Passive mechanical properties of the lumbar multifidus muscle support its role as a stabilizer

Fibre and extracellular matrix contributions to passive forces in human skeletal muscles: An experimental based constitutive law for numerical modelling of the passive element in the classical Hill-type three element model

The forces that allow body movement can be divided into active (generated by sarcomeric contractile proteins) and passive (sustained by intra-sarcomeric proteins, fibre cytoskeleton and extracellular matrix (ECM)). These are needed to transmit the active forces to the tendon and the skeleton. However, the relative contribution of the intra- and extra- sarcomeric components in transmitting the passive forces is still under debate. There is limited data in the literature about human muscle and so it is difficult to make predictions using multiscale models, imposing a purely phenomenological description for passive forces. In this paper, we apply a method for the experimental characterization of the passive properties of fibres and ECM to human biopsy and propose their clear separation in a Finite Element Model. Experimental data were collected on human single muscle fibres and bundles, taken from vastus lateralis muscle of elderly subjects. Both were progressively elongated to obtain two stress-strain curves which were fitted to exponential equations. The mechanical properties of the extracellular passive components in a bundle of fibres were deduced by the subtraction of the passive tension observed in single fibres from the passive tension observed in the bundle itself. Our results showed that modulus and tensile load bearing capability of ECM are higher than those of fibres and defined their quantitative characterization that can be used in macroscopic models to study their role in the transmission of forces in physiological and pathophysiological conditions.

Age-associated changes in the mechanical properties of human cadaveric pelvic floor muscles

Proper function of the female pelvic floor requires intact pelvic floor muscles (PFMs). The prevalence of pelvic floor disorders (PFDs) increases substantially with age, in part due to clinically identified deterioration of PFM function with age. However, the etiology of this decline remains largely unknown. We previously demonstrated that PFMs undergo age-related fibrotic changes. This study sought to determine whether aging also impacts PFMs' passive mechanical properties that are largely determined by the intramuscular extracellular matrix. Biopsies from younger (≤52y) and older (>52y) female cadaveric donors were procured from PFMs, specifically coccygeus (C) and two portions of the levator ani - iliococcygeus (IC) and pubovisceralis (PV), and the appendicular muscles - obturator internus (OI) and vastus lateralis (VL). Muscle bundles were subjected to a passive loading protocol, and stress-sarcomere length (L_s) relationships calculated. Muscle stiffness was compared between groups using 2-way ANOVA and Sidak pairwise comparisons, α < 0.05. The mean age was 43.4 ± 11.6y and 74.9 ± 11.9y in younger (N = 5) and older (N = 10) donors, respectively. In all PFMs, the quadratic coefficient of parabolic regression of the stress-L_s curve, a measure of stiffness, was lower in the younger versus older group: C: 33.7 ± 13.9 vs 87.2 ± 10.7, P = 0.02; IC: 38.3 ± 12.7 vs 84.5 ± 13.9, P = 0.04; PV: 24.7 ± 8.8 vs 74.6 ± 9.6, P = 0.04. In contrast, non-PFM stiffness was not affected by aging: OI: 14.5 ± 4.7 vs 32.9 ± 6.2, P = 0.8 and VL: 13.6 ± 5.7 vs 30.1 ± 5.3, P = 0.9. Age-associated increase in PFM stiffness is predicted to negatively impact PFM function by diminishing muscle load-bearing, excursional, contractile, and regenerative capacity, thus predisposing older women to PFDs.

A micromechanical muscle model for determining the impact of motor unit fiber clustering on force transmission in aging skeletal muscle

This study used a micromechanical finite element muscle model to investigate the effects of the redistribution of spatial activation patterns in young and old muscle. The geometry consisted of a bundle of 19 active muscle fibers encased in endomysium sheets, surrounded by passive tissue to model a fascicle. Force was induced by activating combinations of the 19 active muscle fibers. The spacial clustering of muscle fibers modeled in this study showed unbalanced strains suggesting tissue damage at higher strain levels may occur during higher levels of activation and/or during dynamic conditions. These patterns of motor unit remodeling are one of the consequences of motor unit loss and reinnervation associated with aging. The results did not reveal evident quantitative changes in force transmission between old and young adults, but the patterns of stress and strain distribution were affected, suggesting an uneven distribution of the forces may occur within the fascicle that could provide a mechanism for muscle injury in older muscle.

Effect of corporal suspension and pendulum exercises on neuromuscular properties and functionality in patients with medullar thoracic injury

BACKGROUND

Traumatic spinal cord injury (TSCI) is one of the most devastating injuries that has a physical impact on patients. The CHORDATA® method involves suspension and pendulous exercises and has been clinically used to treat patients with TSCI. Although empirically used to treat neurological patients, there is no scientific evidence of the efficacy of this method.

PURPOSE

To evaluate the chronic effects of CHORDATA® method on torque, muscle activation, muscle thickness, and functionality in patients with traumatic spinal cord injury.

METHODS

Twenty-six male patients with medullar thoracic injury were randomly categorised into two groups: intervention group (n = 14) and control group (n = 12). Rehabilitation program comprised of 16 sessions of body suspension and pendulum exercises (twice/week). The maximal voluntary isometric trunk flexion and extension torques, muscle activation and thickness (external and internal oblique, rectus and transversus abdominis, longissimus, and multifidus muscles), and functionality (adapted reach test) were evaluated before and after of rehabilitation program.

FINDINGS

A significant increase was observed in maximal voluntary isometric torque (flexion, 58%; extension, 76%), muscle activation of the rectus abdominis muscle, and muscle thickness of all intervention group muscles, without changes in the control group. Compared to the pre-intervention period, the intervention group also showed improvement in functionality at post-intervention, but no such differences were noted in the control group.

INTERPRETATION

The corporal suspension and pendulum exercises training improved rectus abdominis muscle activation, trunk muscles structure and strength, and reaching capacity in medullar thoracic injury patients.

