Muscle Strength

Shrawan Kumar

• Editorial: CRC Press
• Año: 2004
• ISBN:
  9781134202195
• Edición: 1
• Categoría: Medicina
• Categoría: Fisioterapia, medicina física y rehabilitación

Muscle strength is an important topic for ergonomics practitioners and physiologists to understand, especially as it relates to workplace injuries. Muscle strength and function is at the heart of many injuries that lead to reduced productivity and economic strain on the worker, the company, and society as a whole.This comprehensive source o

1
Introduction and Terminology

Shrawan Kumar

CONTENTS

• 1.1 Ergonomic Relevance of Strength
• 1.2 Strength
  • 1.2.1 What Is
  • 1.2.2 How Is It Created?
• 1.3 Evolution of Strength
• 1.4 Terminology Used in
  • 1.4.1 Scientific Classification
  • 1.4.2 Applied Terminology
    • 1.4.2.1 Static
    • 1.4.2.2 Dynamic
    • 1.4.2.3 Psychophysical Strength
• 1.5 Strength Parameters and
  • 1.5.1 Peak vs. Average
  • 1.5.2 Force Transmission
• 1.6 Occupational Injury Considerations
  • 1.6.1 Differential Fatigue
  • 1.6.2 Cumulative Load Theory
  • 1.6.3 Overexertion Theory
• 1.7 Summary
• References

1.1 Ergonomic Relevance of Strength

Industrial activity around the world is performed by two-thirds of the world population over 10 years of age spending one third of their lives on work (WHO, 1995). This enormous workforce, through its contribution, generates a wealth of 21.3 trillion U.S. dollars, which sustains the socioeconomic fabric of the international society. Given the uneven distribution of population between developed and developing countries, a large majority of humanity is resorting to mechanical force application to accomplish their tasks. Additionally, in spite of the progression of technology, there are a great number of jobs in developed countries that require muscle force application (e.g., mining, forestry, agriculture, drilling and exploration, manufacturing and service sector). In many recreational activities and sports, as well as industry, force application is the primary requirement after skill. Thus, force application is essential to getting things done, more in some activities than others. The quantity, duration, and frequency of force application at a job provide an essential tool to gauge the job demands. A determination of such demands, when compared against the worker's ability to exert force (strength), provides ergonomists a meaningful measure of stress on workers. Even in developed countries a large number of tasks are not automated, due to the cost and complexity. Consequently, the responsibility of strength exertion falls on the workers. It has been estimated that, in order to produce one ton of product, a worker has to lift or manually handle anywhere between 80 and 320 tons of raw materials. Workers in an industrial economy are a very important resource, and for industrial health, they must be protected. The values for strength requirement and strength available are important data, which need to be used in job design for worker health and safety and industrial productivity.

As early as 1978 Chaffin et al. reported that worker safety is seriously jeopardized when the job strength requirement exceeds the isometric strength of the worker. The opinion for using strength data, especially in designing manual materials handling jobs, is particularly well supported (Ayoub and McDaniel, 1974; Chaffin and Park, 1973; Chaffin et al., 1978; Davis and Stubbs, 1980; Garg et al., 1980; Kamon et al., 1982; Keyserling et al., 1980; Kroemer, 1983; Kumar, 1991a and b, 1995; Kumar and Garand, 1992; Mital et al., 1993).

1.2 Strength

1.2.1 What Is It?

Strength in the context of this book is human ability to exert physical force and is measured as the maximum force one can exert in a single maximal voluntary contraction (MVC). However, it must be recognized that the overall manifestation of force will be dependent on the length of the lever arm on which the force is acting. The product of the force and moment arm is designated as torque, and the product of force and time is called impulse. If one measures the product of torque and time, it is called impulse over time or cumulative load.

Force (F) = Newtons (N)

Torque (T) = N × meters (Nm)

1.2.2 How Is It Created?

A detailed description of the mechanism of force generation is provided in Chapters 2 and 4. However, a very brief version follows. The force is generated by contraction of skeletal muscles. In fact, force is generated by smooth (involuntary) muscles as well. However, since smooth muscles are postural muscles and under the control of autonomic nervous system, we do not exercise them for industrial use. Therefore, the entire discussion of muscle strength in this book relates to skeletal (voluntary) muscles. Contraction of these muscles can be produced at will in any configuration. The fundamental property of life “irritability” is expressed in the physical domain through these muscle contractions. Such muscle contractions allow us to conduct all our voluntary activities — work, leisure, and activities of daily living.

