Leonardo da Vinci (1452–1519) and Giovanni Alfonso Borelli (1608–1679) explained the functioning of the human body by combining anatomic, physiologic, and mechanic know-ledge of their times. Current anthromechanic modeling relies on the assumption that the body behaves according to the laws of Newtonian mechanics. In the late 1800s, Harless determined the masses of body segments, Braune and Fischer investigated the interactions
between mass distribution and external impulses applied to the human body, and von Meyer discussed statics and dynamics of the human body. (Kroemer et al. [52] described the modeling history.) Recently, models have been developed that describe, among other topics, responses of the human body to vibrations and impacts, the exertion of static and dynamic muscle strength, movements of astronauts in space, functions of the spinal column, hemodynamics, and prosthetic devices.
In our attempts to understand the workings of the human body, we regularly use a model that has several subdivisions [27,53–58]. Each of the compartmental models contains specific disciplinary knowledge areas. Some of these divisions derive from traditional disciplines. Among the classic foundations is anthropometry, discussed above, which describes the sizes of the total body and its segments. Of special anthromechanic interest are (a) the measures of segment links, the long bones, (b) their connections and articula-tions (body joints), and (c) the volumes and mass properties of segments.
Human physiology is another classic discipline of great importance to anthromechanics.
Physiology explains the structure and functions of muscles. These are the body's engines;
they can rotate body segments about their connecting articulations, or they can stiffen the body against external onslaught. To do their work, muscles require energy. Metabolism provides that energy by catabolizing organic molecules acquired via food and drink.
Metabolism, in turn, relies on other body functions: one is blood circulation, which transports chemically stored energy and oxygen to the muscles. Circulation also removes metabolic by-products (especially carbon dioxide, water, and heat) from the muscles to the lungs, and heat and water to body surfaces for release.
The control of involuntary and conscious actions of the human is done by the nervous and hormonal systems—despite their importance, these control functions are (by definition) not included in biomechanical submodels, but are part of the overall ergonomic model of the human, together with psychological properties. Kroemer et al. [27,34] provide overviews of the mutually dependent physiological processes as they concern the human factors engineer (for more details see books on human physiology).
The internal-combustion engine, as used in automobiles, provides a useful analogy for how the human body generates and transforms energy into motion and work.
. In the cylinder of the engine, an explosive combustion of a fuel–air mixture transforms chemically stored energy into physical kinetic energy and heat. The energy moves the pistons of the engine, and gears transfer their motion to the wheels of the car.
. In the ‘‘human machine,’’ muscle fibers are both cylinders and pistons: bones and joints are the gears. The fuels, mostly carbohydrates and fats in the nutrients, need oxygen to yield energy. When the muscles work, they produce metabolic by-products, especially carbon dioxide, water, and heat, that all need removal.
As we utilize such compartmental models, we should ponder what Asimow wrote in 1963 [59]. I am slightly paraphrasing what he said on page 13 of his book dealing with the structures and workings of the human body:
When developing models we must realize that selecting certain features, drawing distinctions, and making classifications usually imposes artificial divisions of our own choosing upon a universe that is, in many ways, all in one piece. We do so because it helps us in our attempted understanding of the intricate system. It breaks down a set of objects and phenomena too complex to be grasped in their entireties into smaller realms that we can deal with one by one. There is nothing objectively‘‘true’’ about such models, however, and the only proper criterion of their value is their usefulness.
2.2.2.1 Basic anthromechanic model
Compartmental models were of great value in the emergence of the ergonomics literature in the 1950s and 1960s (see the listing in Edholm's 1967 [60] book on‘‘the biology of work’’ for examples). Today's models build on their many predecessors; while more up-to-date, most have the same paradigms, uses, and limitations. The current anthromechanic model still has as its basic structure the skeleton of long bones (links), which connect in the articulations (joints). Engines (muscles) cross the articulations to provide stability to the structure and power for motion and work. Submodels represent components of the body, for instance
1. Sizes and surfaces of body contours 2. Volumes, masses of body segments 3. Structural members, lever arms (bones) 4. Joints, bearing surfaces in the body
5. Generators or consumers of energy (organs) 6. Engines, dampers, locks (muscles)
7. Cables transmitting muscle forces (tendons, ligaments) 8. Pulleys, sliding surfaces (tendon sheaths, bursae)
Assessment of body contours, the first item in the list, is primarily in the domain of anthropometry. However, surface characteristics also describe features of item 2, segments volumes and the magnitude and distribution of mass, which are properties of great interest in human mechanics. Length of bones, item 3, is commonly an anthropometric topic; yet other bone properties, especially their abilities to sustain mechanical strains, fall into the anthromechanical domain (see Zernike's Chapter 6 for detailed information).