Supersonic Shear Imaging Elastography in Skeletal Muscles: Relationship Between In Vivo and Synthetic Fiber Angles and Shear Modulus

OBJECTIVES

To verify a relationship between the pennation angle of synthetic fibers and muscle fibers with the shear modulus (μ) generated by Supersonic shear imaging (SSI) elastography and to compare the anisotropy of synthetic and in vivo pennate muscle fibers in the x₂ -x₃ plane (probe perpendicular to water surface or skin).

METHODS

First, the probe of Aixplorer ultrasound scanner (v.9, Supersonic Imagine, Aix-en-Provence, France) was placed in 2 positions (parallel [aligned] and transverse to the fibers) to test the anisotropy in the x₂ -x₃ plane. Subsequently, it was inclined (x₁ -x₃ plane) in relation to the fibers, forming 3 angles (18.25 °, 21.55 °, 36.86 °) for synthetic fibers and one (approximately 0 °) for muscle fibers.

RESULTS

On the x₂ -x₃ plane, μ values of the synthetic and vastus lateralis fibers were significantly lower (P < .0001) at the transverse probe position than the longitudinal one. In the x₁ -x₃ plane, the μ values were significantly reduced (P < .0001) with the probe angle increasing, only for the synthetic fibers (approximately 0.90 kPa for each degree of pennation angle).

CONCLUSIONS

The pennation angle was not related to the μ values generated by SSI elastography for the in vivo lateral head of the gastrocnemius and vastus lateralis muscles. However, a μ reduction with an angle increase in the synthetic fibers was observed. These findings contribute to increasing the applicability of SSI in distinct muscle architecture at normal or pathologic conditions.

Overexpression of striatal D2 receptors reduces motivation thereby decreasing food anticipatory activity

Dopamine has been implicated in circadian timing underlying the food entrainable oscillator (FEO) circuitry and overexpression of the dopamine D2 receptor (D2R) in the striatum has been reported to reduce motivation to obtain food rewards in operant tasks. In the present study, we explored both of these mechanisms by examining food anticipatory activity (FAA) in dopamine D2 receptor-overexpressing (D2R-OE) mice under various durations of food availability. First, we noted that at baseline, there were no differences between D2R-OE mice and their littermates in activity level, food intake, and body weight or in circadian activity. Under conditions of very restricted food availability (4 or 6 hr), both genotypes displayed FAA. In contrast, under 8-hr food availability, control mice showed FAA, but D2R-OE mice did not. Normalization of D2R by administration of doxycycline, a tetracycline analogue, rescued FAA under 8-hr restricted food. We next tested for circadian regulation of FAA. When given ad libitum access to food, neither D2R-OE nor controls were active during the daytime. However, after an interval of food restriction, all mice showed elevated locomotor activity at the time of previous food availability in the day, indicating circadian timing of anticipatory activity. In summary, motivation is reduced in D2R-OE mice but circadian timing behavior is not affected. We conclude that an increase in striatal D2R reduces FAA by modulating motivation and not by acting on a clock mechanism.

Lumbar Muscle Structure Predicts Operational Postures in Active-Duty Marines

Background The relationship between lumbar spine posture and muscle structure is not well understood. Objectives To investigate the predictive capacity of muscle structure on lumbar spine posture in active-duty Marines. Methods Forty-three Marines were scanned in this cross-sectional study, using an upright magnetic resonance imaging scanner while standing without load and standing, sitting, and prone on elbows with body armor. Cobb, horizontal, and sacral angles were measured. Marines were then scanned while unloaded in supine using a supine magnetic resonance imaging scanner. The imaging protocol consisted of T2 intervertebral disc mapping; high-resolution, anatomical, fat-water separation, and diffusion tensor imaging to quantify disc hydration and muscle volume, fat fraction, and restricted diffusion profiles in the lumbar muscles. A stepwise multiple linear regression model was used to identify physiological measures predictive of lumbar spine posture. Results The multiple regression model demonstrated that fractional anisotropy of the erector spinae was a significant predictor of lumbar posture for 7 of 18 dependent variables measured, and explained 20% to 35% of the variance in each model. Decreased fractional anisotropy of the erector spinae predicted decreased lordosis, lumbosacral extension, and anterior pelvic tilt. Conclusion Fractional anisotropy is inversely related with muscle fiber size, which is associated with the isometric force-generating capacity of a muscle fiber. This suggests that stronger erector spinae muscles predict decreased lordosis, lumbosacral extension, and anterior pelvic tilt in a highly trained population. J Orthop Sports Phys Ther 2018;48(8):613-621. Epub 17 May 2018. doi:10.2519/jospt.2018.7865.

Fabrication of Micromolded Gelatin Hydrogels for Long-Term Culture of Aligned Skeletal Myotubes

Cultured skeletal myotubes are a powerful in vitro system for identifying mechanisms of skeletal muscle development and disease. However, skeletal myotubes routinely delaminate from conventional culture substrates after approximately 1 week, which significantly hampers their utility for in vitro disease modeling and drug screening. To address this problem, we fabricated micromolded gelatin hydrogels as culture substrates that are more biomimetic than conventional substrates. On micromolded gelatin hydrogels, C2C12 skeletal myoblasts align and differentiate into skeletal myotubes that are stable in culture for multiple weeks. With this protocol, we detail three key steps: (1) Fabrication of micromolded gelatin hydrogels; (2) Culture of mouse C2C12 myoblasts and differentiation into myotubes; and (3) Quantification of myotube morphology. These substrates have many applications for skeletal muscle disease modeling and drug screening over longer time scales.

Investigation of the passive mechanical properties of spine muscles following disruption of the thoracolumbar fascia and erector spinae aponeurosis, as well as facet injury in a rat

BACKGROUND CONTEXT

Muscle tissue is known to remodel in response to changes to its mechanical environment. Alterations in passive mechanical properties of muscles can influence spine stiffness and stability.

