| The Biomechanics of Resistance Exercise |
| - Anatomy o It encompasses the study of components that make up the musculoskeletal “machine” - Biomechanics o It focuses on the mechanisms though which these components interact to create movement |
| The Musculoskeletal System The Skeleton - Muscles function by pulling against bones that rotate about joints and transmit force through the skin to the environment - Muscles can only pull, but not push - There are approximately 206 bones in the body, though the number can vary - The relatively light, strong structure provides leverage, support, and protection - Axial skeleton o Head, spinal column, and chest - Appendicular skeleton o Arm, shoulders, legs, and pelvis - Joint (Junctions of bones) o Fibrous joints (e.g., sutures of the skull) allow virtually no movement o Cartilaginous joints (e.g., intervertebral discs) allow limited movement o Synovial joints (e.g., elbow and knee) allow considerable movement § Low friction and large range of motion § Articulating bone ends are covered with smooth hyaline cartilage § The entire joint is enclosed in a capsule filled with synovial fluid ? Sport and exercise movements occur mainly about the synovial joints ? virtually all joint movement consists of rotation about points or axes o Uniaxial joint (elbow) § Operate as hinges (rotating about only one axis) o Biaxial joints (ankle and wrist) § All movement about two perpendicular axes o Multiaxial joints (shoulder and hip ball-and-socket joints, knee) § Allow movement about all three perpendicular axes that define space - Spinal column (Several vertebral bones separated by flexible disks that allow movement to occur) o Cervical vertebrae (7) in neck region o Thoracic vertebrae (12) in the middle to upper back o Lumbar vertebrae (5) in lower back o Sacral vertebrae (5) in rear part of the pelvis o Coccygeal vertebrae (3 to 5) in vestigial internal extending downward from the pelvis Skeletal Musculature - Proximal attachment is called the muscle origin - Distal attachment is called the insertion - Fleshy attachments o Most often found at the proximal end of a muscle, muscle fibers are directly affixed to the bone, usually over a wide area so that force is distributed rather than localized - Fibrous attachments o Such as tendons, blend into and are continuous with both the muscle sheaths and the connective tissue surrounding the bone o They have additional fibers that extend into bone itself, making for a very strong union o Tendons are capable of sustaining forces as high as 12,000 N (2,700 lb) per cm² of cross-sectional area - Movements of muscle actions o Agonist § The muscle most directly involved in bringing about a movement o Antagonist § A muscle that can slow down or stop the movement o Synergist § When it assists indirectly in a movement § Synergists are required to control body motion when agonist is muscle that crosses two joints Levers of the Musculoskeletal System - Lever o A rigid or semirigid body that, when subjected to a force whose line of action does not pass through its pivot point, - Fulcrum o The pivot point of a lever - Moment arm (Force, lever arm, or torque arm) o The perpendicular distance from the line of action of the force to the fulcrum. The line of action of a force is an infinitely long line passing through the point of application of the force, oriented in the direction in which the force is exerted - Torque (Moment) o The degree to which a force tends to rotate an object about a specified fulcrum; quantitatively defined as the magnitude of a force times the length of its moment arm - Muscle force o Force generated by biomechanical activity that tends to draw the opposite ends of a muscle towards each other - Resistive force o Force generated by a source external to the body (e.g., gravity, inertia, friction) that acts contrary to muscle force - Mechanical advantage o The ratio of the moment arm through which an applied force acts to that through which a resistive force acts. A mechanical advantage greater than 1.0 allows the applied (muscle) force to be less than the resistive force to produce an equal amount of torque. A mechanical advantage of less than 1.0 is a disadvantage in the common sense. - First-class lever o A lever for which muscle force and resistive force act on opposite sides of the fulcrum - Second-class lever o A lever for which the muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through a moment arm longer than that through which the resistive force acts. Due to its mechanical advantage the required muscle force is smaller than the resistive force. - Third-class lever o A lever for which the muscle force and resistive force act on the same side of the fulcrum, with the muscle force acting through a moment arm shorter than that through which the resistive force acts. The mechanical advantage is thus less than 1.0, so the muscle force has to be greater than the resistive force to produce torque equal to that produced by the resistive force. o Most human limbs are operated as third-levers by the muscles that rotate the limbs about body joints. This