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The humerus bone in the upper arm and the radius bone in the lower arm can be modeled as a lever with the elbow joint as the pivot point. Movement about the elbow results from a combination of internal and external forces acting on these bones. The primary internal force is generated from contractions of skeletal muscles; a 1-cm² cross-sectional area of skeletal muscle can generate a maximum of 7 × 10⁶ dynes of force (1 dyn = 1 g⋅cm/s²) . Friction between the humerus and the radius at the elbow is another internal force that can affect arm movements. As people age, cartilage degeneration decreases the lubrication between bones, which increases the coefficients of static and kinetic friction.The contraction of the biceps, a skeletal muscle in the upper arm, exerts a pulling force on the radius in the lower arm. If the torque generated by this muscle exceeds that of any opposing internal and external forces, the arm curls about the elbow. This specific type of movement is known as a concentric muscle contraction.The following experiments were performed with a 30-year-old male subject. Electrodes were placed around the subject's biceps and attached to an electromyograph, a device that measures the electrical activity of skeletal muscle contractions. The estimated weight of the subject's lower arm is 45 N, and the subject's lower arm segments to the elbow are shown in Table 1.Table 1 Lower Arm Segment Lengths
Experiment 1The subject kept his entire arm stationary and in a fixed position while holding balls of varying masses. The upper arm was perpendicular to the ground, and the lower arm was parallel to the ground (Figure 1) .
Figure 1 The fixed position of the arm during Experiment 1Experiment 2The subject kept his upper arm stationary and perpendicular to the ground while lifting and lowering balls of varying masses.
Adapted from N. Ozkaya, Fundamentals of Biomechanics. (C) 2016 Springer.
-Assuming the weight of the arm remains the same, how would the friction at the elbow change if the subject were to repeat the experiments 20 years later?

Question

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The humerus bone in the upper arm and the radius bone in the lower arm can be modeled as a lever with the elbow joint as the pivot point. Movement about the elbow results from a combination of internal and external forces acting on these bones. The primary internal force is generated from contractions of skeletal muscles; a 1-cm² cross-sectional area of skeletal muscle can generate a maximum of 7 × 10⁶ dynes of force (1 dyn = 1 g⋅cm/s²) . Friction between the humerus and the radius at the elbow is another internal force that can affect arm movements. As people age, cartilage degeneration decreases the lubrication between bones, which increases the coefficients of static and kinetic friction.The contraction of the biceps, a skeletal muscle in the upper arm, exerts a pulling force on the radius in the lower arm. If the torque generated by this muscle exceeds that of any opposing internal and external forces, the arm curls about the elbow. This specific type of movement is known as a concentric muscle contraction.The following experiments were performed with a 30-year-old male subject. Electrodes were placed around the subject's biceps and attached to an electromyograph, a device that measures the electrical activity of skeletal muscle contractions. The estimated weight of the subject's lower arm is 45 N, and the subject's lower arm segments to the elbow are shown in Table 1.Table 1 Lower Arm Segment Lengths

Experiment 1The subject kept his entire arm stationary and in a fixed position while holding balls of varying masses. The upper arm was perpendicular to the ground, and the lower arm was parallel to the ground (Figure 1) .

Figure 1 The fixed position of the arm during Experiment 1Experiment 2The subject kept his upper arm stationary and perpendicular to the ground while lifting and lowering balls of varying masses.
Adapted from N. Ozkaya, Fundamentals of Biomechanics. (C) 2016 Springer.
-Assuming the weight of the arm remains the same, how would the friction at the elbow change if the subject were to repeat the experiments 20 years later?

Quizplus · Accepted Answer

11ecba2d_12d8_b7dd_a3bf_ddc8a5cc135b_MD0008_00 Friction is the force that resists sliding between two surfaces, and it can be either static or kinetic. Static friction is involved when no sliding occurs, and kinetic friction is involved when sliding occurs. Both static friction F_s and kinetic friction F_k depend on their coefficients of friction μ_s and μ_k: F_s/k = μ_s/kN where N is the normal force, the perpendicular force one surface exerts on another. The passage states that cartilage degeneration increases with age, resulting in an increase in both the static and kinetic friction coefficients. Therefore, the older subject will experience increased static and kinetic friction at the elbow. Because the arm is stationary during Experiment 1, the static frictional force would increase. (Choice B) Although kinetic friction is expected to increase due to cartilage degeneration, there is no movement in Experiment 1; therefore, there will be no kinetic friction. (Choices C and D) Both static friction and kinetic friction are expected to increase because the coefficients of each will increase. In addition, there is no static friction at the elbow when the arm is moving. Educational objective: Static and kinetic friction are forces that resist sliding between two surfaces. Static friction occurs only when the two surfaces do not slide, and kinetic friction occurs only when the two surfaces slide relative to each other. The magnitude of each depends on their respective coefficients of friction; therefore, an increase in the coefficients will increase the frictional force.

Passage The Humerus Bone in the Upper Arm and the Radius