Measuring Muscle Visco-Elasticity in Horizontal Bed Rest Model for Better Understanding of Sedentary Life Style

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May 21, 2013

R. Viir, Institute of Exercise Biology and Physiotherapy, University of Tartu, TartuEstonia

 RehabCenter, RheumatismFoundationHospital, Heinola, Finland


M. Pääsuke, Institute of Exercise Biology and Physiotherapy, University of Tartu, TartuEstonia

K. Rajaleid, Center for Health Equity Studies, StockholmUniversity/ Karolinska Institutet, Stockholm, Sweden

*Note: The source for this paper is the 2008 ISU 12th Annual Symposium – ‘Space Solutions to Earth’s Global Challenges’


The aim of this study was to use the upper trapezius muscle as a representative of the musculoskeletal support system to determine the effect on muscle tension due changing from upright position to a lying position and whether this change could be useful in the prevention of musculoskeletal disorders. Twenty two female subjects participated in this study. Myometric measurements of the upper trapezius muscle on both sides of the body were recorded in a standing, sitting and supine position. Changing from upright positions to a supine position reduced the upper trapezius muscle support function by up to one fifth as characterized with diminishing of muscle tone and stiffness. The change in tone and stiffness of the trapezius muscle is of significance to the well being of sedentary workers. Introducing regular brief breaks of simple unchallenging movements while being in a supine position should enhance recovery from prolonged sitting.

1. Background

1.1 Model from History

In his treatise De Motu Animalium the father of modern biomechanics Giovanni Alfonso Borelli stated already 300 hundred years ago:  ‘…muscular tensions inevitably have to be involved in holding posture’ (1). Also, father of occupational medicine Bernardino Ramazzini said the following about causation of clerk’s neck & shoulder complaints: “Yet ’tis certain that in each City and Town, vast Numbers of Persons still earn their Bread by writing. The Diseases of Persons incident to this Craft arise from three Causes: First, constant sitting, secondly, the perpetual Motion of the Hand in the same manner and thirdly, the Attention and Application of the Mind” (2). Ramazzini listed several other maladies related to prolonged sitting.

Great minds didn’t have more or less sophisticated devices like EMG (Electromyograph), MRE (Magnetic resonance elastography) or MYOTON (Myometric measuring device, as fully described by Vain et al. [10]) to characterise skeletal muscle in different functions and its properties. Indeed their tools were those like inspection and palpation, and system-biological deep pondering upon their observations integrability.

How did scientists characterise the skeletal muscle ensuring the sitting posture in 1700. Obviously, there was interest for it. One could use the tactile sense of the fingers and establish the change in e.g. upper trapezius (UT) muscle in between these positions by palpation. The change can clearly be established – palpation, sensing with fingers is a good method. Anybody can make this discovery palpating their own UT muscles in these positions – much in line with Borelli and Ramazzini. Still, the result of the palpation study cannot be shared with other investigators – this is the limitation of the sensing method – that are why it cannot be used in modern scientific research.

After 300 years passed, Ariëns and her colleagues in 2001 (3) used questionnaire study to conclude that sitting position is an independent risk factor for developing neck pain for the people that sit 95% of the working time.

1.2 Model from Space Life Studies

Human is quite an extraordinary mammal in a sense that during the whole life, each day we change the direction of the longitudinal axis relative to most significant environmental factor – the gravitational force. From morning till night our longitudinal axis relative Earth’s radius have to be supported against gravitational force. By night, the situation is changed drastically by 90° when we lie horizontally, which eliminates the need for the vertical support function. Humans spend in this position one third of the lifetime. Lying down works as the most natural recovering intervention to start the next day. If we were to imagine viewing the whole human life during a one-hour movie, the most noticeable activity would be change between (standing,) sitting and lying. If anyone would observe this from outer space, it could be considered a major factor for understanding our physiology.

On the Earth, the approach of horizontal bed rest microgravity simulation studies is based on the same changing of the longitudinal axis by 90°; the rhythm of the change is different. Staying in horizontal position for couple of months without the need to counteract with the gravitational force causes disuse of muscles and osteoporosis in bones as well as numerous other changes in the whole organism also in organ systems and the cellular level; but the applications of the bed rest (BR) research reveal the crucial role of gravity to health (4). In principle, studying the effect of microgravity on life deals with support function qualitative and quantitative loading. Zero-G spaceflight provokes inherent supportlessness condition in human body with its consequences.

