Skeletal System PDF

SKELETAL SYSTEM EXP01Y2 CHAPTER 16 SKELETAL SYSTEM The skeletal system includes the bones and cartilage that provide the framework for the muscles and organs of the body. The skeletal system adapts to exercise training in much the same way as other body systems. A healthy skele...

SKELETAL SYSTEM EXP01Y2 CHAPTER 16 SKELETAL SYSTEM The skeletal system includes the bones and cartilage that provide the framework for the muscles and organs of the body. The skeletal system adapts to exercise training in much the same way as other body systems. A healthy skeleton is important for preventing sports-related injuries and major health problems, including osteoporosis. Exercise physiologists are concerned with issues such as: Maximization of peak bone mass Prevention and treatment of osteoporosis Impact of heavy exercise training on the skeleton of growing prepubescent athletes. SKELETAL TISSUE Bone tissue, also called osseous tissue, is a dynamic, living tissue that is constantly undergoing change. Bone remodeling (bone turnover) refers to the continual process of bone breakdown (resorption) and the formation (deposition) of a new bone. Bone remodeling has important roles in regulating blood calcium levels and replacing an old bone with a new bone to ensure the integrity of the skeletal system. The mass and shape of bones depend largely on the stress placed on them. The more bones are stressed (by mechanical loading during physical activity), the more they increase in volume and mass, specifically at the site of mechanical loading. The concept that a bone adapts to changes in mechanical loading is described by Wolff 's Law. FUNCTIONS The skeletal system provides a number of important structural and physiological functions. Structurally, the skeletal system provides rigid support and protection for vital organs and allows for locomotion. Physiologically, skeletal tissue: Provides a site for blood cell formation (haematopoiesis in bone marrow), Plays a role in the immune function (providing the site for white blood cell formation) Serves as a dynamic storehouse for calcium and phosphate. FUNCTIONS The ability of a bone to perform its structural functions relates directly to its role in storing calcium. Because calcium is essential for many processes in the body, bone is broken down (resorbed) as needed to maintain blood calcium levels. The body sacrifices bone mineral (calcium) when it is needed to maintain blood calcium levels. Ca2+ & PO4 – (stored in bone) are essential for nerve conduction, heart and muscle contraction, blood clotting, and energy formation. REGULATION OF BLOOD CALCIUM The skeletal system (bone), the digestive system (stomach and intestines), and the urinary system (kidneys) operate together to regulate and maintain blood calcium levels. Adequate ingestion and absorption of calcium are required through the digestive system to provide the necessary calcium to be deposited in the bone. In turn, because of the importance of calcium in so many vital processes of the body, bone mass is broken down to maintain blood calcium within normal limits (9–11 mg⋅dL−1). The kidneys regulate blood calcium by filtering and reabsorbing it. The primary hormones involved in regulating blood calcium levels and bone remodeling are parathyroid hormone (PTH), calcitonin, and vitamin D (calcitrol). REGULATION OF BLOOD CALCIUM MODELING Bone modeling is the process of altering the shape of a bone and adjusting the bone strength through bone resorption and bone formation. Micromodeling involves the microscopic level of cell organization that occurs during formation; it determines what kind of tissue will be formed. Macromodeling controls if, when, and where new tissue will form or old tissue will be removed. This process ensures that the bone’s shape matches its role. Modeling is largely responsible for bone growth during the years in which the skeleton is growing. REMODELING Bone remodeling involves a continual process of bone turnover, maintenance, replacement, and repair. It reflects the balance between the coupled processes of bone resorption and bone formation. This continuous process occurs due to the coupled actions of bone cells Osteoclasts responsible for bone resorption Osteoblasts responsible for bone formation Remodeling occurs in response to stress on the skeleton throughout the adult years. Physical activity influences bone strength and mass through remodeling, which is accomplished largely because of the activity of bone cells. BONE CELLS The three types of bone cells are osteoclasts, osteoblasts, and osteocytes. These cells are the living part of a bone. Osteoclasts are large, multinucleated bone cells that cause the resorption of bone tissue. Osteoclasts secrete enzymes that disintegrate the bone matrix. As the bone is degraded, the mineral salts (primarily calcium and phosphate) dissolve and move into the bloodstream. BONE CELLS Osteoblasts are bone cells that cause the deposition of bone tissue. Osteoblasts produce an organic bone matrix that becomes calcified and hardens as minerals are deposited in it. BONE CELLS Osteocytes (which are mature osteoblasts surrounded by calcified bone) help regulate the process of bone remodeling. Osteocytes appear to initiate the process of calcification. The actions of osteoclasts and osteoblasts are coupled. Osteoclasts must first cause bone resorption before the osteoblasts can form a new bone. BONE CELLS The figure (16.3) shows the major events of a bone- remodeling From the resting phase, the osteoclasts are stimulated and cause the resorption of bone, forming a cavity. Osteoblasts