Lecture 4 Biomechanics of Resistance Exercise

MUSCULOSKELETAL SYSTEM Skeleton Muscles function by pulling against bones that rotate about joints and transmit force through the skin to the environment. The skeleton can be divided into the axial skeleton and the appendicular skeleton. Skeletal Musculature A system of muscles enables the skeleton to move. Origin = proximal (toward the center of the body) attachment Insertion = distal (away from the center of the body) attach-ment

KEY TERMS agonist: The muscle most directly involved in bringing about a movement; also called the prime mover. antagonist: A muscle that can slow down or stop the movement.

FA = force applied to the lever; MAF = moment arm of the applied force; FR = force resisting the lever’s rotation; MRF = moment arm of the resistive force.

KEY TERM first-class lever: A lever for which the muscle force and resistive force act on opposite sides of the fulcrum.

O = fulcrum; FM = muscle force; FR = resistive force; MM = moment arm of the muscle force; MR = moment arm of the resistive force. Mechanical advantage = MM /MR = 5 cm/40 cm = 0.125, which, being less than 1.0, is a disadvantage. ● The depiction is of a first-class lever because muscle force and resistive force act on opposite sides of the fulcrum. During isometric exertion or constant- speed joint rotation, FM · MM = FR · MR ●Because MM is much smaller than MR, FM must be much greater than FR; this illustrates the disadvantageous nature of this arrangement.

KEY TERM second-class lever: 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.

A SECOND-CLASS LEVER (THE FOOT) Figure 4.4 (next slide) The slide shows plantarflexion against resistance (e.g., a standing heel raise exercise). FM = muscle force; FR = resistive force; MM = moment arm of the muscle force; MR = moment arm of the resistive force. When the body is raised, the ball of the foot, the point about which the foot rotates, is the fulcrum (O). Because MM is greater than MR, FM is less than FR.

The slide shows plantarflexion against resistance (e.g., a standing heel raise exercise). – FM = muscle force; FR = resistive force; MM = moment arm of the muscle force; MR = moment arm of the resistive force. –When the body is raised, the ball of the foot, the point about which the foot rotates, is the fulcrum (O). –Because MM is greater than MR, FM is less than FR.

KEY TERM third-class lever: 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.

A THIRD-CLASS LEVER (THE FOREARM) Figure 4.5 (next slide) The slide shows elbow flexion against resistance (e.g., a biceps curl exercise). FM = muscle force; FR = resistive force; MM = moment arm of the muscle force; MR = moment arm of the resistive force. Because MM is much smaller than MR, FM must be much greater than FR.

–The slide shows elbow flexion against resistance (e.g., a biceps curl exercise). – FM = muscle force; FR = resistive force; MM = moment arm of the muscle force; MR = moment arm of the resistive force. –Because MM is much smaller than MR, FM must be much greater than FR.

– (a) The patella increases the mechanical advantage of the quadriceps muscle group by maintaining the quadriceps tendon’s distance from the knee’s axis of rotation. – (b) Absence of the patella allows the tendon to fall closer to the knee’s center of rotation, shortening the moment arm through which the muscle force acts and thereby reducing the muscle’s mechanical advantage.

EXAMPL The patellofemoral joint reaction force C is the equilibrant to the force of the quadriceps muscles. For reasonable assumption, we may consider the patella as a moveable pulley, and the tendinous attachments to the patella as the two supporting strands (figure 1), thus Fm and Ft are equal in magnitude. Q: In the position shown in the figure, how much force develops the quadriceps muscles when lb compression force acts on the patellofemoral joint?

MOMENT ARM AND MECHANICAL ADVANTAGE Figure 4.7 (next slide) During elbow flexion with the biceps muscle, the perpendicular distance from the joint axis of rotation to the tendon’s line of action varies throughout the range of joint motion. When the moment arm (M) is shorter, there is less mechanical advantage.

As a weight is lifted, the moment arm (M) through which the weight acts, and thus the resistive torque, changes with the horizontal distance from the weight to the elbow.

MUSCULOSKELETAL SYSTEM Variations in Tendon Insertion tendon insertion: The points at which tendons are attached to bone. Tendon insertion farther from the joint center results in the ability to lift heavier weights. This arrangement results in a loss of maximum speed. This arrangement reduces the muscle’s force capability during faster movements.

MUSCULOSKELETAL SYSTEM Anatomical Planes of the Human Body The body is erect, the arms are down at the sides, and the palms face forward. The sagittal plane slices the body into left- right sections. The frontal plane slices the body into front-back sections. The transverse plane slices the body into upper-lower sections.

HUMAN STRENGTH AND POWER Basic Definitions strength: The capacity to exert force at any given speed. power: The mathematical product of force and velocity at whatever speed.

HUMAN STRENGTH AND POWER Biomechanical Factors in Human Strength Neural Control Muscle force is greater when: (a) more motor units are involved in a contraction, (b) the motor units are greater in size, or (c) the rate of firing is faster. Muscle Cross-Sectional Area The force a muscle can exert is related to its cross-sectional area rather than to its volume. Arrangement of Muscle Fibers Variation exists in the arrangement and alignment of sarcomeres in relation to the long axis of the muscle.