The mechanism of contraction of skeletal muscle has been a subject of vigorous debate in recent years. The initial mechanism (proposed by Huxley, 1957) suggested that the actin filaments in each sarcomere, at the time of contraction, slide to the middle by repeated bonding, pulling in, release, and rebonding of the actin and myosin filaments. This caused shortening of each of the sarcomeres and, in turn, the entire muscle fiber. The phenomenon of contraction involves electrical impulses coming via the nerves as commands, releasing the neurotransmitters at motor end plate in the myoneural cleft. This alters the ionic balance of the sarcolemma (cell membrane) of the muscle fiber, causing the sarcoplasmic reticulum to release calcium ions, which are essential to bind actin and myosin. This bondage triggers a ratchet action, pulling the actin past the myosin. The bondage is released by adenosine triphosphate (ATP) to repeat the process.

The above-mentioned shortening of all sarcomeres in any given muscle fiber can reduce the length of muscle to anywhere between two-thirds to half its original length. Since muscles have a stable point of origin from one bone and are inserted across, at least, one joint on another bone, they exert a force on the joint to move through their connective tissue attachment (tendons). This application of force across a joint moves that joint. By a complex series of coordinated movements, we are able to do anything we want.

The alternate theory and supporting evidence of muscle contraction are presented in Chapter 4. The evidence for the alternate theory is mounting but it is by no means fully established and universally accepted.

1.3 Evolution of Strength

Ability to exert strength is not one of the fundamental properties of life. During the course of evolution a series of adaptations occurred in organisms, which were primarily driven by the necessity of food. In some very primitive organisms (which lived in ocean or other aquatic media) the nutritional needs were met by medium itself, for example planktons. The protozoans could also exhaust the nutritional supply by just staying at one place. They achieved their locomotion through streaming of their protoplasm and rolling along to reach additional sources. In others, for example paramecia, there were multitudes of cilia, which beat in a coordinated and rhythmic manner to cause the organisms to move from one place to the other more rapidly than amoebae. Yet another higher level of organization among protozoans was demonstrated by development of colonies, as seen in volvox. However, these colonies did not have any functional differentiation, but by increasing their sizes they were able to achieve even faster movement.

It was not until the multicellular organisms developed that the phenomenon of functional differentiation began to manifest. Clearly, if each cell of a multicellular organism were to do everything, it would have been very inefficient. Thus, differentiation of neuron and nervous system; stomodeum, alimentary canal, and proctodeum; gills and lungs; gonads and locomotor systems took place. With reference to the locomotor system, it became more essential due to bilaterian body plan (Shankland and Seaver, 2002). Also, the growing sizes of the organisms could not be supported by external cuticle as exoskeleton. With the increase of the size of organisms there was a need for an internal skeleton, which many structures could be anchored to or suspended from. The notochord changed into the axial skeleton to provide support and protection to the dorsal nervous system (Kent, 1992; Walker and Liem, 1994). With a need to locomote to different surroundings in search of food, the fishes developed fins, and other classes of the vertebrate phylum differentiated further their pectoral and pelvic girdles, with specialized skeletal limbs to suit their habitat. However, skeleton alone would not have been much use without skeletal muscles, which are the engines of movement. Whereas an undulating and sequential concentric contraction and expansion can be seen in many invertebrates, e.g., annelids, the contractile structure differentiated into a sophisticated skeletal muscle. These muscles have the architecture, physiology, and mechanical properties described in Chapters 2 and 4. In addition, there was a need for the organisms to stay in one place as intact organisms when they were not locomoting. To meet this requirement, development of involuntary muscles took place, which in all probability preceded the development of the skeletal muscle. As such, we find in our muscles slow and fast (or Type I and Type II) fibers, where division of labor has been such that postural loads are borne by Type I, and motion and force application is largely achieved through the Type II muscles.

In ergonomics, we are mostly interested in the Type II muscle and the internal load it develops. However, postural muscle's contribution for the maintenance of posture over a long period of time is also of considerable consequence. In activities where we involve Type II muscles, in static contractions, over and above the level provide by Type I, muscles have proven to be of considerable consequence.

