Elsevier

Journal of Biomechanics

Volume 42, Issue 13, 18 September 2009, Pages 2011-2016
Journal of Biomechanics

Experimental determination of sarcomere force–length relationship in type-I human skeletal muscle fibers

https://doi.org/10.1016/j.jbiomech.2009.06.013Get rights and content

Abstract

The objectives of this study were to measure the active and passive force–length (FL) relationships in type-I human single muscle fibers and to compare the results to predictions from the sliding filament model (the “standard model”). We measured isometric forces in chemically skinned fibers at different sarcomere lengths (SLs) in separate maximal activations. The experimental tolerance interval for optimal SL was calculated to be (2.37, 2.95 μm), which included the prediction by the standard model (2.64, 2.81 μm). Average passive slack length was 2.22±0.08 μm, and the passive FL relationship was well described by an exponential function. Best fit lines were used to estimate the ascending and descending limbs from the active FL data using the average SL obtained from a digital image of the fiber. The experimental descending limb was also estimated using the shortest SL to address the possible effects of sarcomere inhomogeneity (SI). The experimental slopes of the ascending and descending limbs, 0.42 Fo/μm and −0.52 Fo/μm (vs. −0.55 Fo/μm with the shortest SL) respectively, Fo being the maximal isometric force, were significantly less in magnitude than those from the standard model. These results suggested that the difference between experimental and standard models was not fully explained by SI and other factors could be important. The broader experimental FL curve compared to the standard model implies that human muscle has functionally a wider operating length range where its force-generating capacity is not compromised.

Introduction

The isometric relationship between muscle length and force is a fundamental property of skeletal muscle. It is used as a basis for almost all models of muscle contractile properties and can be used to predict the kinematic range of optimal function, such as the joint angles for maximal strength. Therefore, precise measurement of the force–length (FL) property is crucial for understanding in vivo muscle function.

Measurement of the FL curve at the most elemental level of muscle, i.e., the sarcomeric FL curve, is important because it is assumed to be generally applicable to all skeletal muscles. In intact frog single fibers, measurements of the sarcomeric FL curve by Gordon et al. (1966) matched the prediction from the sliding filament model with thick (myosin) and thin (actin) filament length estimates from electron micrographs. Specifically, the model predicted the length range over which force was maximal (i.e., the “plateau region”) and the slopes of the ascending and descending limbs. In this paper, we will refer to the FL curve based on the sliding filament model as the “standard model”.

To our knowledge, there has been no direct experimental verification of the sliding filament model in human muscle based upon the work in frog fibers with filament length estimates from electron microscopy. Therefore, the objectives of this study were to measure the active and passive FL curves within a limited sarcomere length (SL) range encompassing the plateau region in type-I chemically skinned human single muscle fibers and to compare our results with the standard FL model. Specifically, our aims were to: (1) calculate tolerance limits for optimal sarcomere length (SLo); (2) estimate the lines of the ascending and descending limbs; (3) estimate slack length (SLp); and (4) estimate the passive exponential FL curve.

Section snippets

Single fiber preparation and experimental apparatus

Two thin strips of the lateral gastrocnemius muscle were harvested from a human subject with no previous musculoskeletal abnormalities during an ankle fracture repair surgery. The strips were maintained at a length with slight tension and were immediately washed with a series of relaxing and skinning solutions (see below for respective compositions) containing 1% (w/v) Triton X-100 at ~3 °C for complete removal of the connective tissue. These strips were further dissected into smaller bundles

Results

We obtained data which satisfied the criteria for quality control from 10 type-I fibers out of 31 fibers tested. The mean diameter of the fibers was 0.103±0.006 mm, and mean maximal isometric stress was 133±26 kPa (at an average SL=2.7 μm). More measurements were made within the range of 2.4–3.0 μm (Fig. 1). Upon fiber activation, force rise was marked by an initial rapid phase followed by a plateau in all trials (Fig. 2) and sarcomeres in the central region shortened from their passive lengths (

Discussion

The most extensive measurements of sarcomeric FL properties have been made using frog single fibers. There has been general visual agreement between the standard model and experimental data obtained either with intact fibers or segments of skinned fibers (Gordon et al., 1966; Moss, 1979). In mammalian single fibers, FL measurements have only been made from rodent muscle (Edman, 2005; Stephenson and Williams, 1982; ter Keurs et al., 1978). This study represents the first attempt to compare FL

Conflict of interest

I confirm that there have been no conflicts of interest interfering with the manuscript, “Experimental determination of sarcomere force–length relationship in type-I human skeletal muscle fibers” by S.K. Gollapudi and D.C. Lin.

Acknowledgements

We wish to thank Richard Lasher, Drs. H. Graeme French, Anita Vasavada, and Kenneth B. Campbell for their helpful support and comments on the manuscript. Funding was provided by the Whitaker Foundation.

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