PURPOSE

This study aimed to determine whether passive muscle elastic moduli and passive muscle stresses increased 28 days following mechanical disruption of the thoracolumbar fascia and erector spinae aponeurosis, and injury induced by facet joint compression.

STUDY DESIGN

Male Sprague Dawley rats were randomly assigned to three groups (Incision n=8; Injury n=8; and Control n=6).

METHODS

The thoracolumbar fascia and erector spinae aponeurosis were incised in the Incision and Injury groups to expose the left L5-L6 facet joint. In the Injury group, this facet was additionally compressed for three minutes to induce facet injury and cartilage degeneration. Twenty-eight days after surgery, rats were sacrificed and muscle samples were harvested from lumbar and thoracic erector spinae and multifidus for mechanical testing.

RESULTS

Histologic staining revealed mild cartilage degeneration and boney remodeling in the Injury group. However, the hypotheses that either (1) disruption of the thoracolumbar fascia and erector spinae aponeurosis (Incision group) or (2) the addition of facet compression (Injury group) would increase the passive elastic modulus and stress of surrounding muscles were rejected. There was no effect of surgery (Incision or Injury) on the passive elastic modulus (p=.6597). Passive muscle stresses were also not different at any sarcomere length between surgical groups (p>.7043).

CONCLUSION

Disruption of the thoracolumbar fascia and erector spinae aponeurosis and mild facet damage do not lead to measurable changes in passive muscle mechanical properties within 28 days. These findings contribute to our understanding of how spine muscles are affected by injury and fundamental aspects of the initial stages of spine surgery.

Skeletal muscle mechanics, energetics and plasticity

The following papers by Richard Lieber (Skeletal Muscle as an Actuator), Thomas Roberts (Elastic Mechanisms and Muscle Function), Silvia Blemker (Skeletal Muscle has a Mind of its Own: a Computational Framework to Model the Complex Process of Muscle Adaptation) and Sabrina Lee (Muscle Properties of Spastic Muscle (Stroke and CP) are summaries of their representative contributions for the session on skeletal muscle mechanics, energetics and plasticity at the 2016 Biomechanics and Neural Control of Movement Conference (BANCOM 2016). Dr. Lieber revisits the topic of sarcomere length as a fundamental property of skeletal muscle contraction. Specifically, problems associated with sarcomere length non-uniformity and the role of sarcomerogenesis in diseases such as cerebral palsy are critically discussed. Dr. Roberts then makes us aware of the (often neglected) role of the passive tissues in muscles and discusses the properties of parallel elasticity and series elasticity, and their role in muscle function. Specifically, he identifies the merits of analyzing muscle deformations in three dimensions (rather than just two), because of the potential decoupling of the parallel elastic element length from the contractile element length, and reviews the associated implications for the architectural gear ratio of skeletal muscle contraction. Dr. Blemker then tackles muscle adaptation using a novel way of looking at adaptive processes and what might drive adaptation. She argues that cells do not have pre-programmed behaviors that are controlled by the nervous system. Rather, the adaptive responses of muscle fibers are determined by sub-cellular signaling pathways that are affected by mechanical and biochemical stimuli; an exciting framework with lots of potential. Finally, Dr. Lee takes on the challenging task of determining human muscle properties in vivo. She identifies the dilemma of how we can demonstrate the effectiveness of a treatment, specifically in cases of muscle spasticity following stroke or in children with cerebral palsy. She then discusses the merits of ultrasound based elastography, and the clinical possibilities this technique might hold. Overall, we are treated to a vast array of basic and clinical problems in skeletal muscle mechanics and physiology, with some solutions, and many suggestions for future research.

Changes in sarcomere lengths of the human vastus lateralis muscle with knee flexion measured using in vivo microendoscopy

Sarcomeres are the basic contractile units of muscle, and their lengths influence muscle force-generating capacity. Despite their importance, in vivo sarcomere lengths remain unknown for many human muscles. Second harmonic generation (SHG) microendoscopy is a minimally invasive technique for imaging sarcomeres in vivo and measuring their lengths. In this study, we used SHG microendoscopy to visualize sarcomeres of the human vastus lateralis, a large knee extensor muscle important for mobility, to examine how sarcomere lengths change with knee flexion and thus affect the muscle׳s force-generating capacity. We acquired in vivo sarcomere images of several muscle fibers of the resting vastus lateralis in six healthy individuals. Mean sarcomere lengths increased (p=0.031) from 2.84±0.16μm at 50° of knee flexion to 3.17±0.13μm at 110° of knee flexion. The standard deviation of sarcomere lengths among different fibers within a muscle was 0.21±0.09μm. Our results suggest that the sarcomeres of the resting vastus lateralis at 50° of knee flexion are near optimal length. At a knee flexion angle of 110° the resting sarcomeres of vastus lateralis are longer than optimal length. These results show a smaller sarcomere length change and greater conservation of force-generating capacity with knee flexion than estimated in previous studies.

Non-invasive assessment of human multifidus muscle stiffness using ultrasound shear wave elastography: A feasibility study

There is a lack of numeric data for the mechanical characterization of spine muscles, especially in vivo data. The multifidus muscle is a major muscle for the stabilization of the spine and may be involved in the pathogenesis of chronic low back pain (LBP). Supersonic shear wave elastography (SWE) has not yet been used on back muscles. The purpose of this prospective study is to assess the feasibility of ultrasound SWE to measure the elastic modulus of lumbar multifidus muscle in a passive stretching posture and at rest with a repeatable and reproducible method. A total of 10 asymptotic subjects (aged 25.5 ± 2.2 years) participated, 4 females and 6 males. Three operators performed 6 measurements for each of the 2 postures on the right multifidus muscle at vertebral levels L2-L3 and L4-L5. Repeatability and reproducibility have been assessed according to ISO 5725 standard. Intra-class correlation coefficients (ICC) for intra- and inter-observer reliability were rated as both excellent [ICC=0.99 and ICC=0.95, respectively]. Reproducibility was 11% at L2-L3 level and 19% at L4-L5. In the passive stretching posture, shear modulus was significantly higher than at rest ( µ < 0.05). This preliminary work enabled to validate the feasibility of measuring the shear modulus of the multifidus muscle with SWE. This kind of measurement could be easily introduces into clinical routine like for the medical follow-up of chronic LBP or scoliosis treatments.