1.3 Model from Robots

For biomechanics, design of bipedal robots is mutually stimulating. Their applications represent experiments, which may support or falsify ideas about the organization and function of living motion and support systems. Bipedal robot – being also a model of a human – needs support and locomotion system. Robots must have flexible waist, which together with their legs have to be supplied with muscle-like property of elasticity/damping ability. This gives to robots self-stability and humanoid-like economic smooth gait, the same time diminishing the need of “constant neural” control (5, 6, and 7).

Bipedal walking – a priori most physiological motion series for human – is absent in weightlessness of Space. The floor is absent. Efforts are made with promising results to offset Earth based rhythmic ground reactions with the Dynamic Foot Stimulus device to combat musculoskeletal disorders in microgravity (8, 9).

On the Earth, we should think of lifelong unassisted natural walking as the most harmonically orchestrated function of a human body in synchrony with breathing, heart beating and triggering pleasure giving raise of theta rhythms from hippocampal neurones (10).

2. Study population and methods

2.1 Study population

The participants were 22 employed working female patients with widespread pain in their rehabilitation cycle in Rheumatism Foundation Hospital. Each of the participants gave their informed written consent. The study was vetted by the Ethical Committee of the Päijät–Häme Hospital District to ensure that it was conducted under the terms of the Declaration of Helsinki.

2.2 Methods

In step 1, each participant stood in relaxed position, upper extremities pendent on sides. They were asked to focus on a mark 4 meters away in order to fix the tilt of the head and the angle of the neck. Non-toxic marks were highlighted on the skin above the middle of the upper trapezius belly halfway from the acromion to the seventh cervical (C7) process.

In step 2, a four-legged wooden chair with a padded backrest and seat without armrests, and without height adjustability was used. Each participant sat in a comfortable relaxed upright position, hands in lap. Again, they were asked to focus on a mark 4 meters away descended on the eyes’ level of participants.

In step 3, the participant lay comfortably supine on a padded examination table.

In all steps the myometric measuring device (figure 1), as fully described by Vain et al. (11) Viir et al. (12) and Gavronski et al. (13), was applied to the marks, and 20 consecutive measurements (at an interval of 1–2 seconds between each) were done twice in two series. Participant was allowed to fidget between two measurement series. Then we compared the results obtained in upright positions with those obtained in the supine position. The differences, significant at the 95% level, are reported on the basis of a Student’s t-test.

2.3 Myometry method

Briefly, the Myoton device registers mechanical oscillations of the tissue provoked by the device itself—the testing-end component of the apparatus works also as a sensor for tissue response. An acceleration transducer, situated on the testing-end, allows the muscle deformation evoked by mechanical impact (0.4 N) to be recorded. Because the neural activation of the skeletal muscle may occur after 25 ms and the properties of the muscle may change (13), the duration of the impact is set at 15 ms to avoid neural reactions and nonelastic deformations of the tissue. In response to the given impact, the tissue (in our case, muscle tissue), together with the testing end, performs natural damping oscillations, governed by the viscoelastic properties of the biological tissue.

Whereas electromyography registers the parameters of electrical activity of the skeletal muscle, the parameters measured by the Myoton device reflect the conditions (i.e., the workability restoration time of muscles during work and after it), and the character of mechanical tension transmission from the sarcomere to the bone levers (15).


Figure 1. The Myoton device and the myometric method (illustration designed by Ivo-Ott Hirvesoo). The principle of myometry lies in giving the muscle under investigation a dosed local mechanical impulse shortly followed by quick release, and recording the mechanical response of the muscle. [Figure reprinted with the permission of Journal of Mechanics in Medicine and Biology [12] and the World Scientific Pub­lishing Co Pte, Ltd, Singapore]

2.4 Muscle tone and established parameters

The term “muscle tone” is used to describe the mechanical firmness of skeletal muscle that exists when the muscles are in a steady-state condition, with no voluntary contraction, and can be characterized by certain mechanical properties of the muscle (e.g., stiffness and elasticity).

Three parameters were calculated – muscle frequency, stiffness, and elasticity. The frequency of the damping oscillation characterizes the state of the tissue under mechanical stress. The higher value (Hz) means the tenser the muscle. Muscle tension increases through both contraction and stretching. By definition, the oscillation frequency of the nonactive muscle is muscle tone.

Stiffness reflects the resistance of the tissue to the force that changes its shape. The higher the value (N/m), the more force is needed to modify the shape of tissue. During contraction or loading, the stiffness of skeletal muscle increases.

