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Effects of Mild Fatigue on Repeated Sit-to-stand Movement: A Muscle Synergy and Kinematic Analysis

This study demonstrates that while mild perceived fatigue during repeated sit-to-stand movements preserves the spatial structure of muscle synergies, it induces selective timing adjustments in specific synergies that are closely linked to compensatory changes in center of mass control.

Original authors: Hironori Otomune, Shota Niwa, An Qi, Noriaki Hattori

Published 2026-07-07

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CC BY 4.0

Original authors: Hironori Otomune, Shota Niwa, An Qi, Noriaki Hattori

Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). ⚕️ This is an AI-generated explanation of a preprint that has not been peer-reviewed. It is not medical advice. Do not make health decisions based on this content. Read full disclaimer

The Big Picture: The "Tired Chair" Experiment

Imagine you are sitting in a chair, and you have to stand up and sit down again, over and over, until your legs feel a bit "heavy" and you feel like you've worked a little hard (but not exhausted). This is exactly what 17 healthy young men did in this study.

The researchers wanted to know: When we get slightly tired, does our brain change how we move, or does it just change when we use our muscles?

To answer this, they looked at two things:

The Muscle Team (Spatial Structure): Which muscles work together as a team?
The Timing (Temporal Activation): When exactly does that team start working?

The "Musical Band" Analogy

Think of your body's muscles like a band playing a song (the act of standing up).

The Musicians (Spatial Structure): In this study, the researchers found that the "band lineup" didn't change. Even when the musicians were tired, the same groups of instruments (muscles) were still playing together in the same combinations. The "sheet music" for who plays with whom stayed exactly the same.
The Conductor (Temporal Activation): However, the timing did change. The conductor (your brain) started tapping the baton slightly later for certain parts of the song.

What They Found

1. The Team Stays the Same
The researchers identified four main "muscle teams" (called synergies) that work together to get you out of a chair:

Team 1: Leaning forward (getting ready to stand).
Team 2: Pushing off the seat.
Team 3: Straightening the legs and body.
Team 4: Wiggling slightly to stay balanced once standing.

When the participants got tired, these four teams didn't break up or swap members. The "spatial pattern" (who is in the team) remained stable.

2. The Timing Shifts
While the teams stayed the same, the moment they started working shifted. Specifically, the researchers found a connection between when the teams started and how far forward the person leaned.

The "Center of Activation" (CoA): Imagine a spotlight on a stage. The "Center of Activation" is where the spotlight is brightest. If the spotlight moves to the right, the action is happening later in the song.
The Discovery: The study found that people who shifted their "spotlight" later (delayed the muscle team's start time) also leaned their body further forward before standing up.

3. The "Forward Momentum" Strategy
When you stand up, you have to lean your body forward (like a pendulum) to generate momentum before you push up with your legs.

The study suggests that when people get slightly tired, they don't necessarily change which muscles they use. Instead, they subtly adjust the timing.
Some people waited a split second longer to engage their "leaning" muscles (Team 1) and their "straightening" muscles (Team 3). This allowed them to lean further forward, using gravity to help them stand up, rather than relying purely on tired leg muscles to push up immediately.

The Key Takeaway

The paper concludes that mild fatigue doesn't break your movement "software" (the muscle teams); it just tweaks the "clock" (the timing).

Before you actually fail or stumble, your body makes tiny, individual adjustments. Some people might delay their muscle activation to lean further forward and use momentum to save energy. The researchers found that measuring this timing shift (the Center of Activation) is a very sensitive way to see how the body is compensating for tiredness, even when the overall movement looks normal to the naked eye.

In short: When you get a little tired standing up, your brain doesn't rewrite the song; it just changes the tempo slightly to keep the show going.

Technical Summary: Effects of Mild Fatigue on Repeated Sit-to-Stand Movement

Problem Statement
Previous research on fatigue-induced movement changes has predominantly focused on high levels of fatigue, such as exhaustive exercise or task failure. Consequently, it remains unclear how motor control adapts at a mild level of perceived fatigue before overt performance deterioration occurs. The sit-to-stand (STS) movement is a complex, biomechanically demanding task requiring precise coordination of forward momentum, seat-off, vertical extension, and stabilization. While the muscle synergy framework suggests that the central nervous system simplifies control by recruiting groups of muscles as functional modules, it is unknown whether mild fatigue alters the spatial structure (muscle weighting) or the temporal activation patterns of these synergies during repeated STS movements. Furthermore, the relationship between potential fatigue-related adaptations in synergy timing and kinematic strategies (specifically anterior-posterior center of mass displacement) requires investigation.