Prolonged Culture of Aligned Skeletal Myotubes on Micromolded Gelatin Hydrogels

In vitro models of skeletal muscle are critically needed to elucidate disease mechanisms, identify therapeutic targets, and test drugs pre-clinically. However, culturing skeletal muscle has been challenging due to myotube delamination from synthetic culture substrates approximately one week after initiating differentiation from myoblasts. In this study, we successfully maintained aligned skeletal myotubes differentiated from C2C12 mouse skeletal myoblasts for three weeks by utilizing micromolded (μmolded) gelatin hydrogels as culture substrates, which we thoroughly characterized using atomic force microscopy (AFM). Compared to polydimethylsiloxane (PDMS) microcontact printed (μprinted) with fibronectin (FN), cell adhesion on gelatin hydrogel constructs was significantly higher one week and three weeks after initiating differentiation. Delamination from FN-μprinted PDMS precluded robust detection of myotubes. Compared to a softer blend of PDMS μprinted with FN, myogenic index, myotube width, and myotube length on μmolded gelatin hydrogels was similar one week after initiating differentiation. However, three weeks after initiating differentiation, these parameters were significantly higher on μmolded gelatin hydrogels compared to FN-μprinted soft PDMS constructs. Similar results were observed on isotropic versions of each substrate, suggesting that these findings are independent of substrate patterning. Our platform enables novel studies into skeletal muscle development and disease and chronic drug testing in vitro.

WISE 2005: Aerobic and resistive countermeasures prevent paraspinal muscle deconditioning during 60-day bed rest in women

Microgravity-induced lumbar paraspinal muscle deconditioning may contribute to back pain commonly experienced by astronauts and may increase the risk of postflight injury. We hypothesized that a combined resistive and aerobic exercise countermeasure protocol that included spinal loading would mitigate lumbar paraspinal muscle deconditioning during 60 days of bed rest in women. Sixteen women underwent 60-day, 6° head-down-tilt bed rest (BR) and were randomized into control and exercise groups. During bed rest the control group performed no exercise. The exercise group performed supine treadmill exercise within lower body negative pressure (LBNP) for 3-4 days/wk and flywheel resistive exercise for 2–3 days/wk. Paraspinal muscle cross-sectional area (CSA) was measured using a lumbar spine MRI sequence before and after BR. In addition, isokinetic spinal flexion and extension strengths were measured before and after BR. Data are presented as means ± SD. Total lumbar paraspinal muscle CSA decreased significantly more in controls (10.9 ± 3.4%) than in exercisers (4.3 ± 3.4%; P < 0.05). The erector spinae was the primary contributor (76%) to total lumbar paraspinal muscle loss. Moreover, exercise attenuated isokinetic spinal extension loss (−4.3 ± 4.5%), compared with controls (−16.6 ± 11.2%; P < 0.05). In conclusion, LBNP treadmill and flywheel resistive exercises during simulated microgravity mitigate decrements in lumbar paraspinal muscle structure and spine function. Therefore spaceflight exercise countermeasures that attempt to reproduce spinal loads experienced on Earth may mitigate spinal deconditioning during long-duration space travel.

Cross-Sectional Nakagami Images in Passive Stretches Reveal Damage of Injured Muscles

Muscle strain is still awanting a noninvasive quantitatively diagnosis tool. High frequency ultrasound (HFU) improves image resolution for monitoring changes of tissue structures, but the biomechanical factors may influence ultrasonography during injury detection. We aim to illustrate the ultrasonic parameters to present the histological damage of overstretched muscle with the consideration of biomechanical factors. Gastrocnemius muscles from mice were assembled and ex vivo passive stretching was performed before or after injury. After injury, the muscle significantly decreased mechanical strength. Ultrasonic images were obtained by HFU at different deformations to scan in cross and longitudinal orientations of muscle. The ultrasonography was quantified by echogenicity and Nakagami parameters (NP) for structural evaluation and correlated with histological results. The injured muscle at its original length exhibited decreased echogenicity and NP from HFU images. Cross-sectional ultrasonography revealed a loss of correlation between NP and passive muscle stretching that suggested a special scatterer pattern in the cross section of injured muscle. The independence of NP during passive stretching of injured muscle was confirmed by histological findings in ruptured collagen fibers, decreased muscle density, and increased intermuscular fiber space. Thus, HFU analysis of NP in cross section represents muscle injury that may benefit the clinical diagnosis.

Stiffness of resting lumbar myofascia in healthy young subjects quantified using a handheld myotonometer and concurrently with surface electromyography monitoring

This study aimed to non-invasively quantify passive stiffness of superficial myofascia at a lower lumbar (L3-L4) anatomical level in young healthy male and female subjects and investigate its possible morphological variation. Resting prone lumbar myofascial measurements were quantified using MyotonPro(®) and statistically analyzed in 20 young healthy individuals over 3-weekly intervals, concurrently with surface electromyography (sEMG). Averaged mean ± SE stiffness (Newton/meter) over three weeks was significantly (p < 0.001) greater in males (247.8 ± 11.3) than females (208.4 ± 11.3), on the right (237.7 ± 12.8) than left sides (218.5 ± 12.3), at 10-min (231.4 ± 9.1) than initial baseline (224.8 ± 9.1) values. A polymorphism of stiffness values in 10 male and 10 female subjects was suggested by box plot analyses of the 3 weekly measurements and greater inter-individual than intra-individual variances. Greater knowledge of lumbar myofascial stiffness can improve understanding of their contributions in health and chronic low back disorders.