Properties of body articulations, item 4, particularly their mobility and ability to function under mechanical loading, are topics of great bioengineering importance (see Walji's Chapter 8, Olver's Chapter 15 , and Gielo-Perzak's Chapter 15 for detailed information).
This topic has much economical importance as well, such as in design and implantation of artificial body joints, mostly for hip and knee as well as the digits of the hand. The estimate for 2004 was that nearly 480,000 manufactured knee and 235,000 replacement hip joints were implanted in Americans with damaged joints. More women than men received artificial joints, and the rate of replacements in elderly patients was increasing strongly (http:==www.cdc.gov=nchs=data=ad=ad371.pdf). A related bioengineering task is hip joint surfacing, as opposed to hip joint replacement. In this operation, the surgeon usually reshapes the head of the femur and caps it with a prosthetic thatfits into a manufactured lining in the socket of the hip.
Muscles, item 6, are the body's‘‘engines’’ that, activated by nervous signals, stiffen and power the human body and thus enable it to move and perform work on external objects.
Because skeletal muscle generates force, movement, power, and work within the body, it is also a source of physical loading of other body tissues, such as tendons, ligaments, joints, and nerves (see the example of carpal tunnel syndrome discussed later). Skeletal muscle usually does not damage itself by overuse because, when fatigued, it fails to contract before loading approaches the danger of cellular damage. Muscle recovers from fatigue with time, in minutes or hours. Skeletal muscle has its own self-repair and adaptation mechanisms that maintain its structure and remodel itself over time. If damage or injury to skeletal muscle occurs, it happens because external loadings exceed the tolerance levels of the muscles' passive and active contractile structures [61,62]. Knowledge of the tolerance limits, a concern in occupational biomechanics, is of great importance for the design of tasks that the human can do safely (see Herzog's Chapter 7 for detailed information).
Tendons and their enclosures, sheaths, are also important elements of components of the anthromechanic model. Tendons connect the ends of muscle with bone. Tendons cannot
contract but serve as nearly nonstretchable‘‘cables’’ that transmit force between muscle and bone. Tendon sheaths enclose tendons where needed to provide guidance. To reduce friction, the body produces synovialfluid as lubricant between the tendon and its sheath.
When the tendon glides in its sheath (the displacement can be a centimeter in the wrist), the magnitude of friction against the surrounding tissue depends on the amount of tension in the tendon, the friction coefficient, and the arc of contact. Ligaments are in many respects similar to tendons; they transmit tension (force) from bone to bone. The repair or replace-ment of injured ligareplace-ments, especially in the knee joint, is a major task of orthopedic surgery (see Woo's Chapter 4 and Frank's Chapter 5 for detailed information on tendons and ligaments).
A brief discussion of two major components of the body's engine, muscle and the attached tendons, illuminates the anthromechanic approach. In cross section, skeletal muscle shows a lattice of myofibrillar bundles. Longitudinally, they show repeating trains (sarcomeres) of interdigitated thick and thin protein filaments. These actin and myosin filaments are the contractile structures that can actively shorten against resistance: an overwhelming external force can stretch them. While contracting or stretched, skeletal muscle generates internal tension or force, which tendons transmit to bones.
In contrast to muscle tissue, tendons (and ligaments) do not appreciably stretch because they consist of dense tissue, collagenfibers, that form parallel bundles. Loose connective tissues wrap around the collagenfibers. In some areas of the body, such as the wrist, the tendon wrapping forms a double layer lined with synovial cells. This sheath provides synovialfluid, a lubricant that facilitates smooth gliding of the tendon. As Table 2.9 shows, tendons (and ligaments) have properties that are quite different from those of muscle, especially in terms of stiffness, viscoelasticity, strength, and failure resistance—many physiology texts provide further details (for numerical data, see [62] or [56] as well as Chapters 6, 7, and 9 of this book).
2.2.2.2 Modeling and measuring muscle strength
By anthromechanic analogy, skeletal muscles are the engines of the body: when contracting theirfilaments, they generate force (tension, power) and, as a dynamic result, move body segments by turning limbs (links) in their intermediate joints.
