Dealing with time-varying recruitment and length in Hill-type muscle models

Hamouda, A; Kenney, LPJ; Howard, D

Abstract

Hill-type muscle models are often used in muscle simulation studies and also in the design
and virtual prototyping of functional electrical stimulation systems. These models have to
behave in a sufficiently realistic manner when recruitment level and contractile element (CE)
length change continuously. For this reason, most previous models have used instantaneous
CE length in the muscle’s force vs. length (F-L) relationship, but thereby neglect the
instability problem on the descending limb (i.e. region of negative slope) of the F-L
relationship. Ideally CE length at initial recruitment should be used but this requires a
multiple-motor-unit muscle model to properly account for different motor-units having
different initial lengths when recruited. None of the multiple-motor-unit models reported in
the literature have used initial CE length in the muscle’s F-L relationship, thereby also
neglecting the descending limb instability problem. To address the problem of muscle
modelling for continuously varying recruitment and length, and hence different values of
initial CE length for different motor-units, a new multiple-motor-unit muscle model is
presented which considers the muscle to comprise 1000 individual Hill-type virtual motorunits,
which determine the total isometric force. Other parts of the model (F-V relationship
and passive elements) are not dependent on the initial CE length and, therefore, they are
implemented for the muscle as a whole rather than for the individual motor-units. The results
demonstrate the potential errors introduced by using a single-motor-unit model and also the
instantaneous CE length in the F-L relationship, both of which are common in FES control
studies.

Citation

Hamouda, A., Kenney, L., & Howard, D. (2016). Dealing with time-varying recruitment and length in Hill-type muscle models. Journal of Biomechanics, 49(14), 3375-3380. https://doi.org/10.1016/j.jbiomech.2016.08.030

• Journal Article Type: Article
• Acceptance Date: Aug 23, 2016
• Online Publication Date: Aug 27, 2016
• Publication Date: Oct 3, 2016
• Deposit Date: Sep 6, 2016
• Publicly Available Date: Aug 27, 2017
• Journal: Journal of Biomechanics
• Print ISSN: 0021-9290
• Electronic ISSN: 1873-2380
• Publisher: Elsevier
• Volume: 49
• Issue: 14
• Pages: 3375-3380
• DOI: https://doi.org/10.1016/j.jbiomech.2016.08.030
• Publisher URL: http://dx.doi.org/10.1016/j.jbiomech.2016.08.030
• Related Public URLs: http://www.journals.elsevier.com/journal-of-biomechanics/