Kinesiology and Biomechanics Flashcards

During closed-chain knee extension, which of the following accurately describes the arthrokinematic behavior of the tibiofemoral joint?

Answer: The femoral condyles roll anteriorly and slide posteriorly on the fixed tibia

In closed-chain knee extension (e.g., standing up from a chair), the tibia is fixed and the femur moves. The femoral condyles roll anteriorly (in the direction of femoral motion) while simultaneously sliding posteriorly (opposite to roll) to maintain joint congruency. This concave-on-convex rule governs all synovial joint arthrokinematics.

A massage therapist notices a client has a unilateral anterior pelvic tilt. Which combination of muscle imbalances most precisely explains this pattern according to Janda's lower crossed syndrome?

Answer: Shortened hip flexors and erector spinae; inhibited gluteus maximus and rectus abdominis

Janda's lower crossed syndrome describes a predictable pattern where the hip flexors (iliopsoas, rectus femoris) and lumbar erector spinae are overactive/shortened, while the gluteus maximus and deep abdominals (not rectus abdominis) are inhibited and lengthened. This imbalance produces anterior pelvic tilt and increased lumbar lordosis.

The screw-home mechanism of the knee joint involves terminal external tibial rotation during open-chain extension. Which structure is primarily responsible for initiating UNLOCKING of this mechanism at the start of flexion?

Answer: Popliteus muscle

The popliteus muscle is classically described as the 'key' that unlocks the screw-home mechanism. At the initiation of knee flexion from full extension, the popliteus internally rotates the tibia (or externally rotates the femur in closed-chain) to disengage the locked position, allowing flexion to proceed. The ACL and LCL provide passive stability but do not actively unlock the joint.

A client presents with weakness during resisted shoulder abduction between 60° and 120° with pain localized to the acromion region. Applying the biomechanical concept of the painful arc syndrome, which rotator cuff structure is MOST likely being compressed during this range?

Answer: Supraspinatus tendon and subacromial bursa

The painful arc (60°–120° of abduction) corresponds to the supraspinatus tendon and subacromial bursa passing beneath the coracoacromial arch. Below 60° and above 120°, these structures are not impinged. The infraspinatus, subscapularis, and teres minor occupy different anatomical positions that do not produce this specific arc pattern under the acromion.

When analyzing gait biomechanics, excessive contralateral pelvic drop (Trendelenburg sign) during single-leg stance on the right limb indicates weakness in which muscle group on which side?

Answer: Right hip abductors (gluteus medius and minimus)

During right single-leg stance, the right hip abductors must contract eccentrically to prevent the unsupported (left) pelvis from dropping. If the right gluteus medius and minimus are weak, they cannot stabilize the pelvis against the body weight, causing contralateral (left) pelvic drop. The weakness is ipsilateral to the stance leg, not contralateral.

In biomechanical terms, a massage therapist applies a sustained compressive force perpendicular to a muscle belly. Which viscoelastic property best describes the progressive tissue deformation that occurs over time under this constant load, eventually reaching a new equilibrium length?

Answer: Creep

Creep is the time-dependent deformation of viscoelastic tissue under a constant load — the tissue gradually elongates until it reaches a new equilibrium. Stress relaxation, by contrast, occurs when tissue is held at a constant length and the internal stress (force) decreases over time. Hysteresis refers to energy loss during a loading/unloading cycle, and elastic recoil is the immediate return of tissue to its original length after load removal.