Chapter Four Mechanics of the Musculoskeletal System 2023-2024 PDF

Chapter Four Mechanics of the Musculoskeletal System MECHANICS OF THE MUSCULOSKELETAL SYSTEM TISSUE LOADS RESPONSE OF TISSUES TO FORCES STRESS agenda STRAIN STIFFNESS AND MECHANICAL STRENGTH VISCOELASTICITY BIOMECHANICS OF THE PASSIVE MUSCLE–...

Chapter Four Mechanics of the Musculoskeletal System MECHANICS OF THE MUSCULOSKELETAL SYSTEM TISSUE LOADS RESPONSE OF TISSUES TO FORCES STRESS agenda STRAIN STIFFNESS AND MECHANICAL STRENGTH VISCOELASTICITY BIOMECHANICS OF THE PASSIVE MUSCLE–TENDON UNIT (MTU) BIOMECHANICS OF BONE BIOMECHANICS OF LIGAMENTS THREE MECHANICAL CHARACTERISTICS OF MUSCLE FORCE–VELOCITY RELATIONSHIP FORCE–LENGTH RELATIONSHIP FORCE–TIME RELATIONSHIP STRETCH-SHORTENING CYCLE (SSC) FORCE–TIME PRINCIPLE NEUROMUSCULAR CONTROL THE FUNCTIONAL UNIT OF CONTROL: MOTOR UNITS REGULATION OF MUSCLE FORCE PROPRIOCEPTION OF MUSCLE ACTION AND MOVEMENT SUMMARY  introduction Many professionals interested in human movement function need information on how forces act on and within the tissues of the body. The deformations of muscles, tendons, and bones created by external forces, as well as the internal forces created by these same structures, are relevant to understanding human movement or injury. This chapter will provide an overview of the mechanics of biomaterials, specifically muscles, tendons, ligaments, and bone. The neuromuscular control of muscle forces and the mechanical characteristics of muscle will also be summarized. The application of these concepts is illustrated using the Force–Time Principle of biomechanics. An understanding of the mechanics of musculoskeletal tissues is important in understanding the organization of movement, injury, and designing conditioning programs. 2023-2024 Chapter 4 3  TISSUE LOADS When forces are applied to a material, like human musculoskeletal tissues, they create loads. Engineers use various names to describe how loads tend to change the shape of a material. These include the principal or axial loadings of compression, tension, and shear (Figure 4.1). Compression is when an external force tends to squeeze the molecules of a material together. Tension is when the load acts to stretch or pull apart the material. For example, the weight of a body tends to compress the foot against the ground in the stance phase of running, which is resisted by tensile loading of the plantar fascia and the longitudinal ligament in the foot. Shear is a right-angle loading acting in opposite directions. A trainer creates a shearing load across the athletic tape with scissor blades or their fingers when they tear the tape. Note that loads are not vectors (individual forces) acting in one direction, but are illustrated by two arrows (Figure 4.1) to show that the load results from forces from both directions. When many forces are acting on a body they can combine to create combined loads called torsion and bending (Figure 4.2). In bending one side of the material is loaded in compression while the other side experiences tensile loading. When a person is in single support in walking (essentially a one-legged chair), the femur experiences bending loading. The medial aspect of the femur is in compression while the lateral aspect is in tension. 2023-2024 Chapter 4 4 Note that loads are not vectors (individual forces) acting in one direction, but are illustrated by two arrows (Figure 4.1) to show that the load results from forces from both directions. When many forces are acting on a body they can combine to create combined loads called torsion and bending (Figure 4.2). In bending one side of the material is loaded in compression while the other side experiences tensile loading. When a person is in single support in walking (essentially a one- legged chair), the femur experiences bending loading. The medial aspect of the femur is in compression while the lateral aspect is in tension. 2023-2024 Chapter 4 5 RESPONSE OF TISSUES TO FORCES The immediate response of tissues to loading depends on a variety of factors. 1- The size and direction of forces, as well as 2- the mechanical strength and shape of the tissue, affect how the material structure will change. We will see in this section that mechanical strength and muscular strength are different concepts. This text will strive to use “muscular” or “mechanical” modifiers with the term strength to help avoid confusion. Several important mechanical variables explain how musculoskeletal tissues respond to forces or loading. 2023-2024 Chapter 4 6  Stress How hard a load works to change the shape of a material is measured by Force mechanical stress. Mechanical stress is symbolized by the Greek letter sigma (σ) and is defined as the force( F ) per unit area( A ) within a material (σ = F/A). 𝐹 Mechanical stress is similar to the concept of pressure and has the same units 𝜎= (N/m2 and lbs/in2). Stress 𝐴 In the SI system, one Newton per meter squared is one Pascal (Pa) of stress or pressure. Mechanical stress is not vector quantity, but an even more complex quantity called Area a tensor. Tensors are generalized vectors that have multiple directions that must be accounted for, much like resolving a force into anatomically relevant axes along a longitudinal axis and at right angles (shear). The maximum force capacity of skeletal muscle is usually expressed as a maximum stress of about 25–40 N/cm2.This force potential per unit of cross-sectional area is the same across genders, with females tending to have about two-thirds of the muscular strength of males because they have about two-thirds as much muscle mass as males. 