Physics and the Human Body

It's interesting to think that our bodies can be described using mechanics and mathematics.However despite how odd it may seem,to compare human physiology to mechanical movements, there surely is an explanation for just about everything with regard to human movement and functionality. Topics include the mechanics of the static body and the body in motion, the materials properties of the body, muscles in the body, the energetics of body metabolism, fluid flow in the cardiovascular and respiratory systems, the acoustics of sound waves in speaking and hearing, vision and the optics of the eye, the electrical properties of the body, and the basic engineering principles of feedback and control in regulating all aspects of function. All such topics aid us in the understanding of how all the things we take for granted in the human body truly work. Biomechanics and Human Movement (Biomechanics being the science concerned with the internal and external forces exerted on the human body and the effects produced from these forces), gives explanation for how simple movements and flexion work through a mathematical understanding.

Two Branches of Biomechanics

Biomechanics is separated into two different categories, or "branches," of study. There is Kinematics, which is the study of human movement from a geometrical standpoint. And there is also Kinetics, the branch of biomechanics concerned with what causes the body to move the way it does. The two branches work had in hand to describe and explain human movements such as Joint Kinematics, Knee Flexion, Ankle Dorsal Flexion, Joint Kinetics, Joint Angles, and the Instantaneous 3D bone pose. There are several Estimable Quantities (referring to simplified equations, using estimated values of its dependent variables), used in discussing the Biomechanics of the Human Body. Such quantities include the Instantaneous 3D bone pose as stared above, which is a virtual representation of the skeletal system in motion, Relative Movement between adjacent bones, dealing with joint kinetics, Forces transmitted by muscles-tendons-ligaments-bones, which also is a aspect of joint kinetics, and Muscular mechanical work/power, which dealing with systems energy, falling under the category of joint energetics. In sum our bodies movements can be described using Kinematics and Kinetics, the prevalent tools in explanation and understanding of how we work.

Joint Kinematics

Anatomical planes and axis are used to describe the human body in Joint Kinematics. The planes include, the Sagittal Plane, a plane parallel to the sagittal suture. The Sagittal plane divides the body into left and right. The transverse plane (also called the horizontal plane, axial plane, or trans axial plane) is an imaginary plane that divides the body into superior and inferior parts. And the Frontal (coronal) plane, which is any vertical plane that divides the body into ventral and dorsal (belly and back) sections.

In the study of Joint Kinematics,body segments are linked to each other at the joints. The joint structure determines the types of joint motions allowed at the joint. Joint motion is actually the relative motion of the distal segment to the proximal that together form a joint. Analytically speaking, it is advantageous to view joint motions as the relative motions between the rigid bodies. Body segments are considered to be ridged bodies, for the purpose of describing the motion of the body, such segments include, the the foot, shank (leg), thigh, pelvis, thorax, hand, forearm, upper-arm and head. Joints between adjacent segments include the ankle (talocrural plus subtalar joints), knee, hip, wrist, elbow and shoulder. Position is used to describe the location of the body segment in conjuncture to the axial planes stated above. A body segment or joint in space is measured in meters, and a related measure called "displacement" refers to the position with respect to starting position. In two dimensions, the position is given in Cartesian coordinates (system is a coordinate system that specifies each point uniquely in a plane by a pair of numerical coordinates, which are the signed distances to the point from two fixed perpendicular directed lines, measured in the same unit of length,) with horizontal variables followed by the vertical position. Heres a quick example of the use of Joint Kinematics and labeling movement. Kinematic measurements are limited in what they can tell us about the causes of motion is - for this we need to look at the kinetics. However, they do provide a description of the motion which can be valuable for certain purposes.

Kinematics of Walking and Running

One important observation we can make from looking at the kinematics of the body is the amount of up-down and sideways motion. In actions such as walking and running, the body is attempting to move horizontally across the ground. However any other motion, especially vertical motion, does not help this objective, and uses upward precious energy.If the body had wheels it could avoid vertical motion altogether, but since we have legs, there must be some vertical motion. The reason for this is that at heel-strike and toe-off the two legs make up the sides of a triangle, while during mid-stance the stance leg is vertical. This has the effect of lowering the upper-body (often called the HAT segment for Head-Arms-Trunk) at heel-strike and toe-off (which together make up the phase known as double stance, when both feet are in contact with the ground), and raising it during mid-stance:

The body's center of mass (CoM), is located in the pelvis. In order for the CoM to rise up between heel-strike and mid-stance, energy must be expanded, the energy is not conserved when it drops at the toe but rather expended. This is an up and down motion of the CoM.

Knee Flexion

When talking about Flexion we are talking about the action of bending or the condition of being bent especially the bending of a limb or joint, in this case the knee. The knee is the largest joint in the body and one of the joints most frequently injured. Most daily activities as well as sports activities require full functional movement of this joint. The knee consists of six articulating surfaces (any surface of a skeletal formation (bone, cartilage) that makes normal direct contact with another skeletal structure as part of a synovial joint,) including, the two condyles (the round prominence at the end of a bone) of the femur, the two condyles of the tibia, the posterior surface of the patella, and a patellar surface on the anterior surface of the femur. The knee joint is essentially a hinge joint capable of flexion and extension, however the knee is not a simple hinge. During flexion and extension the femur moves forwards and backwards respectively over the tibial plateau and during the late stages of extension there is some medial rotation at the joint. (Conversely during the early stages of flexion there is lateral rotation. The joint where the femur and tibia meet must be able to flex, extend and rotate while withstanding large amounts of force exerted on it and relative sliding between components during vigorous activity.

Ankle Dorsal Flexion

Ankles are stability joints that must very quickly absorb force, then help shift and stabilize weight for the next movement. The ankle joint is a synovial hinge (class of "hinge" in the body including ankle, elbow and knee) composed of the talus, fibula, and tibia. The ankle absorbs 85% of the weight pressing down on the foot during standing. This joint allows for dorsiflexion and plantar flexion. Dorsiflexion, is flexion of the foot in an upward direction in the case of the ankle in the positive direction. Where as plantar flexion is the movement of the foot that flexes the foot or toes downward toward the sole, in this case negative.

Joint Kinetics

Kinetics can be defined as being the scientific study of the turnover, or rate of change, of a specific factor in the body, commonly expressed as units of amount per unit time. The Human body is a system of rigid segments inter-linked at the joints. One of the most important aspects of this type of system is the interaction between the inter-linked segments at the joints. The joints (ligaments, joint capsule, etc.) themselves are passive structures that only apply constraining forces, when they are stretched, on to the bony structures involved in the joints. The active components of the system are the skeletal muscles that produces forces through voluntary contraction. Through a process called inverse dynamics one can asses the interaction between the segments through the joint & muscle and the work done by the muscles. Inverse dynamics literally means that one tries to figure out the cause of motion based on the effects (motion) and the inertial properties of the object in motion. One can quantify the human (or animal) motion through motion analysis. The inertial properties of the segments can be obtained from direct or indirect methods. The human body is most accurately described as being the "linked segment system," in which the natural assumption is that the joints are all pin joints and there is no friction at the joint. Thus, the forces produced by the elements of the joint such as ligaments and joint capsules are all concentric about the joint centers. In other words, all these forces pass through the joint centers. That muscles are the only elements that can produce eccentric forces about the joint centers. And that all muscles are uni-articular and there is no intervening structure that can act as pulley. In other words, muscles are straight and directly attach to the segments. Such a procedure will be later describe through the use of Joint Energetics.