ABC - Prosthetic Assistant Exam Review Guide
Questions and Answers (Solved Papers)
1. A patient with a unilateral transtibial amputation presents with excessive knee flexion during
the stance phase of gait on the prosthetic side. The prosthetic foot is a dynamic-response foot with a
heel height appropriate for the patient's shoes. Which of the following is the most likely cause of
this gait deviation?
A. Excessive plantarflexion of the prosthetic foot at initial contact
B. Insufficient heel lever arm length
C. Prosthetic socket set too anterior relative to the foot
D. Excessive toe lever arm length
Answer: C
Rationale: An anteriorly set socket shifts the ground reaction force anterior to the knee, creating an
external knee flexor moment. This causes the knee to flex during stance. Options A and D affect knee
extension moment; option B affects stability but not necessarily excessive flexion.
2. In a patient with a transradial amputation who uses a myoelectric prosthesis, which of the
following electrode placement strategies is most appropriate to achieve independent control of
wrist pronation/supination and hand open/close when using two-site control?
A. Place electrodes over the flexor carpi ulnaris and extensor carpi radialis longus
B. Place electrodes over the pronator teres and supinator
C. Place electrodes over the flexor digitorum superficialis and extensor digitorum communis
D. Place electrodes over the palmaris longus and brachioradialis
Answer: C
Rationale: For myoelectric control, the most reliable signals for hand open/close come from the finger
flexors and extensors (flexor digitorum superficialis and extensor digitorum communis). Option A
targets wrist movers, not hand function. Option B targets forearm rotators, which would control wrist
rotation, not hand. Option D includes brachioradialis, which is an elbow flexor.
3. A patient with a unilateral transfemoral amputation fitted with a microprocessor-controlled
knee (MPK) reports occasional falls during stair descent. Gait analysis reveals that the patient
tends to lead with the prosthetic limb on stairs. Which of the following adjustments is most likely
to improve safety?
A. Increase swing phase flexion resistance
B. Decrease stance phase flexion resistance
C. Enable stair mode that increases knee resistance during weight acceptance
D. Increase toe lever arm length of the prosthetic foot
Answer: C
Page 1
,Rationale: MPKs often have a stair descent mode that provides increased stance phase resistance to prevent knee buckling
when weight is applied on a flexed knee. Option A addresses swing phase, not stance. Option B would reduce stability. Option
D changes foot function but does not directly address knee control during stair descent.
4. Which of the following best describes the biomechanical advantage of using a hydraulic ankle
unit in a transtibial prosthesis compared to a rigid ankle-foot system?
A. It increases energy return during push-off by storing elastic energy
B. It provides controlled dorsiflexion during stance, reducing peak knee extensor moment
C. It allows for variable cadence walking by adapting plantarflexion resistance
D. It eliminates the need for a heel wedge in patients with knee osteoarthritis
Answer: B
Rationale: Hydraulic ankle units provide controlled dorsiflexion during mid to late stance, which reduces
the knee extensor moment and compensates for loss of ankle motion. Option A describes
dynamic-response feet. Option C is partially true but not the primary biomechanical advantage. Option
D is a potential clinical benefit but not the core biomechanical advantage.
5. A patient with bilateral transtibial amputations is being considered for a
microprocessor-controlled ankle-foot prosthesis. Which of the following patient characteristics
would be a contraindication for this device?
A. Body mass index of 32 kg/m²
B. Active participation in recreational walking 3 times per week
C. Inability to independently manage the charging and programming of the device
D. Presence of mild peripheral neuropathy in the residual limbs
Answer: C
Rationale: Microprocessor-controlled ankles require the user to charge the device and understand basic
programming. Inability to manage these tasks could lead to device failure or unsafe use. Options A and
D are not contraindications; many users with higher BMI or neuropathy benefit. Option B indicates
appropriate activity level.
6. During the fabrication of a transtibial total surface bearing (TSB) socket, a prosthetic assistant
notices that the rectified positive mold has a prominent bulge over the fibular head. What is the
most likely consequence of this fabrication error?
A. Increased comfort due to relief over the fibular head
B. Excessive pressure on the fibular head, leading to pain and potential skin breakdown
C. Reduced socket suspension due to air gap in the fibular head area
D. Improved rotational control of the socket
Answer: B
Rationale: In TSB sockets, the positive mold is rectified to add relief over bony prominences. A bulge
indicates insufficient relief, causing increased pressure on the fibular head. Option A is opposite; option
C would occur if there was too much relief; option D is unrelated.
