Biomechanics Problems

1. Assume that the upper ankle joint is being maintained in a neutral position. The tibialis anterior is known to exert a 75 Newton force at its distal attachment on the dorsomedial aspect of the first cuneiform. The angle of tibialis anterior is 14 degrees to the long axis of the foot.

Question: What is the magnitude of the compressive force exerted at the navicular-first cuneiform joint?

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2. The interphalangeal (IP) joint of hallux is viewed from the medial side and the flexor hallucis longus (FHL) is acting at nine degrees to the long axis of the great toe. FHL is known to be pulling across the IP joint with a force of 28 pounds.

Question: Which force exerted by FHL acting across the IP joint is greater: the compressive or rotary force?

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3. Imagine the hip is being viewed from the side while the subject is prone lying and the femur is parallel with the horizon. The angle of insertion of gluteus maximus is 42 degrees and the gluteal tuberosity is 8 inches from the hip axis for flexion/extension. The weight of the entire lower extremity is 33.5 lbs. and its center of gravity is located 18 inches from the hip axis.

Question: If gluteus maximus exerts a force of 35.3 lbs during an isometric hip extension, what will be the magnitude of the compressive force exerted on the hip joint?

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4. A patient with osteoarthritis of the patellofemoral joint is to have a patelloplasty. A polyethylene prosthesis will be cemented to the articular surface of the patella. The anterior-to-posterior dimension of the patellar prosthesis is 1.1 cm. With the knee joint maintained in extension and the leg parallel with the horizon, the forcearm distance of quadriceps has been measured and is 5.3 cm. The tibial tubercle is measured on a lateral X-ray of the knee and is 7.6 cm from the knee axis for flexion and extension. The center-of-gravity of the leg, ankle and foot (LAF), using Dempster's method is calculated to be 14.8 cm from the knee axis for flexion and extension. The LAF weighs 2.4 kg.

Question 4-a: Calculate the mechanical advantage of quadriceps before and after surgery with the knee maintained in the position described above.
Question 4-b: Stated as a percentage, what improvement in mechanical advantage would you expect?
Question 4-c: What is the angle of insertion before and after surgery?

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