The human body contains more than two hundred skeletal muscles. Connective tissue (fascia) enwraps them; it imbeds nerves and blood vessels. At the ends of the muscle, the tissues combine to form tendons, which usually attach to bones.
Thousands of individual muscle fibers run, more or less parallel, the length of the muscle. Skeletal muscle fibers appear striped (striated) crosswise: thin and thick, light
Table 2.9 Estimated Tolerances (in megapascals) of Human Tissue
Tissue Estimated Ultimate Stress (MPa)
Muscle Tension 30–60
Ligament Tension 20
Tendon Tension 60–100
Bone, longitudinal loading Tension 130
Compression 190 Shear 70
Bone, transverse loading Tension 50
Compression 130 Note: Numbers rounded from those listed by Marras 2006, Marras and Radwin 2006.
and dark bands cross the fiber in regular patterns, which repeat along its length. One such thick dark stripe appears to penetrate thefiber like a membrane or disc: this is the so-called z-disc (from the German zwischen, between). The distance between two adjacent z-lines defines the sarcomere. Its length at rest is approximately 250 Å (1 Å ¼ 1010m), meaning that there are about 40,000 sarcomeres in series within 1 mm of muscle fiber length.
Within each muscle fiber, thread-like myofibrils (from the Greek mys, muscle) lie in parallel by the hundreds or thousands. Each of these, in turn, consists of bundles of myofilaments. A network of tubular channels, sacs, and cisterns, which connect with a larger tubular system in the z-discs,fill the spaces between the filaments. The network of blood vessels and nerves in the fascia is the‘‘plumbing and control’’ system of the muscle, the sarcoplasmic reticulum. It providesfluid transport between the cells inside and outside the muscle and carries chemical and electrical messages.
As just mentioned, two of the myofibrils, myosin and actin, form the contracting microstructure of the muscle: they can slide along each other, pulling the z-discs closer together. Consequently, sarcomeres in series (and those parallel) shorten, and as a result, the whole muscle shortens. The only active action a muscle can take is to contract. After a contraction, the muscle returns to its resting length, primarily through a recoiling of its shortened tissues. An overwhelming force can stretch the muscle beyond its resting length.
The magnitude of a muscle's pull depends, originally, on the intensity of myofibril contraction; the resulting pull force on the bones derives from the tension in the muscle, or tendon, divided by its cross section. Since human skeletal bone does not move linearly within the body but rotates about an articulation, the muscle–tendon pull force generates a torque on the bone.
While these conditions are clear in physics and physiologic terminology, the commonly used term ‘‘muscle strength’’ can be rather diffuse. In may refer to the force or torque exerted by a body part to an object external to the body, usually by hand or foot. Or it may denote an event within the muscle. In this case, it may refer to an isometric effort, where the muscle tenses but cannot shorten; since nothing moves, this isometric exertion denotes, in physics terms, a static condition. However, if the muscle actually contracts (shortens), it does rotate the attached limb, generating a dynamic condition.
Figure 2.2 shows, schematically, the generation and control of muscle strength exertion.
Feedforward of excitation signals from the central nervous system (CNS) stimulates muscle motor units to contract, generating tension. That is transmitted via tendons to bones which act as levers hinged in body joints. These internally generated torques can then be applied by hand, foot, or other body segment to an object outside the body in the form of impacts, forces, or torques. Three feedback paths help in the control of exertion. The reflex loop F1
originates at interoceptors and leads directly to the CNS. The other two loops start at exteroceptors and lead to a comparator where they modify the input to the CNS.
F2provides kinesthetic signals related to touch, body position, and motion. F3is similar but reports specifically on task execution, especially through sound and vision.
Such model can help in the understanding of how we generate and control our muscular activities, mostly through unconscious and automated mechanisms. The model also shows where we may find opportunities to observe internal events and possibly influence or predict the outcome: electroencephalograms (EEGs) may describe the efferent signals emanating from the CNS. Electromyograms (EMGs) reflect activations of the muscle's motor units. Observations of body geometry allow calculating muscle and tendon tension and internally developed torques. (It is usually easy to measure force and torque at the point where a body part applies it to an outside object, often a handle or pedal.) However, such complete and exacting model is still to be developed. Current approaches eliminate such ‘‘nonmechanical’’ variables as motivation and instead concentrate on muscle size,
lever arms, pull angles, body posture, and similar concrete anthromechanical parameters, often with the body stationary.