then appear and deposit the bone matrix where the cavity exists The matrix is called osteoid until it is calcified. Calcification of the new bone occurs as calcium and phosphate minerals are deposited in the osteoid. The bone then returns to the resting or quiescent phase. BONE CELLS Bone remodeling may result in a greater bone mass, the same bone mass, or a reduction in bone mass. This increased mass strengthens the bone and accounts for the increase in BMD that commonly occurs during this period of life. When bone remodeling is in equilibrium, the amount of bone resorbed equals the amount of bone formed; thus, BMD remains relatively constant. The remodeling of bone provides for skeletal growth and involves a constant turnover of bone throughout life. Through young adulthood, typically, more bone is formed than is resorbed, increasing the bone mass. In older adults and those with certain diseases, the amount of bone resorbed is greater than the amount of bone formed, decreasing BMD. Bone remodeling is a complex process regulated by hormonal and local factors HORMONAL CONTROL Bone remodeling reflects the interrelationship between the structural and the physiological functions of bone. Calcium is necessary not only to provide structural integrity of bone but also for the proper functioning of the heart, skeletal muscles, and nervous tissue. Only about 1g of calcium is present in the extracellular fluid of the body, compared to approximately 1150g of calcium present in bone tissue. Excess calcium in the blood leads to the release of calcitonin (from the thyroid gland), which causes deposition of calcium in the bone. This deposition decreases the blood calcium level and increases BMD. Conversely, when the blood calcium level drops below normal, PTH stimulates osteoclast activity, causing calcium to be released from its storage site, the bone. This release of calcium from the bone causes the blood calcium level to increase and BMD to decrease.  HORMONAL CONTROL Hormone Stimulus for Effect on Bone Effect on Blood Release Calcium Levels Increased blood Bone deposition Decreased blood Calcitonin calcium levels (increased calcium) calcium levels Decreased blood Bone resorption Increased blood PTH calcium levels (decreased calcium) calcium levels Vitamin D (calcitriol) is important for the absorption of calcium from the intestines. Thus, it leads to an increased blood level of calcium. HORMONAL CONTROL Other hormones that play an important role in skeletal health are the sex steroids (oestrogen and testosterone) and growth hormone. These hormones stimulate the protein formation necessary for bone growth and are responsible for the eventual closure of the epiphyseal plate, which determines the bone length and thus a person’s height. Oestrogen promotes calcium retention and acts as an inhibiting agent of PTH. The loss of the protective role of oestrogen on the skeletal system after menopause or during secondary amenorrhea has important consequences for females. Decreased oestrogen causes increased bone resorption. Growth hormone and insulin-like growth factor (IGF-1) also play an important role in bone formation and remodeling in children. Hormones are themselves stimulated by other factors, including physical activity and nutritional status. EXERCISE RESPONSE Physical activity increases mechanical forces on bones eventually leading to physiological changes in bone cells allowing bone to be (re)modeled. Mechanotransduction is the process by which a bone responds to a mechanical force on it. Bending causes both compressive stress and tensile stress that alter the hydrostatic pressure in different regions of the bone tissue, causing movement of fluid in this tissue. Fluid flows through the small canals and spaces within the bone matrix (lacunocanalicular system) and around osteocytes; this flow aids in the transport of nutrients and waste. This fluid movement also exerts a shear stress that may stimulate an osteogenic response, resulting in the formation of a new bone. EXERCISE RESPONSE Physical activity causes specific changes in bone physiology within minutes. Soon after a mechanical load is placed on bone cells, they release prostacyclin; this is followed within minutes by an increase in enzymes related to metabolism. Six to twenty four hours after activity, RNA synthesis increases. There is evidence of increased collagen and mineral deposition on the bone surface within 3– 5 days after a bout of loading APPLICATION OF THE TRAINING PRINCIPLES APPLICATION OF THE TRAINING PRINCIPLES Specificity Overload – a demand placed on the body greater than that to which it is accustomed (training load) Rest/Recovery/Adaptation – the change in physiological function that occurs in response to training. Adaptation occurs during periods of rest, when the body recovers from the acute homeostatic disruptions and/or residual fatigue and, as a result, may compensate to above- baseline levels of physiological functioning (supercompensation). Progression – the change in overload in response to adaptation. Retrogression/Plateau/Reversibility – progress is rarely linear, predictable, or consistent. Maintenance – sustaining an achieved adaptation with the most efficient use of time and effort. Individualization. Warm-Up/Cool-Down. APPLICATION OF THE TRAINING PRINCIPLES Adaptations in any given physiological system are dependent on the extent to which exercise stresses that system. For example, adaptations in the cardiovascular system depend on the intensity, duration, and frequency of aerobic exercise training. Similarly, adaptations in skeletal muscle depend on the load, number