2023-2024 Chapter 4 7  Strain Change in length The measure of the deformation of a material created by a load is called strain. This formation is usually expressed as a ratio of the normal or resting length ( L0 ) of the material. Strain ( Є ) can be calculated as a change in 𝐿 − 𝐿0 length divided by normal length: (L – L0)/ L0. Imagine stretching a rubber band 𝜖= Strain between two fingers. If the band is elongated to 1.5 times its original length, 𝐿0 you could say the band experiences 0.5 or 50% tensile strain. This text will discuss the typical strains in musculoskeletal tissues in percentage units. Most engineers use much more rigid materials and typically talk in terms of units of Resting length microstrain. Think about what can withstand greater tensile strain: the shaft of a tennis racket, the shaft of a golf club, or the shaft of a fiberglass diving board? 2023-2024 Chapter 4 8  Stiffness and Mechanical Strength Engineers study the mechanical behavior of a material by loading a small sample in a materials testing system (MTS), which simultaneously measures the force and displacement of the material as it is deformed at various rates. The resulting graph is called a load-deformation curve (Figure 4.3), which can be converted with other measurements to obtain a stress–strain graph. Load-deformation graphs have several variables and regions of interest stress-strain  ELASTIC REGION The elastic region is the initial linear region of the graph where the slope corresponds to the stiffness or Young's modulus of elasticity of the material. Stiffness or Young’s modulus is defined as the ratio of stress to strain in the elastic region of the curve but is often approximated by the ratio of load to deformation (ignoring the change in dimension of the material). 𝑆𝑡𝑖𝑓𝑓𝑛𝑒𝑠 = 𝑆𝑡𝑟𝑒𝑠𝑠 If the test were stopped within the elastic region the material 𝑆𝑡𝑟𝑎𝑖𝑛 would return to its initial shape. Stiffness = Young’s modulus If the material were perfectly elastic, the force at a given deformation during restitution (unloading) would be the same as in loading. We will see later that biological tissues are not like a perfectly elastic spring, so they lose some of the energy in restitution that was stored in them during deformation. 2023-2024 Chapter 4 10 YIELD POINT OR ELASTIC POINT Beyond the linear region is the plastic region, where deformation increases occur with minimal and nonlinear changes in load. The yield point or elastic limit is the point on the graph separating the elastic and plastic regions. When the material is deformed beyond the yield point the material will not return to its initial dimensions. In biological materials, normal physiological loading occurs within the elastic region, and deformations near and beyond the elastic limit are associated with microstructural damage to the tissue. 2023-2024 Chapter 4 11  PLASTIC REGION Another important variable calculated from these measurements is the strength of the material. The mechanical strength of a material is the measurement of the maximum force or total mechanical energy the material can absorb before failure. The energy absorbed and mechanical work done on the material can be measured by the area under the load-deformation graph. Within the plastic region, the pattern of failure of the material can vary, and the definition of failure can vary based on the interest of the research. Conditioning and rehabilitation professionals might be interested in the yield stress (force at the end of the elastic region) of healthy and healing ligaments. 2023-2024 Chapter 4 12  FAILURE Sports medicine professionals may be more interested in the ultimate strength that is largest force or stress the material can withstand. Sometimes it is of interest to know the total amount of strain energy (see Chapter 6) the material will absorb before it breaks because of the residual forces that remain after ultimate strength. This is failure strength and represents how much total loading the material can absorb before it is broken. This text will be specific in regards to the term strength, so that when used alone the term will refer to muscular strength, and the mechanical strengths of materials will be identified by their relevant adjective (yield, ultimate, or failure). 2023-2024 Chapter 4 13  Viscoelasticity Biological tissues are structurally complex and also have complex The word viscoelastic is a combination mechanical behavior in response to loading. of viscous fluidity and elastic solidity, and thus, biological materials under First, biological tissues are anisotropic, which means that their strength properties are different for each major direction of stress and strain exhibit both viscous loading. and elastic behavior. Second, the nature of the protein fibers and the amount of calcification all determine the mechanical response. Third, most soft connective tissue components of muscle, tendons, and ligaments have another region in their load deformation graph. For example, when a sample of tendon is stretched at a constant rate the response illustrated in Figure 4.4 is typical. Note that the response of the material is more complex (nonlinear) than the Hookean elasticity illustrated in Figure 2.4. The initial increase in deformation with little increase in force Protein fibers fall into three major before the elastic region is called the toe region. groups: The toe region corresponds to the straightening of the wavy 1- collagen fibers that are thick, strong, collagen fiber in connective tissue. flexible, and resist stretch; 2- reticular After the toe region, the slope of the elastic region will vary fibers that are thin and form a depending on the rate of stretch. This means that tendons (and other biological tissues) are not supportive mesh; and 3- elastin fibers that are thin and perfectly elastic but are viscoelastic. elastic. 