Elasticity is the ability of tissue to recover its shape after contraction, and it is characterized by the (unit-less parameter) logarithmic decrement of the oscillation. It describes how much mechanical energy is dissipated in this damping. The smaller values mean more elastic tissue. The higher the value, the more energy dissipated and the less elastic the tissue. Opposite of elasticity is plasticity – a plastic body holds the shape it is given.

3. Results

The individual measurement results were combined for each position, and the similarity to normal distribution appeared. The results of the Student’s t-tests follow. The Tukey – Kramer plot was used because it can combine a display of all the data together with a statistical summary and the concise graphs are easily interpreted.


Figure 2. Tukey-Kramer plots: a) Stiffness (stiff) [N/m] and b) tone (freq) [(Hz] in standing 236.07 N/m and 14.76 Hz, sitting 225.7 N/m and 14.26 Hz, lying 125.8 N/m and 9.57 Hz respectively. Stiffness and tone in standing and lying P < 0.0001, stiffness in lying P = 0.04 and tone in lying P = 0.03.

No significant difference is in all parameters between both sides, no significant difference appeared in any parameters between the pre-post fidgeting measurement series, no significant difference is between parameters of stiffness and tone between standing and sitting positions. Significant difference in stiffness and tone in upper trapezius muscle is between upright and lying supine positions. Positioning doesn’t influence the values of UT elasticity (left side 1.09, right 1.05, P=0.09).

4. Discussion

Waging war on the modern chronic diseases provoked by physical inactivity, i.e. sedentary life style requires recognition of targets. The real enemy is Sitting Immobility.  It is our contention that two terms ‘Sedentary Life Style’ and ‘Sitting Immobility’ are essentially near synonyms. The word “sedentary” is derived from the Latin word “sedentarius,” which means “one that sits,” explained by Booth et al. (16) who have defined that individuals who cannot walk briskly at least 30 min each day are sitting too much of the day.

In a physical sense, common life as we know it, is not a chain of constant sport-like efforts, it is rather carrying our own mass and other weights that cannot be considered large. Thus tensed network of musculoskeletal system related to stiffness and elasticity from muscle tissue’s micro and macro levels has to be adapted to life-span mechanical stress. This process could be seen as the mechanical aging of muscular system.

Alarming news: sedentary lifestyle makes us genetically old before our time: key pieces of DNA telomeres shorten more quickly in people spending sedentary life style – this could signify faster cellular ageing (17). A clear motive that is understandable to everyone would be needed to take optimal care of musculoskeletal system.

Optimally up to 40% of human body mass is skeletal muscle tissue. Being continuously informed about the condition of this locomotion engine (i.e. about quality of viscoelastic property of this) is a challenge that needs to be met. Nobody wants to shorten their lifespan, obviously also not with prolonged sitting, if the effect of this is known.

Common sitting immobility with voluntary noncontracted muscles can hardly be characterised by EMG as the neural activation is seen to be low, or virtually absent, but recording visco-elastic properties is quite easily possible, by myometric test (18).

We exhibit that sitting position requires up to 1/5 more tension in UT muscle as compared to the horizontal position. We recommend utilising this phenomenon for regular recovery of muscles’ visco-elastic properties of the people that sit for prolonged periods. In clinical practice we have also obtained results (besides the ones presented within this paper) indicating reduced duration of the morning stiffness and diminished diffuse swelling in limbs of patients with diagnosed rheumatoid arthritis, when they exercised the horizontal movements in their beds 10 times a day. They even organised a so-called “Baby-like Exercising Patients Association” (19).

This positive practical experience has encouraged us to clarify the underlying causation mechanism. For this we have assessed different subject groups. The results of the present study are similar to the results of the study with the same setup in which 15 young female computer workers participated (18).

The solution we propose to sedentary workers, borrowed from Bed Rest study is not lying the whole”siesta time”. Our proposal is: let’s combine diminishing of support function loading and rhythmical walking simulation in horizontal. For shoulder girdle specific simple unchallenging motions are proposed. Two minutes intermittently both actions per hour for muscular system recovering from continuous position controlling.

We see good prospects for such rehabilitation and prevention programme for addressing the complaints stemming from sitting work; potentially the problems that are also costly for the employers in a global sense could find a simple remedy.

It is our contention that every footstep makes life healthier and longer. Still, the current consensus that a 30 minute brisk walk couple times in a week is sufficient for staying healthy only specifies the minimum level, not optimal. Further, we are looking for results of rehab with walking two or more hours per day. A human with the specific ability for bipedal walking should be ready/capable of this.


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