of reps, rest period, number of sets, and frequency at which load-bearing exercise is performed. APPLICATION OF THE TRAINING PRINCIPLES The adaptation to a mechanical load (physical activity) in bones depends on the strain magnitude, strain rate, distribution of load on the bone, and number of cycles. Bone’s adaptation to physical activity depends on the type of loading. Stress (or load) refers to the external force applied to a bone Strain (deformation) refers to changes in the bone tissue Strain magnitude is the amount of relative change in bone length under mechanical loading. Strain rate is the speed at which strain develops and releases. Distribution of load refers to how strain occurs across a section of bone. Strain cycles are the number of load repetitions. Here are some definitions APPLICATION OF THE TRAINING PRINCIPLES APPLICATION OF THE TRAINING PRINCIPLES The mechanostat theory suggests that a control system operates in which an MES is necessary to maintain a bone and that a higher MES must be surpassed to overload a bone appropriately for positive adaptations (increased BMD and strength). Above the repair MES, bone enters a state of overuse. In the pathological overuse zone, bone suffers from microdamage, and woven (unorganized) bone is added as part of the repair process, leading to increased bone mass but not bone strength APPLICATION OF THE TRAINING PRINCIPLES The precise type and amount of activity for enhancing and maintaining bone health are not fully known at present. However, several recommendations can be made based on the mechanostat theory and research. The overall goals of physical activity relative to skeletal health are to: 1. Increase peak bone mass in adolescents, 2. Minimize age-related bone loss, 3. Prevent falls and fractures SPECIFICITY The specificity principle applies to the specific bones being stressed, the composition of the bone being stressed (cortical versus trabecular), and the type of activity being performed. Research data suggest that the type of exercise or activity performed greatly influences skeletal adaptations. Weight-bearing exercise refers to a movement in which the body weight is supported by muscles and bones, working against gravity. Non– weight–bearing exercise, by contrast, refers to a movement in which the body is supported or suspended, not working against the pull of gravity. Weightbearing or impact-loading activities, such as running and resistance training, are more likely to stimulate increased bone mass than non–weight- bearing activities, such as swimming and cycling. SPECIFICITY Because dynamic resistance training is associated with positive adaptations in skeletal tissue as well as muscular fitness, exercise physiologists generally recommend this type of exercise for maintaining both muscular and skeletal health. Loading seems to have a localized effect (Wolff ’s Law); thus, specific sites can be isolated for impact. Conversely, a general dynamic resistance program that works all the major muscles of the body should benefit the total skeleton. Note that bone tissue does not appear to respond to static resistance exercise (isometric) OVERLOAD The threshold for a stimulus that initiates new bone formation is termed the minimal effective strain (MES) for remodeling A load or force that exceeds this threshold and is repeated a sufficient number of times is thought to cause osteoblasts to secrete osteoid and lead to the formation of a new bone. The MES for bone modeling, and thus the impact load necessary to induce positive skeletal adaptations in humans, is not precisely known, but the stimulus must include forces considerably greater than those of habitual activity. There is strong evidence that weightbearing, impact-loading exercises can lead to an increase in BMD in children and adolescents and also decrease the age-related loss of BMD in adulthood. Impact loads, and thus the strain applied to bones, can be manipulated by increasing repetitions or by increasing the strain magnitude as measured by ground reaction force or joint force. For example, running loads the bones by high repetition, whereas skipping (rope) overloads the bones primarily by intensity (strain magnitude). OVERLOAD For adaptations in skeletal tissue, intensity is apparently more important than repetition. Until the amount of exercise needed to impose an overload is known, the rate of adaptation or the ideal progression necessary to induce additional gains in bone density cannot be determined. However, any type of exercise overload (intensity, duration, or frequency) must begin at a level the individual can safely tolerate and progress gradually. Skeletal adaptations are unique in terms of the slow turnover rate of a bone. Because it takes about 3–4 months for one remodeling cycle to complete the sequence of bone resorption, formation, and mineralization, a minimum of 6–8 months of exercise training are typically required to detect a measurable change in bone mass in humans, using current technology. REST/RECOVERY/ADAPTATION To date, little research has been done with humans to determine the optimal amount of rest and recovery for positive bone adaptations. It is known that inadequate rest and recovery along with excessive repetitive loads can lead to stress fractures. Some researchers have used information from animal studies to develop an osteogenic index (OI) to guide exercise prescription for bone adaptations, but the utility of such a tool has not yet been proven. Useful for those beginning or returning to exercise INDIVIDUALIZATION The individual response principle applies to the skeletal system