Effect of supraspinatus tendon injury on supraspinatus and infraspinatus muscle passive tension and associated biochemistry

BACKGROUND

Injury to the supraspinatus and infraspinatus tendons and the associated atrophic changes to the muscle remain a common clinical problem. Specifically, increased muscle stiffness has been implicated in failure of the repair and poor functional outcomes. We present a comparison of the passive mechanical properties and associated biochemical studies from patients with and without torn supraspinatus tendons.

METHODS

Muscle biopsy samples (n = 40) were obtained from twenty patients undergoing arthroscopic shoulder surgery. Passive mechanical tests of both individual fibers and fiber bundles as well as analysis of titin molecular weight and collagen content were performed.

RESULTS

At the fiber-bundle level, a significant increase in passive modulus was observed between intact supraspinatus samples (mean [and standard error], 237.41 ± 59.78 kPa) and torn supraspinatus samples (515.74 ± 65.48 kPa) (p < 0.05), a finding that was not observed at the single fiber level. Within the torn samples, elastic moduli in the supraspinatus were greater than in the infraspinatus at both the single fiber and the fiber-bundle level. There was a significant positive correlation between bundle elastic modulus and collagen content (r(2) = 0.465) in the supraspinatus muscle as well as a significant positive correlation between tear size and bundle elastic modulus (r(2) = 0.702) in the torn supraspinatus samples.

CONCLUSIONS

Supraspinatus muscle passive tension increases in a tendon tear size-dependent manner after tendon injury. The increase in muscle stiffness appears to originate outside the muscle cell, in the extracellular matrix.

CLINICAL RELEVANCE

Muscle stiffness after rotator cuff tendon injury is more severe with large tears. This finding supports the concept of early intervention, when tendon tears are smaller, and interventions targeting the extracellular matrix.

Skeletal muscle fibrosis and stiffness increase after rotator cuff tendon injury and neuromuscular compromise in a rat model

Rotator cuff tears can cause irreversible changes (e.g., fibrosis) to the structure and function of the injured muscle(s). Fibrosis leads to increased muscle stiffness resulting in increased tension at the rotator cuff repair site. This tension influences repairability and healing potential in the clinical setting. However, the micro- and meso-scale structural and molecular sources of these whole-muscle mechanical changes are poorly understood. Here, single muscle fiber and fiber bundle passive mechanical testing was performed on rat supraspinatus and infraspinatus muscles with experimentally induced massive rotator cuff tears (Tenotomy) as well as massive tears with chemical denervation (Tenotomy + BTX) at 8 and 16 weeks post-injury. Titin molecular weight, collagen content, and myosin heavy chain profiles were measured and correlated with mechanical variables. Single fiber stiffness was not different between controls and experimental groups. However, fiber bundle stiffness was significantly increased at 8 weeks in the Tenotomy + BTX group compared to Tenotomy or control groups. Many of the changes were resolved by 16 weeks. Only fiber bundle passive mechanics was weakly correlated with collagen content. These data suggest that tendon injury with concomitant neuromuscular compromise results in extra-cellular matrix production and increases in stiffness of the muscle, potentially complicating subsequent attempts for surgical repair.

Collagen content does not alter the passive mechanical properties of fibrotic skeletal muscle in mdx mice

Many skeletal muscle diseases are associated with progressive fibrosis leading to impaired muscle function. Collagen within the extracellular matrix is the primary structural protein providing a mechanical scaffold for cells within tissues. During fibrosis collagen not only increases in amount but also undergoes posttranslational changes that alter its organization that is thought to contribute to tissue stiffness. Little, however, is known about collagen organization in fibrotic muscle and its consequences for function. To investigate the relationship between collagen content and organization with muscle mechanical properties, we studied mdx mice, a model for Duchenne muscular dystrophy (DMD) that undergoes skeletal muscle fibrosis, and age-matched control mice. We determined collagen content both histologically, with picosirius red staining, and biochemically, with hydroxyproline quantification. Collagen content increased in the mdx soleus and diaphragm muscles, which was exacerbated by age in the diaphragm. Collagen packing density, a parameter of collagen organization, was determined using circularly polarized light microscopy of picosirius red-stained sections. Extensor digitorum longus (EDL) and soleus muscle had proportionally less dense collagen in mdx muscle, while the diaphragm did not change packing density. The mdx muscles had compromised strength as expected, yet only the EDL had a significantly increased elastic stiffness. The EDL and diaphragm had increased dynamic stiffness and a change in relative viscosity. Unexpectedly, passive stiffness did not correlate with collagen content and only weakly correlated with collagen organization. We conclude that muscle fibrosis does not lead to increased passive stiffness and that collagen content is not predictive of muscle stiffness.

Lumbar muscle dysfunction during remission of unilateral recurrent nonspecific low-back pain: evaluation with muscle functional MRI

OBJECTIVES

After cessation of a low-back pain (LBP) episode, alterations in trunk muscle behavior, despite recovery from pain, have been hypothesized to play a pathogenic role in the recurrence of LBP. This study aimed to identify the presence of lumbar muscle dysfunction during the remission of recurrent LBP, while performing a low-load trunk-extension movement.

METHODS

Thirteen participants with unilateral recurrent LBP were tested at least 1 month after cessation of the previous LBP episode and were compared with a healthy control group without any history of LBP (n=13). Also, differences between previously painful and nonpainful sides were examined. Muscle functional magnetic resonance imaging, based on quantitative T2-imaging, was used to examine muscle tissue characteristics (T2 rest) and muscle recruitment (T2 shift) during prone trunk extension. The lumbar multifidus, erector spinae, quadratus lumborum, and psoas were bilaterally visualized on 2 lumbar levels using a T2-weighted (spin-echo multicontrast) magnetic resonance imaging sequence.