Page 2
,7. A patient with a unilateral transradial amputation uses a body-powered prosthesis with a
voluntary-opening terminal device. The patient complains of difficulty maintaining grip on heavy
objects for extended periods. Which of the following modifications would best address this issue?
A. Switch to a voluntary-closing terminal device
B. Increase the cable housing length
C. Reduce the harness tension
D. Add a wrist flexion unit
Answer: A
Rationale: Voluntary-opening devices require constant cable tension to maintain grip, leading to fatigue.
Voluntary-closing devices maintain grip without continuous effort, as they use elastic bands or springs to
close, and the user opens by pulling. Option B and C would reduce control; option D addresses wrist
position, not grip maintenance.
8. An individual with a unilateral transradial amputation is being fit with a myoelectric prosthesis.
The occupational therapist notes that the patient has poor signal discrimination from the flexor
and extensor muscle groups. Which of the following interventions is most likely to improve
prosthetic control?
A. Increase the gain on the electrodes
B. Use targeted muscle reinnervation surgery
C. Provide biofeedback training to isolate muscle signals
D. Switch to a pattern recognition control system
Answer: C
Rationale: Biofeedback training helps the patient learn to produce distinct EMG signals from agonist and
antagonist muscles. Option A may amplify noise. Option B is invasive and not first-line. Option D is an
alternative if training fails, but training is the first step.
9. A patient with a unilateral transfemoral amputation using a mechanical knee (constant friction)
reports that the prosthesis feels unstable during the swing phase when walking at variable speeds.
Which of the following knee mechanisms would best address this issue?
A. Hydraulic swing phase control
B. Polycentric knee with stance control
C. Weight-activated friction knee
D. Manual locking knee
Answer: A
Rationale: Hydraulic swing phase control automatically adjusts knee flexion and extension resistance
based on walking speed, providing stability at variable cadences. Option B addresses stance stability,
not swing. Option C provides resistance proportional to weight, not speed. Option D locks the knee,
eliminating swing phase instability but limiting function.
10. A patient with a partial foot amputation (Lisfranc level) is being considered for a toe filler
prosthesis. Which of the following is the primary biomechanical goal of this prosthesis?
A. Restore the arch support and prevent midfoot collapse
Page 3
, B. Provide a rigid lever arm for push-off during late stance
C. Prevent anterior translation of the foot within the shoe
D. Distribute plantar pressure and improve balance during stance
Answer: D
Rationale: Toe fillers primarily fill the void in the shoe, redistribute pressure, and provide a wider base of
support for balance. Option A is more relevant for orthotics. Option B is not achieved because the toe
filler does not provide active push-off. Option C is not the primary goal.
11. A transradial prosthesis user reports difficulty controlling terminal device grip force when
lifting a heavy object. The myoelectric control system uses pattern recognition with two EMG
channels. Which modification to the control algorithm would MOST likely improve grip force
modulation without sacrificing response time?
A. Increase the gain on both EMG channels to amplify weak signals
B. Implement a time-domain feature set that includes mean absolute value and zero crossings
C. Add a third EMG channel from the flexor carpi ulnaris to distinguish co-contraction
D. Reduce the window length for feature extraction from 250 ms to 100 ms
Answer: B
Rationale: Time-domain features like mean absolute value and zero crossings provide better force
discrimination than amplitude alone, improving modulation while maintaining speed. Increasing gain
(A) amplifies noise without improving selectivity. Adding a channel (C) increases processing delay.
Reducing window length (D) degrades feature stability and may increase classification errors.
12. During dynamic alignment of a transtibial prosthesis with a microprocessor-controlled ankle,
the patient exhibits excessive knee flexion during mid-stance. Gait analysis reveals that the center
of pressure trajectory is shifted posteriorly. Which alignment adjustment should be performed
FIRST?
A. Translate the foot anteriorly relative to the socket
B. Increase plantarflexion of the prosthetic foot
C. Move the socket in the sagittal plane relative to the foot
D. Reduce the dorsiflexion assist of the microprocessor ankle
Answer: A
Rationale: An anterior translation of the foot shifts the ground reaction force vector anteriorly, extending
the knee. Increasing plantarflexion (B) would further flex the knee. Socket translation (C) is vague;
specific foot translation is standard. Reducing dorsiflexion assist (D) may increase knee flexion
instability.
13. A patient with bilateral transfemoral amputation uses a stubble prosthesis for limited
community ambulation. During stance phase, the prosthetic knee buckles unexpectedly. The socket
fit is adequate. Which of the following is the MOST likely cause?