as well as other body systems; that is, different people respond to the same exercise stress differently depending on their genetic makeup, hormonal and nutritional status, and so on. Individuals with low BMD have the greatest potential for benefit. Additionally, exercise interventions’ goals vary for individuals across the lifespan. Childhood – improve bone acquisition and attain the highest possible peak bone mass Adulthood – build and maintain bone. Older adults – reduce bone loss and prevent falls. RETROGRESSION/PLATEAU/REVERSIBILITY The reversibility principle suggests that if you cease exercising for a time, you lose the benefits of exercising. Studies of immobilized patients and discontinued training indicate that this principle also applies to bones. And lose overall muscular strength MAINTENANCE The increased BMD resulting from exercise training appears to be reduced with the cessation of training. The rate of bone loss is not known, however, nor is the level of activity needed to maintain BMD or the threshold at which bone loss occurs. Intense exercise training during the pubertal years and early adulthood may lead to greater attainment of peak bone mass, which may protect against fractures later in life because more bone mass can be lost before the bone is weakened to the point of fracture. There is evidence that increases in BMC in the femoral neck gained during 7 months of high-impact training in prepubertal children were maintained during a 7-month detraining period. Clearly, activity is needed to maintain BMD, but additional research is necessary to determine the level of activity in various age groups for maintaining improvements in BMD that resulted from exercise training. SKELETAL ADAPTATIONS TO EXERCISE TRAINING SKELETAL ADAPTATIONS TO EXERCISE TRAINING Adaptation of bone to exercise depends largely on the amount of activity and may be represented as a continuum. Measurable skeletal adaptation depend on the type of bone being measured (trabecular or cortical) as well as the type of activity employed. One approach to studying the effects of increased physical activity on bone density is to compare the dominant limb to the nondominant limb. These studies report that the dominant arm has greater BMD or mass than the nondominant arm. This seems true for both females and males and across a wide age span. Furthermore, the difference in BMD between the dominant arm and the nondominant arm appears related to the age at which participants started playing the sport. SKELETAL ADAPTATIONS TO EXERCISE TRAINING SKELETAL ADAPTATIONS TO EXERCISE TRAINING Another approach to studying skeletal adaptations to exercise training has been to compare different athletic groups with one another and with control groups. These studies collectively suggest that individuals involved in athletics or participating in vigorous fitness training have greater BMD than sedentary controls. Individuals involved in weight-bearing or impact-loading sports have higher BMD than those involved in non– weight-bearing activities. Training studies have also been conducted of sedentary individuals beginning an exercise program. BMD measurements were compared before and after the exercise training. Regular physical exercise can delay the physiological decrease in BMD that occurs with aging and reduce the risk of osteoporosis. Weight-bearing exercises, including weight lifting, jumping, and running, were associated with the greatest improvements in bone mass SKELETAL ADAPTATIONS TO EXERCISE TRAINING Skeletal adaptation to exercise depends on the age of the participant. Vigorous exercise helps increase bone mass and strength in children and is thus important for the attainment of peak bone mass. Those who continued an active lifestyle including weight-bearing activity tend to have higher BMD in adulthood than those who ceased participation or are sedentary. No reliable data suggests that exercise training negatively affects either skeletal maturation (measured by ossification) or bone length in growing children. SKELETAL ADAPTATIONS TO EXERCISE TRAINING Overall, studies suggest that weight-bearing and resistance exercise training play an important role in maximizing bone mass during childhood and adolescence, maintaining bone mass through adulthood, attenuating bone loss with aging, and reducing falls and fractures in the elderly. However, if some is good, more is not necessarily better when it comes to exercise training and bone health. As shown in Figure 16.11, excessive physical activity can exceed the adaptive ability of bone, resulting in overuse injuries. Start off slow, progress at a manageable intensity, and allow sufficient time for recovery  Please read me! SKELETAL ADAPTATIONS TO DETRAINING Research clearly shows that a cessation of weight-bearing exercise is detrimental to the skeleton because it results in a loss of BMD. This effect has been clearly shown in astronauts and in patients confined to bed rest or immobilized in a cast. Studies have consistently indicated that weight-bearing bones are affected more and that trabecular bone (measured in the spine) is lost at a greater rate than cortical bone SKELETAL ADAPTATIONS TO DETRAINING Research also suggests that discontinuing weightbearing exercise results in a loss of the positive adaptation that occurs with training. Use it or lose it Collective data strongly suggest that the increased bone mineral resulting from exercise is lost if exercise is not continued; bones respond to activity and inactivity. The positive effects of exercise can be lost if training is reduced or stopped (see textbook for examples)