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Timing and extent of finger force enslaving during a dynamic force task cannot be explained by EMG activity patterns
Mojtaba Mirakhorlo1, Huub Maas1, DirkJan H E J Veeger1,2
1Department of Human Movement Sciences, Faculty of Behavioural and Movement Sciences, Vrije Universiteit, Amsterdam Movement Sciences, Amsterdam, The Netherlands.
Plos One
|August 18, 2017
Summary
Finger enslaving, the inability of fingers to move independently, increases with relative finger motion. Mechanical connections, not neural factors, likely cause this increased force enslaving during index finger flexion.
Area of Science:
- Biomechanics
- Neuroscience
- Human Motor Control
Background:
- Finger enslaving, the inability of digits to move or generate force independently, is primarily studied during isometric tasks.
- Understanding the factors influencing finger enslaving is crucial for rehabilitation and understanding fine motor control.
Purpose of the Study:
- To investigate if the degree of force enslaving between fingers is influenced by relative finger movements.
- To differentiate between neural and mechanical contributions to force enslaving during dynamic tasks.
Main Methods:
- Ten healthy subjects performed index finger flexion against controlled resistance.
- Forces exerted by non-instructed fingers and EMG activity of flexor digitorum superficialis (FDS) and extensor digitorum (ED) were measured.
- Analysis focused on changes during index finger flexion compared to a static baseline.
Main Results:
- Non-instructed fingers exerted increased forces (0.2–1.4 N) during index finger flexion, indicating heightened enslaving.
- These force changes began around 4°–6° of index finger MCP joint flexion, 260–370 ms after movement initiation.
- No significant changes in EMG activity were observed in the FDS of non-instructed fingers, despite increased force output.
Conclusions:
- The observed increase in force enslaving during index finger flexion appears primarily driven by mechanical factors, such as connective tissue linkages.
- The delay in force development and the mismatch between force and EMG suggest passive mechanical coupling plays a significant role.
- While neural factors are not entirely ruled out, mechanical connections are proposed as the main contributors to dynamic force enslaving.