2023-2024 Chapter 4 14 Viscoelastic means that the stress and strain in a material depend on the loading rate, Loading rate is the speed at which forces so the force application's timing affects the material's strain response. impact the body. Figure 4.5 illustrates the response of a ligament stretched to a set length at two speeds, slow and fast. Note that a high rate of stretch results in a higher stiffness than a slow stretch. Muscles and tendons also have increasing stiffness with increasing rates of stretch. What is the great functional significance viscoelasticity of muscles and tendons? A slow stretch will result in a small increase in passive resistance (high compliance) from the muscle, while the muscle will provide a fast increase in passive resistance (high stiffness) to a rapid stretch. This is one of the reasons that stretching exercises should be performed slowly, to minimize the increase in force in the muscle–tendon unit (MTU) for a given amount of stretch. The graph's solid lines represent the ligament's loading response, while the dashed lines represent the mechanical response of the tissue as the load is released (unloading). Why the stretching exercises should be performed slowly? 2023-2024 Chapter 4 15 There are other important properties of Stress relaxation is the decrease in stress over time Hysteresis is the property of viscoelastic materials of have viscoelastic materials: when a material is elongated to a set length. {Stress a different unloading response than their loading response 1- creep, relaxation is a decrease in stress under constant strain.} (Figure 4.5). 2- stress relaxation, and For example, holding a static stretch at a specific joint Hysteresis also provides a measure of the amount of 3- hysteresis. position results in a gradual decrease in tension in the energy lost because the material is not perfectly elastic. muscle from stress relaxation. The area between the loading and unloading is the energy Creep is the gradual elongation (increasing strain) lost in the recovery from that stretch. of a material over time when placed under a If you leave a free weight hanging from a nylon cord, The energy and work are related, constant tensile stress. {Creep is an increased you might return several days later to find the elongation Mechanical work is defined as force times displacement strain under constant stress.} (creep) in the cord has stretched it beyond it initial (Force distances), length. so work can be visualized as an area under a force- displacement graph. Strain Stress Creep and stress relaxation are nonlinear responses and have important implications for stretching (see application box on flexibility and stretching) and risk of injury in repetitive tasks. ‫ مما يقلل من‬،‫ أوضاع العمل التي تمد األربطة‬،‫على سبيل المثال‬ For example, work postures that stretch ligaments, reducing their mechanical and ،‫ ويزيد من تراخي المفاصل‬،‫فعاليتها الميكانيكية وفعالية التحسس‬ proprioceptive effectiveness, increase joint laxity, and likely increase risk of injury. ‫ومن المحتمل أن يزيد من خطر اإلصابة‬ 2023-2024 Chapter 4 16 All these mechanical response variables of biological What are the differences between Creep, Stress relaxation, and Hysteresis? materials depend on precise measurements and characteristics of the samples. The example mechanical strengths and strains mentioned in the next section represent typical values from the literature. Do not assume these are exact values because factors like 1-training, 2-age, and 3-disease all affect the variability of the mechanical response of tissues. Methodological factors like how the human tissues were stored, attached to the machine, or preconditioned (like a warm-up before testing) all affect the results. Remember that the rate of loading has a strong effect on the stiffness, strain, and strength of biological materials. 2023-2024 Chapter 4 17 BIOMECHANICS OF THE PASSIVE MUSCLE–TENDON UNIT (MTU) The mechanical response of the MTU to passive stretching is viscoelastic, so the response of the tissue depends on the time or rate of stretch. At a high rate of passive stretch, the MTU is stiffer than when it is slowly stretched. This is the primary reason why slow, static stretching exercises are preferred over ballistic stretching techniques. ???? A slow stretch results in less passive tension in the muscle for a given amount of elongation compared to a faster stretch. The load in an MTU during other movement conditions is even more complicated because the load can vary widely with 1- activation, 2- previous muscle action, and 3- kind of muscle action. All these variables affect how the load is distributed in the active and ‫يؤدي التمدد البطيء إلى توتر سلبي أقل في العضالت لمدة معينة‬ passive components of the MTU. ‫من االستطالة مقارنة بالتمدد األسرع‬ 2023-2024 Chapter 4 18 The tendon is the connective tissue that links muscle to bone and strongly affects how muscles are used or injured in movement. The tendon is a well-vascularized tissue whose mechanical response is primarily related to the protein fiber collagen. The parallel arrangement of collagen fibers in the tendon and cross-links between fibers make the tendon about three times stronger in tension than muscle. Acute overloading of the MTU usually results in strains (sports medicine term for overstretched muscle, not mechanical strain) and failures at the muscle-tendon junction or the tendon/bone interface. In creating movement, a long tendon can act as a spring in fast bouncing movements..