2. Arthrokinematics
•Arthrokinematics describes the motion that occurs
between the articular surfaces of joints. The shapes of
the articular surfaces of joints range from flat to
curved. Most joint surfaces, however, are at least
slightly curved, with one surface being relatively
convex and one relatively concave.
6. ▪ If the MA = 1
It indicates that the effort arm = the resistance arm
Function : is to change the direction of motion & balance the lever.
▪ If the MA > 1
It indicates that the effort arm > the resistance arm
Function : is to magnify force.
▪ If the MA < 1
It indicates that the effort arm < the resistance arm
Function : is to magnify speed or velocity of motion & range of motion..
7. Pulley System
▪ Change the direction of the force.
▪ Balance forces as the first class of
lever
▪ Magnify forces via increasing the
mechanical advantage
8. How much force must be produced by the biceps
brachii at a perpendicular distance of 3 cm from
the axis of rotation at the elbow to support a
weight of 200 N at a perpendicular distance of 25
cm from the elbow? (Answer: 1667 N).
9. Two people push on opposite sides of a swinging door. If A
exerts a force of 40 N at a perpendicular distance of 20 cm
from the hinge and B exerts a force of 30 N at a
perpendicular distance of 25 cm from the hinge, what is the
resultant torque acting at the hinge, and which way will the
door swing? (Answer: T = 0.5 N-m; in the direction that A
pushes).
10. Question
• If a quadriceps muscle in a healthy, average-sized man, is with
physiologic cross-sectional area of 180 cm2.
Calculate the maximum amount of active force produced by
quadriceps muscle. Assume the maximum force produced per unit
physiologic cross sectional area is 30 N/cm2.
11. Question
• If a adductor pollicis muscle in a healthy, average-sized man, is with
physiologic cross-sectional area of 2.5 cm2.
Calculate the maximum amount of active force produced by
quadriceps muscle. Assume the maximum force produced per unit
physiologic cross sectional area is 30 N/cm2.
13. • Fx on the horizontal line x axis and Fy on the vertical line Y axis
• Fy = Fsinθ
• Fx = Fcosθ
14. • To show the effect of pennation angle on the muscle force,
resolution of the force line of the muscle fiber should be done.
• Resolution is dividing the force line into two components; vertical
component and horizontal component.
15. Effect of pennation angle on the muscle force:
Pennation Angle θ =300 Pennation Angle θ =600
Tension of central tendon = F cos θ
= 5 cos 30
= 5 x 0.8
= 4N
Contractile force = 4N
Tension of central tendon = F cos θ
= 5 cos 60
= 5 x 0.5
= 2.5N
Contractile force = 2.5N
Conclusion: Tension produced through central tendon is greater if the pennation angle is 300 compared
with angle 600. So when the pennation angle of the muscle decreases the muscle force increases. By
increasing the pennation angle, the muscle force decreases.
N.B. The largest angle of pennation in the body is 300.
16. 2. Draw and calculate the 2 component of the following vectors
F1 = 30N
30°
17. Answer
F1 = 30 N
30°
Fx
Fy
• ∵F1 = 30N
• ∵ θ=30°
∴ F y = F1sinθ= 30Sin30= 15 N
∴ F x = F1 cosθ= 30cos30= 26N
18. 3. Draw and calculate the 2 component of the following vectors
F2
20. 4. Draw and calculate the 2 component of the following vectors
60°
F3
21. Answer
60°
F3=30 N
• ∵F3 = 30N
• ∵ θ=30°
∴ F y = F3cosθ= 30cos60= 15 N
∴ F x = F3 sinθ= 30sin60= 25 N
Fy
Fx
22. 5. Draw and calculate the 2 component of the following vectors
F4
23. Answer
F4 = Fx =60N
Fy
• F x = F cosθ
• F y= F sinθ
∵θ= 0
∴ F x = F cos 0 = 60N
∴ F y = F sin0 = 0 N
24. 6.A car is pulled with a force of 60. N at angle of 37° from the horizontal.
• Find the vertical and horizontal components of the applied force
25. Answer
• F x = F cosθ
• F y= F sinθ
∵ F= 60N
∵θ= 37
∴ F x = 60 cos 37° = 48N
∴ F y = 60 Sin 37° = 36N
° 37
◦
F=60N
FX
FY
26. If the forces are acting along the same line but in the opposite
direction as the boys in figure B, the magnitude of the resultant
equals the subtraction of the two forces R=D-E and is directed
towards the bigger one.
27. Composition of forces
It is a method of finding a single force (resultant R) that shows the
combined effect of different forces.
The principles of finding the R is:
• Draw the first force vector.
• From the head of the first vector draw the second.
• From the head of second draw the third and so on.
• The resultant (R) is the arrow from the tail of the first vector to
the tip of the last vector.
• Direction of (R) 1s towards the bigger force.
28. • Another example for the linear forces, is when the forces act in
the opposite directions away from each other. There are forces
acting toward the right (L, M, and N) and another forces (F and G)
acting toward the left. We draw arrows representing each force
with the tip pointing toward the right in L, M, and N and the tip
pointing toward the left in G and F. The total length of L, M and N
represents the single resultant force to the right, and the length of
G and F represents the resultant force to the left. The resultant (R)
of all forces is drawn from the tail of the first vector to the tip of
the last vector and is directed toward the greater force. If the
forces in the right are equal to that in the left, this causes a state
of equilibrium and R=θ.