A. Insufficient hip extensor strength
B. Excessive heel height of the shoe
C. Improper alignment with excessive knee flexion moment
D. Faulty knee friction brake mechanism
Page 4
Questions and Answers (Solved Papers)
1. A patient with a unilateral transtibial amputation presents with excessive knee flexion during
the stance phase of gait on the prosthetic side. The prosthetic foot is a dynamic-response foot with a
heel height appropriate for the patient's shoes. Which of the following is the most likely cause of
this gait deviation?
A. Excessive plantarflexion of the prosthetic foot at initial contact
B. Insufficient heel lever arm length
C. Prosthetic socket set too anterior relative to the foot
D. Excessive toe lever arm length
Answer: C
Rationale: An anteriorly set socket shifts the ground reaction force anterior to the knee, creating an
external knee flexor moment. This causes the knee to flex during stance. Options A and D affect knee
extension moment; option B affects stability but not necessarily excessive flexion.
2. In a patient with a transradial amputation who uses a myoelectric prosthesis, which of the
following electrode placement strategies is most appropriate to achieve independent control of
wrist pronation/supination and hand open/close when using two-site control?
A. Place electrodes over the flexor carpi ulnaris and extensor carpi radialis longus
B. Place electrodes over the pronator teres and supinator
C. Place electrodes over the flexor digitorum superficialis and extensor digitorum communis
D. Place electrodes over the palmaris longus and brachioradialis
Answer: C
Rationale: For myoelectric control, the most reliable signals for hand open/close come from the finger
flexors and extensors (flexor digitorum superficialis and extensor digitorum communis). Option A
targets wrist movers, not hand function. Option B targets forearm rotators, which would control wrist
rotation, not hand. Option D includes brachioradialis, which is an elbow flexor.
3. A patient with a unilateral transfemoral amputation fitted with a microprocessor-controlled
knee (MPK) reports occasional falls during stair descent. Gait analysis reveals that the patient
tends to lead with the prosthetic limb on stairs. Which of the following adjustments is most likely
to improve safety?
A. Increase swing phase flexion resistance
B. Decrease stance phase flexion resistance
C. Enable stair mode that increases knee resistance during weight acceptance
D. Increase toe lever arm length of the prosthetic foot
Answer: C
Page 1
,Rationale: MPKs often have a stair descent mode that provides increased stance phase resistance to prevent knee buckling
when weight is applied on a flexed knee. Option A addresses swing phase, not stance. Option B would reduce stability. Option
D changes foot function but does not directly address knee control during stair descent.
4. Which of the following best describes the biomechanical advantage of using a hydraulic ankle
unit in a transtibial prosthesis compared to a rigid ankle-foot system?
A. It increases energy return during push-off by storing elastic energy
B. It provides controlled dorsiflexion during stance, reducing peak knee extensor moment
C. It allows for variable cadence walking by adapting plantarflexion resistance
D. It eliminates the need for a heel wedge in patients with knee osteoarthritis
Answer: B
Rationale: Hydraulic ankle units provide controlled dorsiflexion during mid to late stance, which reduces
the knee extensor moment and compensates for loss of ankle motion. Option A describes
dynamic-response feet. Option C is partially true but not the primary biomechanical advantage. Option
D is a potential clinical benefit but not the core biomechanical advantage.
5. A patient with bilateral transtibial amputations is being considered for a
microprocessor-controlled ankle-foot prosthesis. Which of the following patient characteristics
would be a contraindication for this device?
A. Body mass index of 32 kg/m²
B. Active participation in recreational walking 3 times per week
C. Inability to independently manage the charging and programming of the device
D. Presence of mild peripheral neuropathy in the residual limbs
Answer: C
Rationale: Microprocessor-controlled ankles require the user to charge the device and understand basic
programming. Inability to manage these tasks could lead to device failure or unsafe use. Options A and
D are not contraindications; many users with higher BMI or neuropathy benefit. Option B indicates
appropriate activity level.
6. During the fabrication of a transtibial total surface bearing (TSB) socket, a prosthetic assistant
notices that the rectified positive mold has a prominent bulge over the fibular head. What is the
most likely consequence of this fabrication error?
A. Increased comfort due to relief over the fibular head
B. Excessive pressure on the fibular head, leading to pain and potential skin breakdown
C. Reduced socket suspension due to air gap in the fibular head area
D. Improved rotational control of the socket
Answer: B
Rationale: In TSB sockets, the positive mold is rectified to add relief over bony prominences. A bulge
indicates insufficient relief, causing increased pressure on the fibular head. Option A is opposite; option
C would occur if there was too much relief; option D is unrelated.
Page 2
,7. A patient with a unilateral transradial amputation uses a body-powered prosthesis with a
voluntary-opening terminal device. The patient complains of difficulty maintaining grip on heavy
objects for extended periods. Which of the following modifications would best address this issue?
