| Bone, Muscle, and Connective Tissue Adaptations to Physical Activity |
| - Bone provides structural support for system, muscle contains contractile units and converts stored chemical energy into mechanical energy needed to produce movement, and connective tissue acts as framework upon which forces generated by contracting musculature can be transmitted to associate bony levers to elicit desired movement. - When the system is stressed, as during physical activity, bone, muscle, and connective tissue must adapt in a coordinated manner to preserve strength and integrity of whole force-generating system. |
| Adaptation of Bone to Exercise - Bone is classified as connective tissue. It is unique connective tissue in that it becomes mineralized and thereby provides rigid support structure. This rigid structure is actually very active tissue that is sensitive to changes in forces it experiences and has capacity for growth and regeneration if damaged. The Biology of Bone Formation - Osteoblast o In response to mechanical loading, bone cells (osteoblasts) migrate to the bone surface that is experiencing the strain and begin the process of bone modeling. - Bone matrix o Proteins (secreted by osteoblasts) form a mechwork. - Periosteum o New bone formation occurs predominately on the outer surface of the bone (periosteum) - The adaptation of bone to mechanical loading occurs at different rates in; o Axial skeleton (spine and proximal femur) o Appendicular skeleton (long bones) o Trabecular bone (spongy) § Dense and forms compact outer shell that is bridged by interconnecting narrow and delicate plates of trabecular bone. o Cortical bone (compact) § It can respond more rapidly to stimuli than cortical bone - Deposition of new collagen fibers in vertebral bone can be expected after 8 to 12 weeks of mechanical loading. The Stimulus for New Bone Formation - Minimal essential strain (MES) o The threshold stimulus that initiates new bone formation o Physical activities that generate forces exceeding the MES are those activities that represent an increase in intensity relative to normal daily activities. o The MES is thought to be a level of strain approximately one tenth of the force required to fracture the bone. o After bone growth occurs, the same force that previously exceeded the MES will not be below the MES threshold and not present a threat to the bone. Effects of Physical Activity on Bone - The total mass of a muscle reflects the forces that the muscle is capable of exerting on the bones to which it is attached. Immobility - Loss of bone matrix and bone mineral density (the quantity of mineral deposited in given area of bone) following period of reduced loading or immobility appears to occur at much more rapid rate than formation and mineralization of new bone. Rapid removal of calcium from bone, which results in net loss of bone mineral content, occurs after only a few weeks of bed rest. Important Training Concepts for Stimulating New Bone Formation - Significant positive correlation between bone mineral density values & strength of attached musculature. - Activities that stimulate muscle hypertrophy and strength gains also appear to stimulate the growth of bone and associated connective tissue. - Specificity refers to employing exercises that directly load a particular region of the skeleton. - Osteoporosis is a disease in which bone mineral density and bone mass become reduced to critically low levels. When bone becomes this compromised, forces that would normally be absorbed by skeleton now result in bone fractures. Sites of fracture that are the most devastating are in axial skeleton (the spine and hip) - Because bone and connective tissue respond to mechanical forces that threaten the supporting structures of contracting musculature, principle of progressive overload – progressively placing greater-than-normal demands on exercising musculature – applies to training to increase bone mass as well as training to improve muscle strength - Stress fractures – microfractures in bone due to structural fatigue – and soft tissue injuries can occur when a force is routinely applied to the bone and related connective tissues before they have had a chance to adapt to the new exercise stimulus. - Young bone may be more responsive to osteogenic stimuli – factors that stimulate new bone formation – than mature bone. - In order to avoid dangerously low bone mass in old age, people should train to maximally elevate their peak bone mass – the maximum bone mass achieved – during early adulthood when the mechanisms for bone growth still function at optimal levels. The Essential Components of Mechanical Loading - The components of the mechanical load that stimulate bone growth in laboratory animals are; o The magnitude of the load (intensity) o The rate (speed) of loading o The direction of the forces o The volume of loading (number of repetitions) - Exercise Prescription Guidelines for Stimulating Bone Growth Variables Specific