RESULTS

Linear mixed model analysis revealed a significantly lower T2 rest (P=0.044) and a significantly higher T2 shift (P=0.034) solely for the multifidus in the LBP group compared with the control group. No significant differences between pain sides were found.

DISCUSSION

Lower T2-rest values have been suggested to correlate with a conversion of the multifidus' fiber typing toward the glycolytic muscle spectrum. Elevated T2 shifts correspond with increased levels of metabolic activity in the multifidus in the LBP group, for which several hypotheses can be put forward. Taken together, these findings provide evidence of concurrent alterations in the multifidus structure and activity in individuals with unilateral recurrent LBP, despite being pain free and functionally recovered.

Human skeletal muscle biochemical diversity

The molecular components largely responsible for muscle attributes such as passive tension development (titin and collagen), active tension development (myosin heavy chain, MHC) and mechanosensitive signaling (titin) have been well studied in animals but less is known about their roles in humans. The purpose of this study was to perform a comprehensive analysis of titin, collagen and MHC isoform distributions in a large number of human muscles, to search for common themes and trends in the muscular organization of the human body. In this study, 599 biopsies were obtained from six human cadaveric donors (mean age 83 years). Three assays were performed on each biopsy - titin molecular mass determination, hydroxyproline content (a surrogate for collagen content) and MHC isoform distribution. Titin molecular mass was increased in more distal muscles of the upper and lower limbs. This trend was also observed for collagen. Percentage MHC-1 data followed a pattern similar to collagen in muscles of the upper extremity but this trend was reversed in the lower extremity. Titin molecular mass was the best predictor of anatomical region and muscle functional group. On average, human muscles had more slow myosin than other mammals. Also, larger titins were generally associated with faster muscles. These trends suggest that distal muscles should have higher passive tension than proximal ones, and that titin size variability may potentially act to 'tune' the protein's mechanotransduction capability.

Passive mechanical properties of rat abdominal wall muscles suggest an important role of the extracellular connective tissue matrix

Abdominal wall muscles have a unique morphology suggesting a complex role in generating and transferring force to the spinal column. Studying passive mechanical properties of these muscles may provide insights into their ability to transfer force among structures. Biopsies from rectus abdominis (RA), external oblique (EO), internal oblique (IO), and transverse abdominis (TrA) were harvested from male Sprague-Dawley rats, and single muscle fibers and fiber bundles (4-8 fibers ensheathed in their connective tissue matrix) were isolated and mechanically stretched in a passive state. Slack sarcomere lengths were measured and elastic moduli were calculated from stress-strain data. Titin molecular mass was also measured from single muscle fibers. No significant differences were found among the four abdominal wall muscles in terms of slack sarcomere length or elastic modulus. Interestingly, across all four muscles, slack sarcomere lengths were quite long in individual muscle fibers (>2.4 µm), and demonstrated a significantly longer slack length in comparison to fiber bundles (p < 0.0001). Also, the extracellular connective tissue matrix provided a stiffening effect and enhanced the resistance to lengthening at long muscle lengths. Titin molecular mass was significantly less in TrA compared to each of the other three muscles (p < 0.0009), but this difference did not correspond to hypothesized differences in stiffness.

MODELING THE EFFECT OF VARIATION IN SAGITTAL CURVATURE ON THE FORCE REQUIRED TO PRODUCE A FOLLOWER LOAD IN THE LUMBAR SPINE

The aim of this study was to investigate how the forces required to stabilize the lumbar spine in the standing posture may be affected by variation in its shape. A two-dimensional model of the lumbar spine in the sagittal plane was developed that included a simplified representation of the lumbar extensor muscles. The shape of the model was varied by changing both the magnitude and distribution of the lumbar curvature. The forces required to produce a resultant load traveling along a path as close to the vertebral body centroids as possible (a follower load) were determined. In general,the forces required to produce a follower load increased as the curvature became larger and more evenly distributed. The results suggest that the requirements of the lumbar muscles to maintain spinal stability in vivo will vary between individuals. This has implications for understanding the role of spinal curvature and muscle atrophy in back pain.

Structure and function of the skeletal muscle extracellular matrix

The skeletal muscle extracellular matrix (ECM) plays an important role in muscle fiber force transmission, maintenance, and repair. In both injured and diseased states, ECM adapts dramatically, a property that has clinical manifestations and alters muscle function. Here we review the structure, composition, and mechanical properties of skeletal muscle ECM; describe the cells that contribute to the maintenance of the ECM; and, finally, overview changes that occur with pathology. New scanning electron micrographs of ECM structure are also presented with hypotheses about ECM structure–function relationships. Detailed structure–function relationships of the ECM have yet to be defined and, as a result, we propose areas for future study.

Psoas muscle architectural design, in vivo sarcomere length range, and passive tensile properties support its role as a lumbar spine stabilizer

STUDY DESIGN

Controlled laboratory and cross-sectional study designs.

OBJECTIVE

To determine psoas major (PM) muscle architectural properties, in vivo sarcomere-length operating range, and passive mechanical properties.

SUMMARY OF BACKGROUND DATA

PM is an important hip flexor but its role in lumbar spine function is not fully understood. Several investigators have detailed the gross anatomy of PM, but comprehensive architectural data and in vivo length-tension and passive mechanical behaviors have not been documented.

METHODS

PM was isolated in 13 cadaver specimens, permitting architectural measurements of physiological cross-sectional area (PCSA), normalized fiber length (Lf), and Lf:muscle length (Lm) ratio. Sarcomere lengths were measured in vivo from intraoperative biopsies taken with the hip joint in flexed and extended positions. Single-fiber and fiber bundle tensile properties and titin molecular weight were then measured from separate biopsies.