A. Switch to a voluntary-closing terminal device
B. Increase the cable housing length
C. Reduce the harness tension
D. Add a wrist flexion unit
Answer: A
Rationale: Voluntary-opening devices require constant cable tension to maintain grip, leading to fatigue.
Voluntary-closing devices maintain grip without continuous effort, as they use elastic bands or springs to
close, and the user opens by pulling. Option B and C would reduce control; option D addresses wrist
position, not grip maintenance.
8. An individual with a unilateral transradial amputation is being fit with a myoelectric prosthesis.
The occupational therapist notes that the patient has poor signal discrimination from the flexor
and extensor muscle groups. Which of the following interventions is most likely to improve
prosthetic control?
A. Increase the gain on the electrodes
B. Use targeted muscle reinnervation surgery
C. Provide biofeedback training to isolate muscle signals
D. Switch to a pattern recognition control system
Answer: C
Rationale: Biofeedback training helps the patient learn to produce distinct EMG signals from agonist and
antagonist muscles. Option A may amplify noise. Option B is invasive and not first-line. Option D is an
alternative if training fails, but training is the first step.
9. A patient with a unilateral transfemoral amputation using a mechanical knee (constant friction)
reports that the prosthesis feels unstable during the swing phase when walking at variable speeds.
Which of the following knee mechanisms would best address this issue?
A. Hydraulic swing phase control
B. Polycentric knee with stance control
C. Weight-activated friction knee
D. Manual locking knee
Answer: A
Rationale: Hydraulic swing phase control automatically adjusts knee flexion and extension resistance
based on walking speed, providing stability at variable cadences. Option B addresses stance stability,
not swing. Option C provides resistance proportional to weight, not speed. Option D locks the knee,
eliminating swing phase instability but limiting function.
10. A patient with a partial foot amputation (Lisfranc level) is being considered for a toe filler
prosthesis. Which of the following is the primary biomechanical goal of this prosthesis?
A. Restore the arch support and prevent midfoot collapse
Page 3
, B. Provide a rigid lever arm for push-off during late stance
C. Prevent anterior translation of the foot within the shoe
D. Distribute plantar pressure and improve balance during stance
Answer: D
Rationale: Toe fillers primarily fill the void in the shoe, redistribute pressure, and provide a wider base of
support for balance. Option A is more relevant for orthotics. Option B is not achieved because the toe
filler does not provide active push-off. Option C is not the primary goal.
11. A transradial prosthesis user reports difficulty controlling terminal device grip force when
lifting a heavy object. The myoelectric control system uses pattern recognition with two EMG
channels. Which modification to the control algorithm would MOST likely improve grip force
modulation without sacrificing response time?
A. Increase the gain on both EMG channels to amplify weak signals
B. Implement a time-domain feature set that includes mean absolute value and zero crossings
C. Add a third EMG channel from the flexor carpi ulnaris to distinguish co-contraction
D. Reduce the window length for feature extraction from 250 ms to 100 ms
Answer: B
Rationale: Time-domain features like mean absolute value and zero crossings provide better force
discrimination than amplitude alone, improving modulation while maintaining speed. Increasing gain
(A) amplifies noise without improving selectivity. Adding a channel (C) increases processing delay.
Reducing window length (D) degrades feature stability and may increase classification errors.
12. During dynamic alignment of a transtibial prosthesis with a microprocessor-controlled ankle,
the patient exhibits excessive knee flexion during mid-stance. Gait analysis reveals that the center
of pressure trajectory is shifted posteriorly. Which alignment adjustment should be performed
FIRST?
A. Translate the foot anteriorly relative to the socket
B. Increase plantarflexion of the prosthetic foot
C. Move the socket in the sagittal plane relative to the foot
D. Reduce the dorsiflexion assist of the microprocessor ankle
Answer: A
Rationale: An anterior translation of the foot shifts the ground reaction force vector anteriorly, extending
the knee. Increasing plantarflexion (B) would further flex the knee. Socket translation (C) is vague;
specific foot translation is standard. Reducing dorsiflexion assist (D) may increase knee flexion
instability.
13. A patient with bilateral transfemoral amputation uses a stubble prosthesis for limited
community ambulation. During stance phase, the prosthetic knee buckles unexpectedly. The socket
fit is adequate. Which of the following is the MOST likely cause?
A. Insufficient hip extensor strength
B. Excessive heel height of the shoe
C. Improper alignment with excessive knee flexion moment
D. Faulty knee friction brake mechanism
Page 4
