Learning Objectives:

• Learn how to qualitatively describe human motion
  • Review the major segments, bones, and muscles of the human body
  • Review anatomical position
  • Review some key spatial and directional terminology that is helpful in qualitatively describing human motion including the (1) three anatomical planes of motion, (2) corresponding axes of rotation, and (3) joint actions that occur in each plane, at each of the major joints
• Understand and apply:
  • relative and absolute angular position
  • angular displacement
  • angular velocity
  • angular acceleration
  • the relationship between angular velocity, radius length, and linear speed of a rotating object
  • tangential and centripetal acceleration 

Qualitatively Describing Human Motion

1. Be familiar with the anatomy of the human musculoskeletal system: segments, bones, and muscles

Body segments you should be familiar with (for the context of this class) are:

Bones you should be familiar with.

Muscles and muscle functions you should be familiar with.

2. Know anatomical position

When qualitatively describing motion, the anatomical position is helpful, and is demonstrated in the above figure. The anatomical position is the standard reference position for the body when describing locations, positions, or movements of body segments.

 

3. Be familiar with the following spatial and directional terminology
 

Anterior/Posterior
Superior/Inferior
Medial/Lateral
Proximal/Distal

 

4. Recognize the cardinal and secondary planes of motion and each corresponding axis

Sagittal Plane--Frontal Axis

Frontal Plane--Sagittal Axis
Transverse Plane--Longitudinal Axis

5. Know the primary segmental actions

Sagittal Plane:
 

Flexion, extension, hyperextension, dorsiflexion, and plantar flexion

Frontal Plane:

Adduction, abduction, radial deviation, ulnar deviation, eversion, inversion, and lateral flexion

Tranverse Plane:

Rotation, horizontal abduction and adduction, and forearm supination and pronation

Other triplanar joint actions to know:

Circumduction

Pronation (subtalar joint): a combination of dorsiflexion, eversion, and abduction

Supination (subtalar joint): a combination of plantarflexion, inversion, and adduction

 

An exercise in describing human motion.

Angular Kinematics

What is angular kinematics? What is an angle (θ)? What is kinematics?

θ = arc length/radius

What are the typical units that are used to describe angles?

Common conversions: 180 ° = 3.14 radians, or 57.3 ° = 1 radian

What is the difference between absolute and relative angles when describing angular motion?

What were the absolute and relative angles discussed in the reading assignment?

What are angular displacement, angular velocity, and angular acceleration?

1. Δθ = θ₂ - θ₁

2. ω = (θ₂ - θ₁)/(Δt)

3. α = (ω₂ - ω₁)/(Δt)

As was the case for linear kinematics, the slope of the angle × time curve equals angular velocity, and the slope of angular velocity × time curve equals angular acceleration
 

Running Example

Relationship Between Linear (d) and Angular Distances (θ):

d = θ·r  

In-class Practice: If a 34 inch baseball bat rotates 180 degrees how far has the sweet spot traveled, assuming that the sweet spot is 31 inches from the axis of rotation? Answer: 97.39 inches

Homework Practice: How far will the head of a 9-iron travel during a complete golf swing? Assume that: (1) the club rotates about a point that is near the shoulders, (2) the length of the forearms and upper arms is 25 inches, (3) the length of the club is 36 inches, (4) the distance between the hand-club interface and proximal end of the club is 7 inches, and (4) the angular displacement that is accomplished is 540 degrees. Answer: ~508 inches

Relationship Between Linear (v) and Angular Velocity (ω):

v = ω·r

In-class Practice: If bat is swung through the aforementioned arc in 0.15 seconds, what was the linear velocity of the sweet spot? Answer: 649 inches/s; What was the angular velocity? 20.94 rad/s, or 1200 degrees/s

If you want to increase v for a rotating object such as a baseball bat, you can increase ω or r.    If you are playing golf, the likely choice is to increase r. The typical length of a 3-iron is 38.5 inches, and each club, from the 3-iron to the wedge, decreases in length by 1/2 inch. With this in mind, how might performance in other sports be increased by increasing r?

Homework Practice: How many revolutions per second will a bicycle tire with a 0.7-m diameter need to perform in order to maintain a linear velocity of 20 mph? 4.06 rev/s

Linear Accelerations Related To A Rotating Object

Tangential Acceleration: The component of of linear acceleration that is tangential to the circular path of a point on a rotating object; this component of linear acceleration influences the linear speed of the object. Tangential acceleration is related to angular acceleration in the following way:

a_T =  αr; or a_T = (v₂-v₁)/Δt

this equation indicates that as angular acceleration occurs, tangential acceleration also occurs. However, when no angular acceleration occurs, does linear acceleration still occur? Yes, but why?

Centripetal Acceleration (a_r): The component of linear acceleration that is directed toward the axis of rotation or the center of rotation; this is the component of angular acceleration that influences the direction of the object

a_r =ω² r, or

a_r = v²/r

Homework Practice: A windmill style softball pitcher executes a pitch in 0.65 s. If her pitching arm is 0.7 m long, what are the magnitudes of the tangential and radial accelerations on the ball just prior to ball release, when the linear velocity of the ball is 20 m/s? Answers: 30.77 m/s/s, and 571.43 m/s/s, respectively; What is the resultant linear acceleration for the ball at this point? Answer: 572.26 m/s/s