‫ يمكن للوتر الطويل أن يكون بمثابة زنبرك فعال في الحركات المرتدة السريعة‬،‫عند إنشاء الحركة‬ A muscle with a short tendon transfers force to the bone more quickly because there is less slack to be taken out of the tendon. ‫االرتخاء‬ The intrinsic muscles of the hand are well suited to the fast finger movements of a violinist because of their short tendons..‫تتناسب عضالت اليد الداخلية تما ًما مع حركات األصابع السريعة لعازف الكمان بسبب أوتارها القصيرة‬ 2023-2024 Chapter 4 19  BIOMECHANICS OF BONE Unlike muscle, the primary loads experienced by most bones are compressive. The mechanical response of bone to compression, tension, and other complex loads depends on the complex structure of bones. Remember that bones are living tissues with blood supplies, made of a high percentage of water (25% of bone mass), and have considerable deposits of calcium salts and other minerals. The strength of bone depends strongly on its Wolff's law was created by 1- The density of mineral deposits, the German anatomist and 2- The density of collagen fibers and surgeon Julius Wolff in the 3- It is also strongly related to dietary habits and 19th century. It is a part of 4- physical activity. bone theory that explains Loading bones in physical activity results in greater osteoblast activity, laying down bone. how bones typically respond to stress. It marks Immobilization or inactivity will result in dramatic decreases in bone density, stiffness, and mechanical the adaptive changes that strength. bones can make internally.‫سيؤدي عدم الحركة أو عدم النشاط إلى انخفاض كبير في كثافة العظام والتصلب والقوة الميكانيكية‬ to become stronger and A German scientist is credited with the discovery that bones remodel (lay down greater mineral stronger in order to resist deposits) according to the mechanical stress in that area of bone. This laying down of bone where it is strain. stressed and reabsorption of bone in the absence of stress is called Wolff's Law. ?? Bone remodeling is well illustrated by the formation of bone around the threads of screws in the hip prosthetic in the x-ray in Figure 4.6. 20XX presentation title 20 The macroscopic structure of bone shows a dense, external layer called cortical (compact) bone and the less-dense internal cancellous (spongy) bone. The mechanical response of bone is dependent on this “sandwich” construction of cortical and cancellous bone. Cortical bone is stiffer (maximum strain about 2%), while cancellous bone is less stiff and can withstand greater strain (7%) before failure. In general, this design results in ultimate strengths of bone of about; Compression 200MPa Tension 125MPa Shear 65MPA This means that an excessive bending load on the femur like in Figure 4.2 would most likely cause a fracture to begin on the lateral aspect that is under tensile loading. Using sports rules to protect athletes from Lateral blows (like blocking rules in American football) is wise because the bone is weakest under shearing loads. 2023-2024 Chapter 4 21 It is also important to understand that the ultimate strength of bone depends on 1-nutritional, 2-hormonal, and 3-physical activity factors. Research done with an elite powerlifter found that the ultimate compressive strength of a lumbar vertebral body (more than 36,000 N or 4 tons) estimated from bone mineral measurements was twice that of the previous maximal value. More recent studies of drop jump training in pre-pubescent children have demonstrated that bone density can be increased, but it is unclear if peak forces, rates of loading, or repetitions are the training stimulus for the increases in bone mass (Bauer, Fuchs, Smith,& Snow, 2001). More research on the osteogenic effects of various kinds of loading and exercise programs could help physical educators design programs that help school children build bone mass. The following section will outline the mechanical response of ligaments to loading. 2023-2024 Chapter 4 22 BIOMECHANICS OF LIGAMENTS o Ligaments are tough connective tissues that connect bones to guide and limit joint motion and provide important proprioceptive and kinesthetic afferent signals. ‫استقبال الحس‬ ‫الحركية‬ o Most joints are not perfect hinges with a constant axis of rotation, so they tend to have small accessory motions and moving axes that stress ligaments in several directions. o The collagen fibers within ligaments are not arranged in parallel like tendons but in various directions. Normal physiological loading of most ligaments is 2–5% of tensile strain, which corresponds to a load of 500 N in the human anterior cruciate ligament, except for “spring” ligaments that have a large percentage of elastin fibers (ligamentum flavum in the spine), which can stretch more than 50% of their resting length. o The maximum strain of most ligaments and tendons is about 8–10%. 20XX presentation title 23 A long-term increase in the mechanical strength of articular cartilage with loads of regular physical activity has also been observed Inactivity, however, results in major decreases in the mechanical strength of ligaments and tendons, with reconditioning to regain this strength taking longer than deconditioning. The ability of the musculoskeletal system to adapt tissue mechanical properties to the loads of physical activity does not guarantee a low risk of injury. There is likely a higher risk of tissue overload when deconditioned individuals participate in vigorous activity or when trained individuals push the envelope, training beyond the tissue's ability to adapt during the rest periods between training bouts. ‫ من المحتمل أن يكون هناك‬.