is why internal muscle forces are much greater than the forces exerted by the body on external objects. ? During movement, the categorization of a lever as first, second, or third class often depends upon the somewhat arbitrary decision of where the fulcrum lies. Variations in Tendon Insertion - There is considerable variability in human anatomical structure, including points at which tendons are attached to bone. A person at which tendons are inserted on bone farther from joint center should be able to lift heavier weights because muscle force acts through longer moment arm and thus can produce greater torque around the joint. - The mechanical advantage gained by having tendons insert farther from the joint center is accompanied by a loss of maximum speed, because with the tendon inserted farther from the joint center, the muscle has to contract more to make the joint move through a given range of motion. |
| Human Strength and Power Basic Definitions - Acceleration o Change in velocity per unit time - Strength o The maximal force that a muscle or muscle group can generate at a specified velocity - Power o The time rate of doing work o The production of the force exerted on an object and the velocity of the object in the direction in which the force is exerted o Power = Work/Time = Force x Distance / Time = Force x Velocity - Work o The product of the force exerted on an object and the distance the object moves in the direction in which the force is exerted. o Work = Force x Distance - Angular displacement o The angle through which an object rotates o Radian (rad); 1 rad = 180° / ? = 57.3° - Angular velocity o The object’s rotational speed, measured in rad/s o Work = Torque x Angular Displacement o Power = Torque x Angular Displacement / Time = Torque x Angular Velocity - What is critical is ability to exert force at speeds characteristic of sport to overcome gravity and accelerate body or implement. For sport movement made relatively slow by high resistance, low-speed strength is critical, whereas for movement that is very fast due to low resistance, high-speed strength is important. Biomechanical Factors in Human Strength - Neural Control o Neural control affects the maximal force output of a muscle by determining which and how many motor units are involved in a muscle contraction (recruitment) and the rate at which the motor units are fired (rate coding) - Muscle Cross-sectional Area o All else being equal, the force a muscle can exert is related to its cross-sectional area rather than to its volume - Arrangement of Muscle Fibers o Pennate muscle is one in which the fibers have a featherlike arrangement o The angle of pennation is defined as the angle between the muscle fibers and an imaginary line between the muscle’s origin and insertion o Pennation appears to prodive some enhancement of force capability for muscle contracting at high speed, particularly at extremes of range of muscle motion, but pennation can be somewhat disadvantageous for generating eccentric, isometric, or low-speed concentric. Force. - Muscle Length o The muscle can generate the most force around its resting length and less force when it is in an elongated or shortened state. - Joint angle o The sharp of a curve of maximal muscle torque versus joint angle may or may not be the same as that of curve of maximal muscle force versus muscle length, for several reasons; § Because of changes in pivot point location and tendon position throughout the movement, the movement arm through which the muscle acts can vary § More than one muscle usually acts together to cause movement about given body joint. At any joint angle, different muscles are at different points in their force-versus-length curves § The length of a muscle crossing two body joints is affected both joint angles - Muscle contraction Velocity o Muscle can produce less force as the velocity of contraction increases - Joint Angular velocity § Concentric muscle action § Isometric muscle action § Eccentric muscle action - Three-Dimensional Strength Relationship Among Torque, Joint Angle, & Angular Velocity o Strength can be obtained when maximal torque capability is depicted as a function of both joint angle and angular velocity in a three-dimensional plot - Strength-to-Mass Ratio o The ratio of the strength of the muscles involved in the movement to the mass of the body parts being accelerated o Force = Mass x Acceleration - Body Size o All else being equal, smaller athletes are stronger “pound-for-pound” than larger athletes. o Classical formula § The lift is divided by body weight to the two-thirds power, thus accounting for the cross-sectional area versus volume relationship. |
