## Examples from Weight Lifting

Weight lifting involves the rotation of a body segment against resistance. Because the exercise is done slowly (about one repetition per second), the inertial effects are neglected. We next present examples from the static analysis of weight lifting. Example 6.4. Deltoids. Deltoids are a shoulder muscle group that is located on the upper side of the arms. Deltoids originate at the bones of the shoulder (clavicle and scapula) and end at the outer midsection of Figure 6.5a,b. A man performing...

## Muscle Groups and Movement

There are layers of muscles in the muscular system. Muscles visible at the body surface are often called externus and superficialis, and they typically serve important functions to stabilize a joint or cause movement. With the naked eye it is often possible to identify the muscle group responsible for a certain action. Major muscle groups of the body are shown in Fig. 1.11. The axial musculature begins and ends on the axial skeleton. Belonging to the group of axial musculature are the muscles...

## O o

The free fall of a mass (mA) onto a mass-spring system with mass mB. where mA is the falling mass, mB is the mass attached to the spring, vo is the velocity of the center of mass of object A just before impact, and Vf is the velocity of the combination after the impact. Because the mass A falls from a distance h its velocity before the collision is given by the equation in which g denotes as usual the magnitude of the gravitational acceleration and e2 is the unit vector in the...

## Skeletal Tree

The human skeleton is divided into two parts the axial and the appendicular (Fig 1.4). The axial skeleton shapes the longitudinal axis of the human body. It is composed of 22 bones of the skull, 7 bones associated with the skull, 26 bones of the vertebral column, and 24 ribs and 1 sternum comprising the thoracic cage. It is acted on by approximately 420 different skeletal muscles. The axial skeleton transmits the weight of the head and the trunk and the upper limbs to the lower limbs at the hip...

## Physical Properties of Skeletal Muscle

Muscles are composed of bundles of long and thin cells that are called muscle fibers. Bundles of skeletal muscle fibers are encased by a dense fibrous connective tissue layer called the epimysium. Bundles are separated from each other by connective tissue fibers of the perimysium, and within each bundle the muscle fibers are surrounded by a delicate network of reticular fibers called the endomysium. Scattered satellite cells lie between the endomysium and the muscle fibers. These cells function...

## Moment of a Force

An important example of vector multiplication is the concept of moment of a force with respect to a point in space. The moment Mo is a measure of the capacity of force F acting on point P to cause rotation about point O. It is defined as follows where rp o is the position vector connecting point O to P (Fig. 3.6b). The magnitude of Mo is the product of the magnitude of the force and the perpendicular distance of the point O from the line of action of the force. Note that in the evaluation of...

## Applications to Human Body Dynamics Pole Vaulting

Pole vaulting is an exciting athletic event in which a vaulter clears a crossbar resting on two metal standards placed approximately 4 m apart (Fig. 8.10). On the ground in front of the crossbar is a small wedge-cut hole called the vaulting box that holds the end of the pole during the vault. Behind the standards is a landing pit that is at least 5 m wide. Back in 1877, the first championship was won with a vault of 2.92 m, but today vaulters reach the sky with much longer and flexible poles....

## Muscle Force in Motion

The first step in the analysis of forces acting on a body segment is to draw a free-body diagram of the segment. To this end, the part of the body is considered as distinct from the entire body, and all forces acting on the part of the body are identified. Then the equations of motion are used to gather information about the unknown muscle forces acting on the body part. The following example on the kicking of a soccer ball illustrates this technique. Example 6.1. Quadriceps Force Before...

## Joints of the Human Body

Human joints can be classified into three groups based on the range of motion permitted at the joint. An immovable joint is called synarthrosis in anatomy. These are the joints found between the bones of the skull and between teeth and the surrounding bone of the jaw. In the skull, the edges of the bones are interlocked and bound together by dense connective tissue. These joints are called sutures. The second group of joints, such as the distal articulation between tibia and fibula, allow for...

## Biarticular Muscles

Biarticular muscles act on two joints. These muscles include some of the major muscles of the upper and lower limbs. Hamstrings, a group of three muscles, constitute an important example for biarticular muscles. These muscles originate in the ischial tuberosity of the hipbone and insert into the bones of the lower leg. When the thigh and the hip are fixed, ham strings flex the knee. They also extend the hip through the movement of the thigh. The third function of the hamstrings is to raise the...

## Problems

The frequency of crack formation during impact of a cadaver head against a flat, rigid surface was measured in a number of studies. A series of free fall drop tests using embalmed cadaver heads showed that a free fall of greater than 50 cm frequently resulted in the fracture of the skull. Consider a similar experiment and drop grapefruits and watermelons from various heights and determine the frequency of fracture. Would a grapefruit be a good model for human head Does the size of...

## Center of Mass and Its Motion

The center of mass of a body B, living or nonliving, is defined by the following equation 2m1 is the total mass in B, rc is the position vector for the center of mass of B, mi is the mass of the ith element in B, and r' is its position vector. These entities are shown in Fig. 3.1. Note that the center of mass is also commonly known as the center of gravity. Let us now perceive center of mass as if it were a particle in space. In reality, the center of mass may not correspond to any point of the...

## Moment Arm and Joint Angle

Moment lever arm of a muscle acting on a joint changes with the angle between the two bones articulating in that joint. Consider, for example, the moment arms of the forearm flexors, biceps, and the brachioradialis muscles shown in Fig. 6.7a. The moment arm of biceps is nearly equal to zero when the angle between the upper arm and the forearm is 180 . The moment arm increases as the joint angle 6 decreases from 180 toward 90 . What is the relation between the moment arm and the joint angle This...

## Bone Cartilage and Ligaments

Bones are the parts of the human body that are most resistant to deformation. Unless they are broken or fractured, bones do not undergo significant shape changes during short periods. As such, they can be considered as rigid bodies in the analysis of movement and motion. In a rigid body neither the distance between any two points nor the angle between any three points changes during motion. The bone matrix is composed of collagen fibers and inorganic calcium salts decorating these fibers Fig....