Force-Tempo Variations

Muscle synergy has been widely acknowledged as a possible strategy of neuromotor control, but current research has ignored the potential inhibitory components in muscle synergies. Our study aims to identify and characterize the inhibitory components within motor modules derived from electromyography (EMG), investigate the impact of aging and motor expertise on these components, and better understand the nervous system's adaptions to varying task demands. We utilized a rectified latent variable model (RLVM) to factorize motor modules with inhibitory components from EMG signals recorded from ten expert pianists when they played scales and pieces at different tempo-force combinations. We found that older participants showed a higher proportion of inhibitory components compared with the younger group. Senior experts had a higher proportion of inhibitory components on the left hand, and most inhibitory components became less negative with increased tempo or decreased force. Our results demonstrated that the inhibitory components in muscle synergies could be shaped by aging and expertise, and also took part in motor control for adapting to different conditions in complex tasks.

Synchronous Muscle Synergy Evaluation of Jaw Muscle Activities during Chewing at Different Speeds, a Preliminary Study

Human mastication is a complex and rhythmic biomechanical process regulated by the central nervous system (CNS). Muscle synergies are a group of motor primitives that the CNS may combine to simplify motor control in human movement. This study aimed to apply the non-negative matrix factorization approach to examine the coordination of the masticatory muscles on both sides during chewing. Ten healthy individuals were asked to chew gum at different speeds while their muscle activity was measured using surface electromyography of the right and left masseter and temporalis muscles. Regardless of the chewing speed, two main muscle synergies explained most of the muscle activity variation, accounting for over 98% of the changes in muscle patterns (variance accounted for >98%). The first synergy contained the chewing side masseter muscle information, and the second synergy provided information on bilateral temporalis muscles during the jaw closing. Furthermore, there was robust consistency and high degrees of similarity among the sets of muscle synergy information across different rate conditions and participants. These novel findings in healthy participants supported the hypothesis that all participants in various chewing speed conditions apply the same motor control strategies for chewing. Furthermore, these outcomes can be utilized to design rehabilitation approaches such as biofeedback therapy for mastication disorders.

Evolution of muscle coordination and mechanical output following four weeks of arm cranking submaximal training

Muscle synergies is extensively studied to understand how the neuromusculoskeletal system deals with abundancy. The synergies represent covariant muscles that acts as building blocks for movement production. Nevertheless, little is known on how those synergies evolve following training, learning and expertise. This study reports the influence a 4-weeks submaximal training of arm-cranking on novice participants' muscle synergies.

METHODS

12 participants performed 8 sessions of submaximal training for 4 weeks. One session consisted in two 30-second-maximal power tests followed by six 2-minutes-bouts at 30% of maximal recorded power. Cranking torque and EMG of 11 muscles were recorded during the entire protocol. After EMG normalization, muscle synergies were extracted using NNMF. Similarity was computed using cross-correlation and cosine similarities and statistical evolution across training was tested using repeated measured ANOVA.

RESULTS

While maximal power increased across training days nor torque management, EMG or muscle synergies were significantly affected by submaximal training. Nevertheless, results suggest slights modifications of muscle synergies across day despite to non-significant differences.

DISCUSSION

Despite the strong complexity of the upper limbs anatomy, our results showed that training didn't induce significant changes in movement realization (mechanical and coordination level). A low-dimensional organization of muscle synergies is selected from the first day and kept through the following training days, despite slight but non-significant modifications.This study supports the hypothesis that motor control for movement production could be simplify using low-dimensional building blocks (muscle synergies). Such building blocks allow stability in movement execution and are slightly adjusted to fit movement requirements with training.