Methodology

Participants: Seventeen healthy young men (mean age 25.9 ± 4.1 years) with no history of neurological, musculoskeletal, or cardiovascular disorders.
Experimental Protocol: Participants performed repeated STS movements at a self-selected pace until their perceived exertion reached a Borg scale score of 13 ("somewhat hard"). This threshold was selected to induce mild fatigue without task failure.
Data Acquisition:
- sEMG: Recorded from 16 trunk and lower-limb muscles at 2,000 Hz.
- Kinematics: 3D motion capture (100 Hz) tracking anatomical landmarks to calculate trunk sagittal angles and center of mass (COM) displacement.
- Kinetics: Ground reaction forces recorded via force plates under the buttocks and feet (100 Hz) to identify movement phases (onset, seat-off, stabilization).
Data Processing:
- Valid repetitions were defined from movement onset to stable standing.
- Pre-fatigue condition: First 10 valid repetitions.
- Post-fatigue condition: Last 10 valid repetitions immediately preceding the Borg score of 13.
- Signals were filtered, rectified, and time-normalized to 101 points (0–100% of the STS cycle).
Muscle Synergy Analysis:
- Non-negative Matrix Factorization (NMF) was used to extract four muscle synergies ( $EMG \approx WH$ ), a number fixed based on previous literature to represent STS subtasks (preparation, seat-off/load transfer, extension, stabilization).
- Temporal Parameters: The Center of Activation (CoA) (temporal center of recruitment) and Activation Dispersion (temporal spread) were calculated for each synergy.
Statistical Analysis:
- Spatial Similarity: Cosine similarity compared pre- and post-fatigue spatial vectors.
- Waveform Comparison: One-dimensional Statistical Parametric Mapping (SPM) compared group-level temporal activation profiles.
- Correlation Analysis: Spearman rank correlations examined relationships between changes in synergy parameters (CoA, dispersion) and changes in kinematics (trunk sagittal angle, anterior-posterior COM displacement). False Discovery Rate (FDR) correction was applied to account for multiple comparisons.

Key Results

Preservation of Spatial Structure: The spatial organization of the four muscle synergies remained largely unchanged after mild fatigue. High cosine similarities (0.81–0.88) were observed between pre- and post-fatigue conditions, and no significant group-level differences were found in spatial muscle weightings.
Group-Level Temporal Stability: Statistical Parametric Mapping (SPM) did not identify significant differences in the group-level temporal activation waveforms between pre- and post-fatigue conditions.
Individual-Specific Timing Shifts: While group averages showed stability, change-score analysis revealed significant individual associations:
- A significant positive correlation existed between the change in CoA of Synergy 1 (trunk flexion-related) and the change in anterior-posterior (AP) COM displacement ( $\rho = 0.77, q = 0.021$ ).
- A significant positive correlation existed between the change in CoA of Synergy 3 (trunk and lower limb extension-related) and the change in AP COM displacement ( $\rho = 0.82, q = 0.006$ ).
- Participants who exhibited a later shift in the CoA of Synergies 1 and 3 demonstrated greater anterior COM displacement after fatigue.
Non-Significant Findings: No significant associations were found between changes in activation dispersion and kinematic variables after FDR correction. Similarly, no significant correlation was found between CoA changes and trunk sagittal angle.

Key Contributions

Refinement of Fatigue Adaptation Theory: The study demonstrates that mild perceived fatigue does not necessarily induce a uniform reorganization of muscle synergy structure or group-level temporal waveforms. Instead, adaptations manifest as participant-specific modulation of synergy timing within a preserved structural framework.
Identification of Sensitive Markers: The Center of Activation (CoA) is identified as a more sensitive marker for early compensatory motor reorganization than group-level waveform comparisons or activation dispersion.
Mechanistic Insight into COM Control: The findings suggest that fatigue-related adjustments in STS performance are linked to the temporal coupling of specific synergies. Specifically, the timing of trunk flexion (Synergy 1) and extension (Synergy 3) modules is adjusted in relation to the strategy for forward COM transfer.

Significance and Claims
The paper claims that mild perceived fatigue during repeated STS movements is characterized not by a breakdown of motor modules, but by selective timing adjustments associated with center of mass control. The authors posit that:

CoA as an Early Marker: Shifts in the CoA may serve as a sensitive indicator of compensatory motor reorganization that occurs before overt performance failure or visible instability.
Individual Variability: Fatigue-related adaptations are highly individual; some participants rely on increased forward momentum (delayed CoA, greater AP COM displacement), while others may adopt different strategies. These nuances are often masked by group-averaged analyses.
Clinical Relevance: Understanding these subtle timing changes is crucial for assessing individuals (such as older adults or those with neurological disorders) who may have reduced capacity to flexibly coordinate COM displacement and extensor activation, potentially leading to inefficiency or instability even before task failure is apparent.

The study concludes that while the spatial architecture of muscle synergies is robust against mild fatigue, the temporal organization is flexibly adjusted to maintain task performance, with the CoA of specific synergies serving as a key variable in this compensatory process.

The Big Picture: The "Tired Chair" Experiment

The "Musical Band" Analogy

What They Found

The Key Takeaway

Technical Summary: Effects of Mild Fatigue on Repeated Sit-to-Stand Movement

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