Influence of tibialis anterior length on force steadiness

February 2024

Parisa Alaei, Msc., Healthy Exercise and Aging Lab, School of Health and Exercise Sciences, University of British Columbia Okanagan, Kelowna, British Columbia, Canada

Take home message

• The length of the tibialis anterior did not influence force steadiness during submaximal contraction of the dorsiflexors.
• The length of tibialis anterior did not impact the neural drive fluctuation and the discharge rate of motor units.
• Rehabilitation and sports trainers can incorporate exercises at different muscle lengths without sacrificing force steadiness, promoting optimal performance.
• Recognizing that force steadiness remains constant across diverse muscle lengths in the tibialis anterior muscle could impact the design and functionality of assistive devices.

Background

• Force steadiness, characterized by the force variation around the mean value when produced at a specific force level, serves as a metric for functional performance.
• The fluctuations in force observed during submaximal contractions are related to the discharge rate characteristics of motor units within the motor neuron pool.
• There is strong evidence linking force steadiness to performance on clinical motor function tests.
• Muscle contractile properties are impacted by the length of the muscle. In cases of shorter muscle lengths, the duration of muscle twitching decreases, leading to a need for a higher motor unit discharge rate to attain a specific torque.

How the study was done

• 17 healthy young men (21.7 ± 1.0 yrs) participated in this study.
• Participants performed submaximal dorsiflexion contractions at 5 target forces (5, 10, 20, 40, and 60% of maximal strength) with 3 different ankle angles (75° = short muscle length, 90° = neutral muscle length, and 105° = long muscle length).
• Force steadiness was measured by the amplitude of force fluctuations in the tibialis anterior muscle.
• Tibialis anterior motor unit discharge rate, motor unit discharge rate variability, and variability in neural drive were recorded with high-density surface electromyography (EMG).

What the researchers found

• Compared to neutral and long muscle length, maximal strength was lowest at the shortest muscle length.
• Force steadiness was similar for all 3 ankle angles at each target force.
• Motor unit discharge rate, motor unit discharge rate variability, and neural drive fluctuation were not different between ankle angles.
• Neural drive fluctuation had a greater influence on force steadiness than the motor unit discharge rate variability. 

Conclusion

• Different muscle lengths had no influence on force steadiness or motor unit discharge rate modulation during steady submaximal contractions in the tibialis anterior muscle.
• Force steadiness was associated with the modulation of motor unit discharge characteristics.
• In rehabilitation and sport training, these results allow exercises at various muscle lengths without compromising force steadiness for optimal performance. 
• For individuals using prosthetics or orthotic devices, the understanding that force steadiness remains consistent across different muscle lengths in the tibialis anterior muscle may influence the design and functionality of these assistive devices.

Reference