‫إن قدرة الجهاز العضلي الهيكلي على تكييف الخواص الميكانيكية لألنسجة مع أحمال النشاط البدني ال تضمن انخفاض خطر اإلصابة‬ ‫ ويتدربون بما يتجاوز قدرة‬،‫خطر أكبر للحمل الزائد لألنسجة عندما يشارك األفراد غير المشروطين في نشاط قوي أو عندما يقوم األفراد المدربون بدفع الظرف‬.‫األنسجة على التكيف خالل فترات الراحة بين نوبات التدريب‬ We will see in the next section that muscle mechanical properties also change in response to activity and inactivity. 20XX presentation title 24 THREE MECHANICAL CHARACTERISTICS OF MUSCLE The force potential of an MTU varies and can be described by three mechanical characteristics. These characteristics deal with the variations in muscle force because of differences in velocity, length, and time relative to activation. 1- Force–Velocity Relationship 2- Force–Length Relationship 3- Force–Time Relationship 2023-2024 Chapter 4 25  Force–Velocity Relationship The Force–Velocity Relationship explains how the force of fully activated muscle varies with velocity. This may be the most important mechanical characteristic since all three muscle actions (eccentric, isometric, concentric) are reflected in the graph. Isometric: A muscular contraction in which the length of the muscle does not change. isotonic: A muscular contraction in which the length of the muscle changes. eccentric: An isotonic contraction where the muscle lengthens. concentric: An isotonic contraction where the muscle shortens. We will see that the force or tension a muscle can create is quite different across actions and the many speeds of movement. The discovery and formula describing this fundamental relationship in concentric conditions is also attributed to A. V. Hill. Hill made careful measurements of the velocity of shortening when the preparation of maximally stimulated frog muscle was released from isometric conditions. 20XX presentation title 26 These studies of isolated preparations of muscle is performed in what we term in vitro (Latin for “in glass”) conditions. Figure 4.7 illustrates the shape of the complete Force–Velocity Relationship of skeletal muscle. The Force–Velocity curve essentially states that the force the muscle can create decreases with increasing velocity of shortening (concentric actions), while the force the muscle can resist increases with increasing velocity of lengthening (eccentric actions). The force in isometric conditions is labeled P0 in Hill's equation. The right side of the graph corresponds to how the tension potential of the muscle rapidly decreases with increases in the speed of concentric shortening. Also note, however, that increasing negative velocities (to the left of Isometric) show how muscle tension rises In o faster eccentric muscle actions. In isolated muscle preparations, the forces that the muscle can resist in fast eccentric actions can be almost twice the maximum isometric force. 20XX presentation title 27  This general shape of a muscle's potential maximum tension has many implications for human movement. First, it is not possible for muscles to create large forces at high speeds of shortening. Muscles can create high tensions to initiate motion, but as the speed of shortening increases their ability to create force (maintain acceleration) decreases. Second, the force potential of muscles at small speeds of motion (in the middle of the graph) depends strongly on isometric muscular strength. This means that muscular strength will be a factor in most movements, but this influence will vary depending on the speed and direction (moving or braking) the muscles are used. Third, the inverse relationship between muscle force and velocity of shortening means you cannot exert high forces at high speeds of shortening, and this has a direct bearing on muscular power. 20XX presentation title 28  So there are major implications for human movement because of the functional relationship between muscle force and velocity. What about training? Does training alter the relationship between muscle force and velocity or does the Force–Velocity Relationship remain fairly stable and determine how you train muscle? It turns out that we cannot change the nature (shape) of the Force– Velocity Relationship with training, but we can shift the graph upward to improve performance. Weight training with high loads and few repetitions primarily shifts the force-velocity curve up near isometric conditions (Figure 4.8),while fast lifting of light loads shifts the curve up near Vmax, which is the maximum velocity of shortening for a muscle. 2025 presentation title 29  FORCE–LENGTH RELATIONSHIP The length of a muscle also affects the ability of the muscle to create tension. The Force-Length Relationship documents how muscle tension varies at different muscle lengths. The variation in potential muscle tension at different muscle lengths, like the Force–Velocity Relationship, also has a dramatic effect on how movement is created. We will see that the Force -Length Relationship is just as influential on the torque a muscle group can make as the geometry (moment arm) of the muscles and joint. 