European Journal of Applied Physiology

, Volume 118, Issue 5, pp 1063–1077 | Cite as

The sag response in human muscle contraction

  • Ian C. Smith
  • Jahaan Ali
  • Geoffrey A. Power
  • Walter Herzog
Original Article

Abstract

Purpose

We examined how muscle length and time between stimuli (inter-pulse interval, IPI) influence declines in force (sag) seen during unfused tetani in the human adductor pollicis muscle.

Methods

A series of 16-pulse contractions were evoked with IPIs between 1 × and 5 × the twitch time to peak tension (TPT) at large (long muscle length) and small (short muscle length) thumb adduction angles. Unfused tetani were mathematically deconstructed into a series of overlapping twitch contractions to examine why sag exhibits length- and IPI-dependencies.

Results

Across all IPIs tested, sag was 62% greater at short than long muscle length, and sag increased as IPI was increased at both muscle lengths. Force attributable to the second stimulus increased as IPI was decreased. Twitch force declined from maximal values across all IPI tested, with the greatest reductions seen at short muscle length and long IPI. At IPI below 2 × TPT, the twitch with highest force occurred earlier than the peak force of the corresponding unfused tetani. Contraction-induced declines in twitch duration (TPT + half relaxation time) were only observed at IPI longer than 1.75 × TPT, and were unaffected by muscle length.

Conclusions

Sag is an intrinsic feature of healthy human adductor pollicis muscle. The length-dependence of sag is related to greater diminution of twitch force at short relative to long muscle length. The dependence of sag on IPI is related to IPI-dependent changes in twitch duration and twitch force, and the timing of peak twitch force relative to the peak force of the associated unfused tetanus.

Keywords

Force–frequency relationship Length–tension relationship Muscle contraction Summation Unfused tetanus 

Abbreviations

ANOVA

Analysis of variance

EMD

Electromechanical delay

HRT

Half relaxation time

IPI

Inter-pulse interval

MVC

Maximum voluntary contraction

Pt

Peak twitch force

RMS

Root mean squared

TPT

Time to peak tension

Notes

Author contributions

ICS and JA conceived and designed the experiments. ICS, GAP and WH developed the experimental tools and methods. ICS and JA performed the experiments at the University of Calgary and analyzed the data. ICS drafted the manuscript which was revised critically by GAP, JA, and WH. All authors have approved the final version of the manuscript. All designated authors qualify for authorship, and all who qualify for authorship are listed.

Compliance with ethical standards

Conflict of interest

The authors declare that they have no competing interests.

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Copyright information

© Springer-Verlag GmbH Germany, part of Springer Nature 2018

Authors and Affiliations

  1. 1.Human Performance Lab, Faculty of KinesiologyUniversity of CalgaryCalgaryCanada
  2. 2.Human Health and Nutritional Sciences, College of Biological SciencesUniversity of GuelphGuelphCanada

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