recommendations Volume 3 – 6 sets of up to 10 repetitions Load 1 – 10 RM Rest 1 – 4 min Variation Typical periodization schemes designed to increase muscle strength and size Exercise selection Structural exercises; squat, deadlift, cleans, bench press, shoulder press Exercise Selection - The choice of exercises should be based on exercises that involve many muscle groups in one exercise (structural exercises), direct the force vectors through the axial skeleton (spine and hip), and allow greater absolute loads to be utilized in training. - Body part exercises may be useful for gently introducing untrained people to resistance exercise. Increased kinesthetic awareness and initial muscle conditioning should be priority goals at the beginning of a new conditioning program for previously untrained people. - Changing the distribution of the force vectors by using a variety of exercises continually presents a unique stimulus for new bone formation within a common region of bone Aerobic Exercise - Most successful in stimulating bone growth involve more intense physical activities such as rowing, stair climbing, running, and running with weight packs or vests. Key to the success of an aerobic exercise program in stimulating new bone formation is that the aerobic activity be significantly more intense than normal daily activities typically experienced by the person. - The intensity of the activity must systematically increase in order to continually overload the bone - Utilizing interval training techniques is one method of providing a greater osteogenic stimulus while still providing the benefits associated with regular aerobic exercise Untrained and Aged People - One must use the same precautionary guidelines that would apply to prescribing resistance exercise to that population. |
| Adaptation of Muscle to Exercise Muscle Growth - Hypertrophy (muscular enlargement that results from resistance training program) and is primarily result of increase in cross-sectional area of existing fibers. - There is increase in number of muscle fibers, which increases overall size of muscle. Here initial muscle undergoes hyperplasia – longitudinal fiber splitting – as response to high-intensity weight training. (Not in humans) Training Programs - Training for Strength o High-resistance, near-maximal muscle contractions extended over a small number of repetitions with a full recovery period between each set o It elicits increases in the cross-sectional are of the exercised muscles, with Type II fiber areas increasing more readily and at a faster rate than Type I fibers. o Initial dominance of Type II fibers is advantage to increasing muscular strength because their growth results in increase in lean body mass, one of the most influential factors in determining maximum strength o Biochemical adaptations to strength training lie in significant increase in muscle glycogen, creatine phosphate, & adenosine triphosphate (ATP) substrate stores. There is also increase in quantity & activity of glycolytic enzymes myokinase and creatine kinase - Training for Muscle Size o Lighter loads, which allows to perform more repetitions than is typical of a strength training program, but heavy enough to elicit concentric or eccentric contraction failure (inability of the muscle to shorten or length under control) within 6 to 12 repetitions o Rest period is of short to moderate duration, since it is important to begin the next set of the exercise before full recovery has been achieved. o Larger muscles may be caused by § Increase in number of muscle fibers § Larger absolute amount of collagen and other noncontractile connective tissue § Greater substrate stores § Increase in diameter of Type I fibers o Percentage of Type II fibers in bodybuilders lower than that found in other aerobic athletes and larger number and size of Type I fibers - Training for Muscular Endurance Using Aerobic Exercise o Submaximal muscle contractions extended over a large number of repetitions with little recovery allowed between each set. Therefore, relative intensity is very low and overall volume is very high o Type I fibers relative to possess an oxidative capacity greater than that of Type II fibers both before and after training. o Endurance training reduces the concentration of glycolytic enzymes and can reduce the overall muscle mass of the hypertrophied (and nonhypertrophied) Type II fibers. o Type IIa fibers (fast oxidative glycolytic fiber) possess greater oxidative capacity than Type IIb fibers (fast glycolytic fibers). o Mitochondria are organelles in cells that are responsible for aerobically producing ATP via oxidation of glycogen. o When larger and more prevalent mitochondria are combined with increase in quantity of oxygen that can be delivered to mitochondria by higher levels of myoglobin (protein that transports oxygen within cell) o Increase in the endurance capability of muscle tissue are caused by § Conversion of Type IIb to Type IIa fiber § Hypertrophy of Type I fibers § Greater myoglobin content |