RESULTS

Architecturally, average PCSA was 18.45 ± 1.32 cm2, average Lf was 12.70 ± 2 cm, and average Lf: Lm was 0.48 ± 0.06. Intraoperative sarcomere length measurements revealed that the muscle operates from 3.18 ± 0.20 μm with hip flexed at 10.7° ± 13.9° to 3.03 ± 0.22 μm with hip flexed at 55.9° ± 21.4°. Passive mechanical data demonstrated that the elastic modulus of the PM muscle fibers was 37.44 ± 9.11 kPa and of fiber bundles was 55.3 ± 11.8 kPa.

CONCLUSION

Analysis of PM architecture demonstrates that its average Lf and passive biomechanical properties resemble those of the lumbar erector spinae muscles. In addition, PM sarcomere lengths were confined to the descending portion of the length-tension curve allowing the muscle to become stronger as the hip is flexed and the spine assumes a forward leaning posture. These findings suggest that the human PM has architectural and physiologic features that support its role as both a flexor of the hip and a dynamic stabilizer of the lumbar spine.

Effects of surgical joint destabilization on load sharing between ligamentous structures in the thoracic spine: a finite element investigation

BACKGROUND

In vitro investigations have demonstrated the importance of the ribcage in stabilizing the thoracic spine. Surgical alterations of the ribcage may change load-sharing patterns in the thoracic spine. Computer models are used in this study to explore the effect of surgical disruption of the rib-vertebrae connections on ligament load-sharing in the thoracic spine.

METHODS

A finite element model of a T7-8 motion segment, including the T8 rib, was developed using CT-derived spinal anatomy for the Visible Woman. Both the intact motion segment and the motion segment with four successive stages of destabilization (discectomy and removal of right costovertebral joint, right costotransverse joint and left costovertebral joint) were analyzed for a 2000 Nmm moment in flexion/extension, lateral bending and axial rotation. Joint rotational moments were compared with existing in vitro data and a detailed investigation of the load sharing between the posterior ligaments carried out.

FINDINGS

The simulated motion segment demonstrated acceptable agreement with in vitro data at all stages of destabilization. Under lateral bending and axial rotation, the costovertebral joints were of critical importance in resisting applied moments. In comparison to the intact joint, anterior destabilization increases the total moment contributed by the posterior ligaments.

INTERPRETATION

Surgical removal of the costovertebral joints may lead to excessive rotational motion in a spinal joint, increasing the risk of overload and damage to the remaining ligaments. The findings of this study are particularly relevant for surgical procedures involving rib head resection, such as some techniques for scoliosis deformity correction.

ISSLS prize winner: Adaptations to the multifidus muscle in response to experimentally induced intervertebral disc degeneration

STUDY DESIGN

Basic science study of the rabbit multifidus muscle response to intervertebral disc degeneration.

OBJECTIVE

To assess changes in passive mechanical properties, associated protein structure, and histology of multifidus in response to disc degeneration produced by experimental needle puncture.

SUMMARY OF BACKGROUND DATA

Relationships have been reported between muscle dysfunction and low back injury; however, little is known about the cause and effect of such relationships.

METHODS

Twelve rabbits were studied; 4 in each of 3 groups: control, 4-weeks postintervertebral disc injury (4-week disc degeneration), and 12-weeks postintervertebral disc injury (12-week disc degeneration). Single multifidus fibers and bundles of fibers were isolated and tested for slack sarcomere length and elastic modulus. Titin isoform mass, myosin heavy chain distribution, and muscle histology were also examined.

RESULTS

Compared to control, individual muscle fibers were 34% stiffer and fiber bundles 107% stiffer in the 12-week disc degeneration group. No changes were detected at 4-week disc degeneration. No statistically significant change was found for MHC distribution in the 12-week disc degeneration group when compared to control, whereas titin isoforms were larger (P < 0.05) in the 12-week disc degeneration group. Histology revealed select regions of multifidus, at 12-week disc degeneration, with increased space between bundles of fibers, which in some instances was partly occupied by adipose tissue.

CONCLUSION

Multifidus becomes stiffer, both in individual fibers and fiber bundles, in response to experimentally induced intervertebral disc degeneration. This cannot be explained by change in fiber-type due to reduced muscle use, nor by the increased size of the protein titin (which would reduce stiffness). We hypothesize that fiber bundles become stiffer by proliferation and/or reorganization of collagen content within the muscle but the basis for fiber stiffening is not known.

Skeletal muscle design to meet functional demands

Skeletal muscles are length- and velocity-sensitive force producers, constructed of a vast array of sarcomeres. Muscles come in a variety of sizes and shapes to accomplish a wide variety of tasks. How does muscle design match task performance? In this review, we outline muscle's basic properties and strategies that are used to produce movement. Several examples are provided, primarily for human muscles, in which skeletal muscle architecture and moment arms are tailored to a particular performance requirement. In addition, the concept that muscles may have a preferred sarcomere length operating range is also introduced. Taken together, the case is made that muscles can be fine-tuned to perform specific tasks that require actuators with a wide range of properties.

Muscle extracellular matrix applies a transverse stress on fibers with axial strain

It is widely assumed that skeletal muscle contraction is isovolumic. This assumption has been verified at the single fiber and at the myofibril level. Model development and mechanical analyses often exploit this assumption when investigating skeletal muscle and evaluating muscle mechanical properties. This communication describes a method whereby individual muscle fibers and bundles of fibers, which include their constituent extracellular matrix (ECM), were tested to define the change in volume with axial strain. The results demonstrate that fibers are isovolumic, but bundles decrease in volume with strain. The loss of volume implicates a transverse force being applied to the fibers by the ECM. The nature and importance of this transverse force warrant further investigation.

