Number of Trials Necessary to Apply Analysis Within the Framework of the Uncontrolled Manifold Hypothesis at Different Levels of Hierarchical Synergy Control

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.

Clinically relevant estimation of minimal number of trials for the uncontrolled manifold analysis

Movement biomarkers are crucial for assessing sensorimotor impairments and tracking the effects of interventions over time. The Uncontrolled Manifold (UCM) analysis has been proposed as a novel biomarker for evaluating movement stability and coordination in various motor tasks across neurological and musculoskeletal disorders. Through inter-trial analysis, the UCM partitions the variance of elemental variables (e.g., finger forces) into components that affect (VORT) and do not affect (VUCM) a performance variable (e.g., total force). A third index, ΔV, is computed as the normalized difference between VORT and VUCM. However, the minimum number of trials required to achieve stable UCM estimates, considering its clinimetric properties, is unknown. This study aimed to determine the minimal number (N) of trials for UCM estimates by computing bootstrap estimates of standard errors (SE) at different N trials using thresholds based on the minimal detectable change (MDC, i.e., the minimum change in an outcome measure beyond measurement error). Thirteen adults (24.6 ± 1.1 years old) performed a finger-pressing coordination task. We computed the 95 % confidence intervals (CI) of bootstrap SE distributions for each UCM estimate and detected the lowest number of trials with the 95 % CI of SE below each MDC threshold. We found the minimal N of trials required was VUCM = 14, VORT = 4 and ΔV = 18. Our findings highlight that a relatively low number of trials (i.e., N = 18) are sufficient to compute all UCM estimates beyond the MDC, supporting the use of the UCM framework in clinical settings where many repetitions of a motor task are not practical.

Size and structure of motor variability in young and old adults performing a rhythmic, repetitive tapping task

The size of motor variability increases with fatigue in repetitive upper limb tasks, and the structure of variability differs with old age. However, the combined influences of old age and fatigue on the size and structure of movement-to-movement variability are unclear. Eighteen young and sixteen old adults performed a fatiguing repetitive tapping task while seated using their dominant arm. Optoelectronic motion capture served to measure upper body angles via forward kinematics. Movement-to-movement variability was measured by the size at joints (standard deviation: SD) and by the structure of the uncontrolled manifold (variance: V_UCM, V_ORT; synergy index: ΔV_z) in the first and final minutes of the task for the early, middle, and late forward movement phases. Outcomes were analyzed by Age*Condition*Phase general estimating equations. Old adults had lower humerothoracic abduction/adduction and flexion/extension SD, wrist flexion/extension SD, V_UCM, and V_ORT, mainly in the early movement phase (p < 0.014). With fatigue, humerothoracic flexion/extension SD increased in young adults only and humerothoracic abduction/adduction SD, wrist pronation/supination SD, wrist flexion/extension SD, and V_UCM increased in both groups. ΔV_z was positive and did not differ with age or fatigue (p > 0.014). Results indicate that fatigue adjustments were mainly in the frontal plane, old age did not affect the ratio of good vs. bad variability, and motor synergy was preserved during fatigue despite less motor flexibility in old age.

Running-Induced Fatigue Changes the Structure of Motor Variability in Novice Runners

Understanding the effects of fatigue is a central issue in the context of endurance sports. Given the popularity of running, there are numerous novices among runners. Therefore, understanding the effects of fatigue in novice runners is an important issue. Various studies have drawn conclusions about the control of certain variables by analyzing motor variability. One variable that plays a crucial role during running is the center of mass (CoM), as it reflects the movement of the whole body in a simplified way. Therefore, the aim of this study was to analyze the effects of fatigue on the motor variability structure that stabilizes the CoM trajectory in novice runners. To do so, the uncontrolled manifold approach was applied to a 3D whole-body model using the CoM as the result variable. It was found that motor variability increased with fatigue (UCMꓕ). However, the UCMRatio did not change. This indicates that the control of the CoM decreased, whereas the stability was not affected. The decreases in control were correlated with the degree of exhaustion, as indicated by the Borg scale (during breaking and flight phase). It can be summarized that running-induced fatigue increases the step-to-step variability in novice runners and affects the control of their CoM.

Uncontrolled manifold analysis of gait kinematic synergy during normal and narrow path walking in individuals with knee osteoarthritis compared to asymptomatic individuals

Knee osteoarthritis (KOA) is a common musculoskeletal disorder resulting in altered gait patterns. Uncontrolled manifold (UCM) analysis has been demonstrated as a useful approach for quantitative analysis of motor variability and synergies. The present study aimed to investigate the changes in the kinematic synergy, controlling the center of mass (COM) position while walking on normal and narrow paths in people with KOA compared to asymptomatic participants. In this cross-sectional study, twenty people with mild to moderate KOA and twenty asymptomatic individuals walked at their comfortable preferred speed across normal and narrow paths on a treadmill. The UCM analysis was performed separately using the lower limb segmental angles as elemental variables and the COM displacement as a performance variable during the stance phase of gait for the frontal and sagittal planes. The results revealed that KOA and asymptomatic individuals could exploit kinematic synergy to control the COM displacement regardless of walking conditions (p < 0.05). Furthermore, the variance within the UCM and synergy index were significantly higher on the narrow path than the normal walking in the mediolateral direction in the KOA group (p < 0.05). The findings of this study suggest that individuals with KOA modify their gait kinematic variability to ensure a stronger kinematic synergy when walking on a challenging narrow path.