2023-2024 Chapter 4 30 Remember that the tension a muscle can create has both active and passive sources, so the length–tension graph of muscle will have both of these components. Figure 4.11 illustrates the Force–Length Relationship for a skeletal muscle fiber. The active component of the Force–Length Relationship (dashed line) is logically associated with the potential number of cross-bridges between the actin and myosin filaments in the Sliding Filament Theory. Peak muscle force can be generated when there are the most cross- bridges. This is called resting length (L0) and usually corresponds to a point near the middle of the range of motion. Potential active muscle tension decreases for shorter or longer muscle lengths because fewer cross-bridges are available for binding. The passive tension component (solid line) shows that passive tension increases slowly near L0 but dramatically increases as the muscle elongates. Passive muscle tension usually does not contribute to movements in the middle portion of the range of motion. Still, it does contribute to motion when muscles are stretched or in various neuromuscular disorders. The exact shape of the Force–Length Relationship slightly varies between muscles because of differences in active (fiber area, angle of pennation) and passive (tendon) tension components. 20XX presentation title 31 The active tension component of the Force–Length Relationship has three regions Plateau Region (Figure 4.12). 1-The ascending limb represents the decreasing force output of the muscle as it is shortened beyond resting length. Movements that require a muscle group to shorten considerably cannot create maximal muscle forces. 2-The plateau region represents the high muscle force region, typically in the midrange of the anatomical range of motion. Movements initiated near the plateau region can potentially create maximal muscle forces. 3-The descending limb represents the decreasing active tension a muscle can make as it is elongated beyond resting length. At extremes of the descending limb, the dramatic increases in passive tension provide the muscle force to bring a stretched muscle back to shorter lengths, even though there are virtually no potential cross-bridge attachment sites. The implications for a muscle working on the ascending limb versus the plateau region of the force–length curve are dramatic. The mechanical work that a muscle fiber can create for a given range of motion can be visualized as the area under the graph because work is force times displacement. Note the difference in work (area) created if the muscle fiber works in the ascending limb instead of near the plateau region (Figure 4.12). These effects also interact with the force–velocity relationship to determine how muscle forces create movement throughout the range of motion. These mechanical characteristics also interact with the time delay in the rise and fall of muscle tension, the force–time relationship. 2023-2024 Chapter 4 32  Force–Time Relationship Another important mechanical characteristic of muscle is related to the temporal delay in the development of tension. The Force–Time Relationship refers to the delay in the development of muscle tension of the whole MTU and can be expressed as the time from the motor action potential (electrical signal of depolarization of the fiber that makes the electromyographic or EMG signal) to the rise or peak in muscle tension. The time delay that represents the Force–Time Relationship can be split into two parts. The first part of the delay is related to the rise in muscle stimulation sometimes called active state or excitation dynamics. In fast and high-force movements the neuromuscular system can be trained to rapidly increase (down to about 20 ms) muscle stimulation. The second part of the delay involves the actual build-up of tension which is sometimes called contraction dynamics. Recall that the contraction dynamics of different fiber types was about 20 ms for FG and 120 ms for SO fibers. When many muscle fibers are repeatedly stimulated, the fusion of many twitches means the rise in tension takes even longer. The length of time depends strongly on the cognitive effort of the subject, training, kind of muscle action, and the activation history of the muscle group. 20XX presentation title 33  This delay in the development of muscle tension has implications for the coordination and regulation of movement. It turns out that the deactivation of muscle (timing of the decay of muscle force) also affects the coordination of movements (Neptune & Kautz, 2001), although this section will limit the discussion of the Force–Time Relationship to a rise in muscle tension. Kinesiology professionals need to know about these temporal limitations so they understand the creation of fast movements and can provide instruction or cues consistent with what the mover's body does. For example, coaches need to remember that when they see high-speed movement in the body, the forces and torques that created that movement preceded the peak speeds of motion they observed. The coach that provides urging to increase effort late in the movement is missing the greater potential for acceleration earlier in the movement and is asking the performer to increase effort when it will not be able to have an effect. Muscles are often preactivated before to prepare for a forceful event, like the activation of plantar flexors and knee extensors before a person lands from a jump. A delay in the rise of muscle forces is even more critical in movements that cannot be preprogrammed due to uncertain environmental conditions. Motor learning research shows that a couple more tenths of a second are necessary for reaction and processing time even before any delays for increases in activation and the electromechanical delay. To make the largest muscle forces at the initiation of an intended movement, the neuromuscular system must use a carefully timed movement and muscle activation strategy. This strategy is called the stretchshortening cycle and will be discussed in the following section. 