The sag response in human muscle contraction
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.
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.
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.
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.
KeywordsForce–frequency relationship Length–tension relationship Muscle contraction Summation Unfused tetanus
Analysis of variance
Half relaxation time
Maximum voluntary contraction
Peak twitch force
Root mean squared
Time to peak tension
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.
Funding for this project was provided by grants from the Canadian Institutes of Health Research (W.H. FDN-143341), the Natural Sciences and Engineering Research Council of Canada (W.H. RGPIN/36674-2013), and the Canada Research Chairs Program (W.H. 950-230603). Additional support was provided by the Killam Foundation (W.H. and G.A.P). I.C.S was supported by a Canadian Institutes of Health Research Fellowship, and G.A.P. was supported by a Canadian Institutes of Health Research Banting Fellowship. Both I.C.S and G.A.P. were supported by postdoctoral fellowships from Alberta Innovates.
Compliance with ethical standards
Conflict of interest
The authors declare that they have no competing interests.
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