Magnitude and endurance of exertion depend not only on muscle mass and how skilled we are in using it but also on our motivation: an example of how physique and psyche interact [63]. Exercising muscles (by purposeful training or just by everyday use) have the effects of increased muscle mass, enhanced strength, and improved skill. Exercising muscles also indirectly furthers the capabilities of the circulatory, metabolic, and respira-tory systems, all needed to support the generation of strength. Not using muscles makes them atrophy and, consequently, reduces the capability of their support functions.
CNS programs Feedback
Generate Efferent signals
Generate Muscle tension
Generate Internal torque(s)
Generate Applied force / torque
Segment strength
Task execution Incomplete Successful
Muscles(s)
Internal transmission Motor units Activate executive programs
subroutines motivation
Activate
Pull angles, lever arms about joint(s) Activate
End F1
F2
F3
Figure 2.2 Muscle strength generation and control.
Figure 2.3 displays an example of the mechanics of the human muscle–bone, engine–gear layout. The biceps and triceps muscles both cross the elbow joint and attach with very short lever arms to the bony structures, radius and ulna, of the forearm. The hand, however, employs a long lever arm from the elbow. Accordingly, the muscles must generate a large force to create a much smaller force at the hand, directly in proportion to the hand's long lever versus the short muscle levers. Note further the opposing actions of the muscles: if one turns the forearm in one direction, the other opposes that action. This agonist=antagonist setup allows fine control of body movements. However, for the modeler this generates a complication, because it is not easily seen, from the outside, whether any measured exertion at the hand is the result of only one muscle pulling, or whether it reflects the difference between simultaneous actions of the two opposing muscles. In fact, there is another com-plication: two more muscles, brachialis and brachioradialis, act synergistically with the biceps in elbow flexion. Hence, lacking knowledge of the relative involvement of each muscle, simply measuring the force at the hand does not provide reliable information about the magnitude of effort in involved muscles.
2.2.2.2.1 Static strength Muscle contraction often shortens the length of the muscle, a concentric action; in an eccentric action, an overwhelming force actually lengthens the
Humerus
Radius Ulna Scapula
Biceps
Biceps pull Triceps
Triceps pull
Figure 2.3 Biceps and triceps pulls.
muscle. If muscle length does not change, the effort is isometric. Since during an isometric effort there is no perceptible change in muscle length, the involved body segments do not move; in physics terms, all forces acting within the system are in static equilibrium, as Newton'sfirst law requires. Therefore, the physiological ‘‘isometric’’ case is equivalent to the‘‘static’’ condition in physics. If muscle length changes, the action is ‘‘dynamic.’’
The static condition is theoretically simple and easily controlled in experiments. It allows rather uncomplicated measurement of muscular effort. Therefore, much of the information currently available on ‘‘human strength’’ describes the outcomes of static (isometric) testing. Accordingly, most of the tables on body segment strength in the physiologic and human factors engineering literature contain static data (see, for example, compilations by Imrhan in Chapter 11; by Kroemer [6,35], Kroemer et al. [27,34], and especially by Kumar [36]). Besides offering the convenience of dealing with statics, measurement of isometric strength yields, for many cases of practical design interest, a reasonable estimate of the maximally possible exertion if there is no body segment motion or very slow movement, especially when eccentric.
2.2.2.2.2 Dynamic strength Dynamic muscular efforts are more difficult to describe and control than static contractions. In dynamic activities, muscle length changes, and therefore involved body segments move. The amount of travel is relatively small at the muscle but amplified along the links of the internal transmission path to the point of application to the outside, for example, at the hand or foot.
In the human body, as a rule the tendon coming from a muscle attaches with a short lever arm to a bone; however, the bone's‘‘business end’’ (such as the hand) is at the end of a long lever arm. Figure 2.3 illustrates that transmission setup: the tendons of both biceps and triceps muscles attach at close distances from the elbow joint; however, the hand is at the end of a long lever arm. This means that the muscle has to generate much more strength than the hand exerts. Given the ‘‘gear ratios’’ of the human body, the human body seems to be designed for speed, not for strength. By the rules of mechanics, the time derivatives of displacement (velocity, acceleration, and jerk) are of importance for both the muscular effort and the external effect; for example, change in velocity determines force and impact, as per Newton's second law.
Definition and experimental control of dynamic muscle exertions are much more
Definition and experimental control of dynamic muscle exertions are much more