20XX presentation title 34  The primary biomechanical advantage of this longer time of force application is safety because the peak force experienced by the body (and consequently the stress in tissues) will be lower than during a short application of force. Moving the body and reaching with the extremities to maximize the time of catching also has strategic advantages in many sports. A team handball player intercepting the ball early not only has a higher chance of a successful catch, but they may prevent an opponent from intercepting. The distance and time the ball is in the air before contacting the catcher’s hands is decreased with good arm extension, so there is less chance of an opponent intercepting the pass. Imagine you are a track coach whose observations of a discus thrower indicate they are rushing their motion across the ring. The Force–Motion Principle and the Force–velocity relationship make you think that slowing the increase in speed on the turns and motion across the circle might allow for longer throws. There is likely a limit to the benefit of increasing the time to apply because maximizing discus speed at an appropriate angle at release is the objective of the event. Are there timing data for elite discus throwers available to help with this athlete, or is there a little art in the application of this principle? Are you aware of other sports or activities where coaches focus on a controlled build-up in speed or unrushed rhythm? 20XX presentation title 35 FORCE–TIME PRINCIPLE The Force–Time Principle for applying biomechanics is not the same as the Force–Time Relationship of muscle mechanics. The Force–Time Principle states that the time available for force application is as important as the size of the forces used to create or modify movement. So the Force–Time Principle is concerned with the temporal strategy of force application in movements, while the Force–Time Relationship (electromechanical delay) states a fact that the tension build-up of muscle takes time. The electromechanical delay is clearly related to how a person selects the appropriate timing of force application. The Force–Time Principle will be illustrated by using forces to slow down an external object and to project or strike an object. Movers can apply forces in the opposite direction of the motion of an object to gradually slow down the object. How does a gymnast maximize the time of force application to cushion the landing from a dismount from a high apparatus? Near complete extension of the lower extremities at touchdown on the mat allows near maximal joint range of motion to flex the joints and more time to bring the body to a stop. 20XX presentation title 36 FORCE–TIME PRINCIPLE So there are sometimes limits to the benefit of increasing the time of force application. In movements with high demands on timing accuracy (baseball batting or a tennis forehand), the athlete should not maximize the time of force application because extra speed is of lower importance than temporal accuracy. If a tennis player prefers a large loop backswing where they use a large amount of time and the force of gravity to create racket head speed, the player will be vulnerable to fast and unpredictable strokes from an opponent. The wise opponent would mix up shot placements, spin, and increase time pressure to make it difficult for the player to get their long stroke in. Suppose a patient rehabilitating from surgery is having difficulty using even the smallest weights in the clinic. How could a therapist use the Force–Time Principle to provide a therapeutic muscular overload? If bodyweight or assistive devices were available, could the therapist have the patient progressively increase the time they isometrically hold various positions? While this approach would tend to benefit muscular endurance more so than muscular strength, these two variables are related and tend to improve the other. Increasing the time of muscle activity in isometric actions or modifying the cadence of dynamic exercises is a common training device used in rehabilitation and strength training. Timing is an important aspect of the application of force in all human movements. In studying the kinetics of human movement (chapters 6 and 7), we will see several examples of how the human body creates forces over time. There will be many examples where temporal and other biomechanical factors make it a poor strategy to increase the time to apply force in sprinting, each foot contact has to remain short (about 100 ms), so increasing the rate of force development (Force–Motion Principle) is more appropriate than increasing the time of force application. It is important to realize that applying a force over a long period can be a useful principle, but it must be weighed with the other biomechanical principles, the environment, and subject characteristics that interact with the purpose of the movement. 