1.4 Terminology Used in Strength

1.4.1 Scientific Classification

The scientific classification of strength had been based entirely on physiological state of the muscle in contraction. The two categories of strength described are: (a) static or isometric (from Greek, iso = equal; metrien = measure or length), and (b) dynamic or isotonic (from Greek, iso = equal; tonic = muscle tone) (Vander et al., 1975). It was, therefore, initially thought that in isometric (static) efforts the length of the muscle remained constant. Externally there is no discernible motion of limb segments or body parts involved in this contraction. This was the primary reason why this contraction was named isometric. However, the question arises that, if the muscle is not shortening, how is it exerting force on the external object. Thus, it is clear and universally accepted that the muscle does contract and shorten because the muscle exerts its force (or torque) on the bone into which its tendon is inserted across a joint. Tendons elongate between 4 and 8% of their initial short length to transmit the force. Since this length is almost miniscule compared to muscle length, there does not appear to be significant change in its length while exerting force in isometric mode. In spite of the fact the term isometric represents the exertion inaccurately, it continues to be used due to its universal popularity, and also because people understand what is meant.

Similarly, the dynamic or isotonic strength or contraction was used on the logic of constant tension. Again, it is well established that, with change in the length of any muscle, its tension (and tone) changes. A dynamic contraction entails a change in joint angle and a change of body parts in space. These changes alter muscle length and hence tension. Therefore, the term isotonic is also a misnomer.

Even a cursory examination of any given situation will reveal the inconsistencies in terms used. As a manual materials handler moves an object from point A to B, the resistance offered by the object may continually change as a result of changing mechanical disadvantage of the load, even if the mass of the object does not change. Thus, the magnitude of the effort required to handle the load (resistance) does not remain constant. As the object is supported by the worker through the range, the tension has to vary with the resistance, hence defying the term isotonic. On the muscle side, in addition to the change in load with respect to the relevant bony articulation, not only does the muscle length change with changing joint angle, but also the disadvantage for the muscle will continually change. The muscle will have to compensate for its varying length and mechanical advantage/disadvantage and also compensate for varying torque of the load. Clearly, such a dynamic in situ condition cannot be managed by a constant tension in the muscle. To complicate the system further, with the preponderance of third-class lever systems in body, the velocity of motion may significantly change with natural changes in muscles contraction. This further changes the inertial property of the object being handled (Kroemer, 1970; Mital and Kumar, 1998a,b). An activity of true constant tension will be an isometric effort where the force is not varied. Furthermore, in all probability, a true isometric condition may not exist at all, as any contraction will change the length of a given muscle.

Initially, any strength measurement involving motion was assigned as a dynamic motion. However, dynamic motion is of two kinds: (a) concentric — shortening contraction, and (b) eccentric — lengthening contraction. The concentric contraction is generally characterized by approximation of proximal and distal segments across a joint, where the length of the contracting muscle continues to shorten through the range of motion. This is exemplified by arm curl, where a load is picked up by the hand and is brought closer to the shoulder by contraction of elbow flexors. In contrast, the eccentric contraction is a lengthening contraction when a muscle or a group of muscles attempts to shorten by contraction but is lengthened by external force. This contraction is exemplified by lowering the weight from shoulder to knuckle height ...

Índice

• Cover Page
• Title Page
• Copyright Page
• Dedication
• Preface
• Contributors
• 1: Introduction and Terminology
• 2: Physiology and Biochemistry of Strength Generation and Factors Limiting Strength Development in Skeletal Muscle
• 3: Cardiac Effects of Strength Training
• 4: Determinants of Muscle Strength
• 5: The Ability to Persist in a Physical Task
• 6: Measurement of Localized Muscle Fatigue in Biceps Brachii Using Objective and Subjective Measures
• 7: Psychophysical Aspects of Muscle Strength
• 8: Isometric, Isoinertial, and Psychophysical Strength Testing: Devices and Protocols
• 9: Isokinetic Strength Testing: Devices and Protocols
• 10: Hand Strength
• 11: Grasp at Submaximal Strength
• 12: Shoulder, Elbow, and Forearm Strength
• 13: Cervical Muscle Strength
• 14: Trunk and Lifting Strength
• 15: Pushing and Pulling Strength
• 16: Design Applications of Strength Data
• 17: Electromyography and Muscle Force
• 18: Myoelectric Manifestations of Muscle Fatigue
• 19: Physical Demands Analysis: A Critique of Current Tools
• 20: Job Accommodation
• 21: Strength and Disability
• 22: Gaps in Knowledge and Future of the Field