Hamstring contractures in children with spastic cerebral palsy result from a stiffer extracellular matrix and increased in vivo sarcomere length

Cerebral palsy (CP) results from an upper motoneuron (UMN)lesion in the developing brain. Secondary to the UMNl esion,which causes spasticity, is a pathological response by muscle - namely, contracture. However, the elements within muscle that increase passive mechanical stiffness, and therefore result in contracture, are unknown. Using hamstring muscle biopsies from pediatric patients with CP (n =33) and control (n =19) patients we investigated passive mechanical properties at the protein, cellular, tissue and architectural levels to identify the elements responsible for contracture. Titin isoform, the major load-bearing protein within muscle cells, was unaltered in CP. Correspondingly, the passive mechanics of individual muscle fibres were not altered. However, CP muscle bundles, which include fibres in their constituent ECM, were stiffer than control bundles. This corresponded to an increase in collagen content of CP muscles measured by hydroxyproline assay and observed using immunohistochemistry. In vivo sarcomere length of CP muscle measured during surgery was significantly longer than that predicted for control muscle. The combination of increased tissue stiffness and increased sarcomere length interact to increase stiffness greatly of the contracture tissue in vivo. These findings provide evidence that contracture formation is not the result of stiffening at the cellular level, but stiffening of the ECM with increased collagen and an increase of in vivo sarcomere length leading to higher passive stresses.

Clinical, biomechanical, and physiological translational interpretations of human resting myofascial tone or tension

BACKGROUND

Myofascial tissues generate integrated webs and networks of passive and active tensional forces that provide stabilizing support and that control movement in the body. Passive [central nervous system (CNS)-independent] resting myofascial tension is present in the body and provides a low-level stabilizing component to help maintain balanced postures. This property was recently called "human resting myofascial tone" (HRMT). The HRMT model evolved from electromyography (EMG) research in the 1950s that showed lumbar muscles usually to be EMG-silent in relaxed gravity-neutral upright postures.

METHODS

Biomechanical, clinical, and physiological studies were reviewed to interpret the passive stiffness properties of HRMT that help to stabilize various relaxed functions such as quiet balanced standing. Biomechanical analyses and experimental studies of the lumbar multifidus were reviewed to interpret its passive stiffness properties. The lumbar multifidus was illustrated as the major core stabilizing muscle of the spine, serving an important passive biomechanical role in the body.

RESULTS

Research into muscle physiology suggests that passive resting tension (CNS-independent) is generated in sarcomeres by the molecular elasticity of low-level cycling cross-bridges between the actomyosin filaments. In turn, tension is complexly transmitted to intimately enveloping fascial matrix fibrils and other molecular elements in connective tissue, which, collectively, constitute the myofascial unit. Postural myofascial tonus varies with age and sex. Also, individuals in the population are proposed to vary in a polymorphism of postural HRMT. A few people are expected to have outlier degrees of innate postural hypotonicity or hypertonicity. Such biomechanical variations likely predispose to greater risk of related musculoskeletal disorders, a situation that deserves greater attention in clinical practice and research. Axial myofascial hypertonicity was hypothesized to predispose to ankylosing spondylitis. This often-progressive deforming condition of vertebrae and sacroiliac joints is characterized by stiffness features and particular localization of bony lesions at entheseal sites. Such unique features imply concentrations and transmissions of excessive force, leading to tissue micro-injury and maladaptive repair reactions.

CONCLUSIONS

The HRMT model is now expanded and translated for clinical relevance to therapists. Its passive role in helping to maintain balanced postures is supported by biomechanical principles of myofascial elasticity, tension, stress, stiffness, and tensegrity. Further research is needed to determine the molecular basis of HRMT in sarcomeres, the transmission of tension by the enveloping fascial elements, and the means by which the myofascia helps to maintain efficient passive postural balance in the body. Significant deficiencies or excesses of postural HRMT may predispose to symptomatic or pathologic musculoskeletal disorders whose mechanisms are currently unexplained.

Regional Myosin heavy chain distribution in selected paraspinal muscles

STUDY DESIGN

Cross-sectional study with repeated measures design.

OBJECTIVE

To compare the myosin heavy-chain isoform distribution within and between paraspinal muscles and to test the theory that fiber-type gradients exist as a function of paraspinal muscle depth.

SUMMARY OF BACKGROUND DATA

There is still uncertainty regarding the fiber-type distributions within different paraspinal muscles. It has been previously proposed that deep fibers of the multifidus muscle may contain a higher ratio of type I to type II fibers, because, unlike superficial fibers, they primarily stabilize the spine, and may therefore have relatively higher endurance. Using a minimally invasive surgical approach, using tubular retractors that are placed within anatomic intermuscular planes, it was feasible to obtain biopsies from the multifidus, longissimus, iliocostalis, and psoas muscles at specific predefined depths.

METHODS

Under an institutional review board-approved protocol, muscle biopsies were obtained from 15 patients who underwent minimally invasive spinal surgery, using the posterior paramedian (Wiltse) approach or the minimally invasive lateral approach. Myosin heavy chain (MyHC) isoform distribution was analyzed using SDS-PAGE (sodium dodecyl sulfate polyacrylamide gel electrophoresis) electrophoresis. Because multiple biopsies were obtained from each patient, MyHC distribution was compared using both within- and between-muscle repeated measures analyses.

RESULTS

The fiber-type distribution was similar among the posterior paraspinal muscles and was composed of relatively high percentage of type I (63%), compared to type IIA (19%) and type IIX (18%) fibers. In contrast, the psoas muscle was found to contain a lower percentage of type I fibers (42%) and a higher percentage of type IIA (33%) and IIX fibers (26%; P<0.05). No significant difference was found for fiber-type distribution among 3 different depths of themultifidus and psoas muscles.

CONCLUSION

Fiber-type distribution between the posterior paraspinal muscles is consistent and is composed of relatively high percentage of type I fibers, consistent with a postural function. The psoas muscle, on the other hand, is composed of a higher percentage of type II fibers such as in the appendicular muscles. Our data do not support the idea of a fiber-type gradient as a function of depth for any muscle studied.