| Adaptation of Connective Tissue to Exercise - Primary structural component of all connective tissues encountered in musculoskeletal system is collagen fiber. - Procollagen is synthesized & secreted by spindle-shaped connective tissue support cell (fibroblast) - True strength of collagen comes from the strong chemical bonding (covalent cross-linking) - Primary type of collagen found in bones, tendons, ligaments, and fascia is called Type I collagen. - The only collagen found in the cartilage at the articulating surfaces of bone is Type II collagen. o Two forms are similar in appearance; they differ in amino acid sequence, which results in differences in their functional properties - It is in extracellular space, not within cells, that collagen molecules align to form collagen fibers. Biology of Tendons, Ligaments, and Fascia - Tendons and ligaments are composed primarily of tightly packed parallel arrangements of collagen bundles. Mature tendons and ligaments contain relatively few cells. - Ligaments contain elastic fibers in addition to collagen. There fibers contain elastin, an extensible protein; certain amount of stretch must be permitted within ligament to allow normal joint motion, degree of elasticity being specific to functional requirement of joint. Thus, ligaments contain fibers that provide strong structural support (collagen) and fibers that confer degree of flexibility to ligament (elastin). - As the bone grows in diameter, tendon or ligament insertion becomes buried in bone surface increasing the strength of this junction. - Fibrous connective tissues that surround & separate different organizational levels within skeletal muscle are referred to as fascia. Fascia has sheets of collagen fibers arranged in different planes to provide resistanve to forces from several different directions. - Individual muscle fibers are surrounded by a layer of connective tissue (endomysium) - Extracellular matrix proteins are part of fascia layer called perimysium, which is connects muscle fibers together into distinct bundles, or fasciculi. - All the fasciculi of muscle are bound together by the fascia layer termed the epimysium Biology of Cartilage - Cartilage is dense connective tissue consisting of cells embedded in a firm matrix. - The main functions of cartilage are o To provide a smooth articulating surface at the interface of bones in a joint o To act as a shock absorber for forces directed through the joint o To aid in the attachment of muscle to the skeleton - Feature unique to cartilage tissue is that it lacks its own blood supply. This means that the cells that produce cartilage, the chondrocytes, must depend on diffusion of oxygen and nutrients from synovial fluid for survival. - Hyaline cartilage (articular cartilage) is found on articulating surfaces of bones. - The internal environment of the cartilage tissue (cartilage matrix), within which the collagen fibers transverse, exists as a gellike material called ground substance. - Fibrous cartilage is a very tough form of cartilage found in the intervertebral disks of the spine and at the junctions where tendons attach to bone. Effects of Physical Activity on Connective Tissue - Primary stimulus for connective tissue growth in mature adult is mechanical forces created during physical activity. - The sites where connective tissues can increase strength and thus load-bearing capacity are o At the junctions between the tendon (or ligament) and bone surface o Within the body of the tendon or ligament o In the network of fascia within skeletal muscle - Stronger muscles pull harder on their bony attachments and cause an increase in bone mass at the tendon- bone junction and along the line over which the forces are distributed. - Specific changes that occur within a tendon that contribute to the increase in its cross-sectional area and strength in response to a functional overload include o An increase in collagen fibril diameter o A greater number of covalent cross-links present within a fiber of increased diameter o An increase in the number of collagen fibrils o An increase in collagen fibril packing density - The response of articular cartilage to physical activity may seem less obvious. However, the fact that articular cartilage gets its nutrient supply by diffusion from synovial fluid links joint mobility to joint health. Movement about a joint creates changes in pressure in the joint capsule that drive nutrients from the synovial fluid toward the articular cartilage of the joint. Immobilization of a joint prevents proper diffusion of oxygen and essential nutrients throughout the joint, resulting in the death of the chondrocytes and resorption of the cartilage matrix - A more encounteraging finding is that long-term adherence to treadmill exercise in experimental animals thickens the cartilage and increases the number of cells and later the total ground substance present in articular cartilage |