| Sources of Resistance to Muscle Contraction Gravity - Weight o The downward force on an object due to gravity o Fg (force sue to gravity) = m (mass) *ag (local acceleration of gravity) o Geographical latitude is much more relevant to gravitational force than is terrestrial altitude Applications to Resistance Training - When weight is directly above/below elbow pivot point there is no resistance torque due to weight - Horizontal distance from the weight to the body joint Weight Stack Machines - As with free weights gravity is the source of resistance for weight stack machines. However, by the means of pulleys, cams, cables, and gears, the machines provide increased control over the direction and patter of resistance. - Some of advantages of the stack machine include the following; o Safety o Design flexibility o Ease of use - Advantages of free weight include the following; o Whole body training o Simulation of real life activities Inertia - In addition to gravitational force, when accelerated, exerts inertial force on the lifter. While the force of gravity acts only downward, inertial force can act in any direction. - Force = mass x acceleration Friction - Resistance force encountered when one attempts to move two objects in contact with each other Fluid Resistance - Resistive force encountered by an object moving through a fluid or by a fluid moving past or around an object or through an orifice o Coefficient of friction x normal force - Two sources of fluid resistance are surface drag, which results from the friction of a fluid passing along the surface of an object, and form drag, which results from the way in which a fluid presses against the front or rear of an object passing through it - The resistive force is greater when the piston is pushed faster - Fluid-resisted machines do not provide an eccentric exercise phase o Force = friction constant x relative velocity Elasticity - The more the elastic component is stretched, the greater the resistance. - Problem with devices using elastic resistance is that every exercise movement begins with low resistance and ends with high resistance. This is contrary to the force capability patterns of virtually all human muscle groups, which show a substantial drop-off force capability toward the end of the range of motion. - The number of bands that can be affixed to device usually limits adjustability of resistance. Electronically Controlled Devices |
| Power Output During resistance Exercise Weight-Lifting Exercise - When the weight is lifted faster, the average power is higher. - Force capability of muscle declines with increasing speed of contraction, muscle power increases Friction-Resisted Exercise - The resistive force of a friction-resisted device stays constant regardless of the movement speed. Fluid-Resisted exercise - As velocity of movement increases, the resistive force increases proportionately Elasticity-Resisted Exercise - Always easy to perform early in the movement and difficult at the end Electronically Controlled Exercise Negative Work and Power - All “negative” power and work occurs during eccentric muscle activity. |
| Lifting Safety Vulnerability of anatomical structures to injury - Bones – stress fractures, breakage - Muscles – soreness, strains, tears - Tendons – strains, tears - Ligaments – sprains, ruptures The Back - Back Injury o 85 – 90% of all spinal-disk herniations occur at disks between lowest two lumber vertebrae (L4 and L5) or between lowest lumber and the top sacral vertebra (L5 and S1) o Disks of people under 40 years old can typically withstand without damage more than twice the compressive force that would damage disks of people over 60 o An arched back (lordosis) has been found to be superior to a rounded back for avoiding injury to vertebrae, disks, facet joints, ligaments, and muscles of the back. o Lower back is rounded, ventral (towards the belly) edges of vertebral bodies squeeze front portions of spinal disks. o Extreme arching of back results in squeezing dorsal (towards the back) portions of disks. o Intraabdominal Pressure and Lifting Belts § Weightlifting belts have been shown to increase intraabdominal pressure during lifting and are therefore probably effective in improving lifting safety - The Shoulders o The glenoid cavity of the shoulder, which holds the head of the humerus, is not a true socket and is quite loose o Particular care must be taken when performing the various forms of the bench, incline, and military presses because of the great stresses they place on the shoulder. - The Knees o Of the various components of the knee, the patella and surrounding tissue are most susceptible to the kinds of forces encountered in resistance training. o Knee Wraps § There has been very little research done one the efficacy of knee wraps § Some detrimental side effects of heavy wraps, including skin damage and chondromalacia patellae, the wearing down and roughening of the posterior surface of the patella - Avoiding Injury During Weight Training o Perform one or more warm-up sets with relatively light weights. This stimulates blood flow to the muscles effecting the movement, increasing the temperature and pliability of ligaments, tendons, and other structures. o Heavy and tight wraps can cause joint injury - Flexibility and Stretching |
| Movement Analysis and Exercise Prescription - Specificity holds that training is most effective when resistance exercises are similar to the sports activity in which improvement is sought Movement-Oriented Exercise Prescription |