20XX presentation title 37 summary Forces applied to the musculoskeletal system create loads in these tissues. Loads are named based on their direction and line of action relative to the structure. Several mechanical variables are used to document the mechanical effect of these loads on the body. How hard forces act on tissue is mechanical stress, while tissue deformation is measured by strain. Simultaneous measurement of force applied to a tissue and its deformation allows biomechanists to determine the stiffness and mechanical strength of biological specimens. Musculoskeletal tissues are viscoelastic. This means that their deformation depends on the rate of loading and they lose some energy (hysteresis) when returning to normal shape. Bones are strongest in compression, while ligaments and tendons are strongest in tension. Three major mechanical characteristics of muscle affect the tension skeletal muscles can create are the force–velocity, Force–length, and force–time relationships. An important neuromuscular strategy used To maximize the initial muscle forces in most movements is the rapid reversal of a Countermovement with an eccentric muscle Action into a concentric action. This strategy Is called the stretch-shortening cycle. The Creation of muscular force is controlled by the recruitment of motor units and modulating their firing rate. Several musculotendon proprioceptors provide length and tension information to the central nervous system to help regulate muscle actions. The Force– Time Principle is the natural application of the mechanical characteristics of muscle. The timing of force application is as important as the size of the forces the body can create. In most movements, increasing the time of force application can enhance safety, but kinesiology professionals must be aware of how this principle interacts with other biomechanical principles. The application of biomechanical principles is not easy, because they interact with each other and also with factors related to the task, individual differences, or the movement environment. 20XX presentation title 38 Thank you Edrees Harki [email protected] 1- Question: What does the Force-Length Relationship document? 4-Question: According to the passage, what provides the muscle force to bring a stretched Answer: a) The effect of muscle length on muscle tension muscle back to shorter lengths? Options: Answer: b) Passive tension a)The effect of muscle length on muscle tension Options: b) The effect of muscle length on muscle force a) Potential cross-bridge attachment sites c) The effect of muscle length on muscle flexibility b) Passive tension d) The effect of muscle length on muscle endurance c) Biomechanical research d) Coordination of muscles 2- Question: What is the active component of the Force-Length Relationship associated with? Answer: a) The number of cross-bridges between actin and myosin filaments 5-Question: The coordination of muscles may be organized around muscles suited to work on Options: which parts of the force-length curve? a) The number of cross-bridges between actin and myosin filaments Answer: a) Ascending limb, plateau, and descending limb b) The length of the muscle fiber Options: c) The angle of pennation a) Ascending limb, plateau, and descending limb d) The range of motion of the muscle b) Ascending limb and plateau only c) Plateau and descending limb only 3- Question: What happens to potential active muscle tension for shorter or longer muscle d) Ascending limb and descending limb only lengths? Answer: c) It decreases 6-Question: According to the passage, how does the length of muscles influence the central Options: nervous system's coordination? a)It increases Answer: d) It influences how the central nervous system coordinates its actions. b) It remains constant Options c) It decreases :a) It determines the range of motion. d) It fluctuates b) It affects the force–velocity relationship. c) It determines the mechanical work created. 3-Question: What does the passive tension component of the Force-Length Relationship show? d) It influences how the central nervous system coordinates its actions. Answer: a) The effect of muscle length on passive muscle tension Options: a) The effect of muscle length on passive muscle tension b) The effect of muscle length on active muscle tension c) The effect of muscle length on muscle endurance d) The effect of muscle length on muscle flexibility 20XX presentation title 40 Question: What does the Force-Time Relationship refer to? Question: What affects the size of the electromechanical delay? a) The rise or peak in muscle tension a) Muscle stimulation b) The delay in the development of muscle tension b) Connective tissue tension c) The cognitive effort of the subject c) Peak force d) The neuromuscular system d) Muscle coordination Answer: b) The delay in the development of muscle tension Answer: b) Connective tissue tension Question: What are the two parts of the time delay in the Force-Time Relationship? Question: What is the purpose of preactivating muscles before a forceful event? a) Active state and contraction dynamics a) To increase reaction time b) Motor action potential and rise in muscle tension b) To decrease muscle forces c) Excitation dynamics and cognitive effort c) To prepare for the event d) Fusion of twitches and activation history d) To delay muscle activation Answer: a) Active state and contraction dynamics Answer: c) To prepare for the event Question: What is the typical delay in peak tension of whole muscle groups? Question: In movements that cannot be preprogrammed, what is the additional time required for a) 20 ms reaction and processing? b) 100 ms a) A couple of seconds c) 500 ms b) A couple of milliseconds d) Over a second c) A couple of minutes Answer: d) Over a second d) A couple of hours Answer: b) A couple of milliseconds Question: What is the term used to describe the strategy of using a carefully timed movement and muscle activation to generate maximum muscle forces? a) Preprogramming strategy b) Delayed activation strategy c) Stretch shortening cycle d) Reaction and processing strategy Answer: c) Stretch shortening cycle 20XX presentation title 41  1- Sarkawt V.Good 2- Bahasht V.good 3- Shayda 4- 5- Mozda 20XX presentation title 42