Experimental determination of sarcomere force–length relationship in type-I human skeletal muscle fibers
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 (F–L) property is crucial for understanding in vivo muscle function.
Measurement of the F–L curve at the most elemental level of muscle, i.e., the sarcomeric F–L curve, is important because it is assumed to be generally applicable to all skeletal muscles. In intact frog single fibers, measurements of the sarcomeric F–L 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 F–L 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 F–L 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 F–L 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 F–L 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 F–L 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, F–L 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 F–L
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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