KEY TERMS pennate muscle: A muscle with fibers that align obliquely with the tendon, creating a featherlike arrangement. angle of pennation: The angle between the muscle fibers and an imaginary line between the muscle’s origin and insertion; 0 ° corresponds to no pennation.

EXAMPLE The role of the Gastrocnemius is to elevate the foot (known as plantar flexion). The medial and lateral parts of the Gastrocnemius generate the same tension and both tend to elevate the tendon. How much force in each part of the Gastrocnemius if we know that the angle of pennation of the muscular fibers is 60 ° and the Achilles tendon tension is 400 N?

KEY TERM concentric muscle action: A muscle action in which the muscle shortens because the con- tractile force is greater than the resistive force. The forces generated within the muscle and acting to shorten it are greater than the external forces acting at its tendons to stretch it.

KEY TERM eccentric muscle action: A muscle action in which the muscle lengthens because the contractile force is less than the resistive force. The forces generated within the muscle and acting to shorten it are less than the external forces acting at its tendons to stretch it.

KEY TERM isometric muscle action: A muscle action in which the muscle length does not change because the contractile force is equal to the resistive force. The forces generated within the muscle and acting to shorten it are equal to the external forces acting at its tendons to stretch it.

Center of Gravity What is the center of gravity? the point around which a body’s weight is equally balanced in all directions also referred to as the center of mass or mass centroid (need not be physically located inside of a body)

Center of Gravity Why is the center of gravity of interest in the study of human biomechanics? it serves as an index of total body motion

Center of Gravity Why is the center of gravity of interest in the study of human biomechanics? the body responds to external forces as though all mass were concentrated at the CG; this is consequently the point at which the weight vector is shown to act in a free body diagram

LOCATING THE HUMAN BODY CENTER OF GRAVITY Locating the CG for a body containing two or more movable, interconnected segments is more difficult than doing so for a non-segmented body, because every time the body changes configuration, its weight distribution and CG location are changed. Every time an arm, leg, or finger moves, the CG location as a whole is shifted at least slightly in the direction in which the weight is moved

Question: The x,y-coordinates of the CGs of the upper arm, forearm, and hand segments are provided on the diagram below. Use the segmental method to find the CG for the entire arm, using the data provided for segment masses from Appendix D.

Stability and Balance What is stability? resistance to disruption of equilibrium: Stability is defined mechanically as resistance to both linear and angular acceleration, or resistance to disruption of equilibrium. What is balance? ability to control equilibrium: An individual’s ability to control equilibrium is known as balance.

Stability and Balance What is the base of support? (area bound by the outermost regions of contact between a body and support surface(s))

Stability and Balance What can increase a body’s stability? increasing body mass increasing friction between the body and the surfaces of contact

Stability and Balance What can increase a body’s stability? increasing the size of the base of support in the direction of an external force

Stability and Balance What can increase a body’s stability? horizontally positioning the center of gravity near the edge of the base of support on the side of the external force

Stability and Balance What can increase a body’s stability? vertically positioning the center of gravity as low as possible The higher the CG, the greater the amount of torque its motion creates about the support surface.

HUMAN STRENGTH AND POWER Biomechanical Factors in Human Strength Strength-to-Mass Ratio In sprinting and jumping, the ratio directly reflects an athlete’s ability to accelerate his or her body. In sports involving weight classification, the ratio helps determine when strength is highest relative to that of other athletes in the weight class. As body size increases, body mass increases more rapidly than does muscle strength. Given constant body proportions, the smaller athlete has a higher strength-to-mass ratio than does the larger athlete.

CAM-BASED WEIGHT-STACK MACHINES Figure 4.14 (next slide) In cam-based weight-stack machines, the moment arm (M) of the weight stack (horizontal distance from the chain to the cam pivot point) varies during the exercise movement. When the cam is rotated in the direction shown from position 1 to position 2, the moment arm of the weights, and thus the resistive torque, increases.

In cam-based weight- stack machines, the moment arm (M) of the weight stack (horizontal distance from the chain to the cam pivot point) varies during the exercise movement. –When the cam is rotated in the direction shown from position 1 to position 2, the moment arm of the weights, and thus the resistive torque, increases.

The force of muscular tension is resolved into two force components one perpendicular to the attached bone and one parallel to the bone. 1- The rotary component: The component of muscle force acting perpendicular to the bone that causes the bone to rotate about the joint center. 2- Dislocating component: The component of muscle force directed parallel to the bone that pulls the bone away from the joint center. 3- Stabilizing component: The component of muscle force directed parallel to the bone that pulls the bone toward the joint center. Nota: a- Depending on whether the angle between the bone and the attached muscle is less than or greater than 90 °. b- The angle of maximum mechanical advantage for any muscle is the angle at which the most rotary force can be produced.