Biomechanics Study Notes Summary & Study Notes

🔬 Biomechanics Foundations

Biomechanics applies mechanical principles to living systems to understand movement, loads, and stability. It integrates two main domains: Kinematics and Kinetics. Kinematics describes motion without regard to forces, while Kinetics analyzes the forces that cause or resist motion.

Example of kinematic description: a knee flexion from 0° to 60° during gait in the sagittal plane. Example of kinetic assessment: evaluating joint reaction forces during a squat under different loads.

Osteokinematic motion refers to gross bone movements at a joint (e.g., flexion, extension, abduction). In contrast, osteology covers the bones themselves and the skeletal system’s functions, including support, protection, and mineral storage.

Wolff's law states that bone adapts to the loads it experiences, increasing or decreasing mass with loading demands. The SAID principle (Specific Adaptations to Imposed Demands) explains that tissues adapt to the specific type of training they receive.

🧭 Osteology & Skeletal System

Function of the skeletal system includes structural support, protection of organs, leverage for movement, mineral storage (calcium and phosphate), and hematopoiesis in marrow. Regular physical activity can positively influence bone mass, especially with weight-bearing and impact activities.

Bone mass and physical activity: mechanical loading stimulates bone formation and remodeling; sedentary lifestyles can lead to bone loss over time. Proper loading patterns are essential for maintaining skeletal health across the lifespan.

Types of bones include long, short, flat, and irregular, each with standard parts: diaphysis, metaphyses, epiphyses, and periosteum. Joints are named for their articulations and are stabilized by capsules and ligaments.

🧭 Joint Anatomy & Function

Fibrocartilage, elastic cartilage, and hyaline cartilage provide varying degrees of stiffness and resilience at joints. Bursa, tendons, ligaments, and cartilage each play distinct roles in joint movement, load transmission, and stability.

Classification of joints: synarthroses (immobile), amphiarthroses (slightly movable), and diarthroses (highly movable). Examples include sutures (synarthroses), pubic symphysis (amphiarthrosis), and the knee or shoulder (diarthroses).

Types of joints include hinge, gliding, pivot, condyloid, saddle, and ball-and-socket. Each type supports characteristic degrees of freedom and anatomical constraints.

Open-packed vs close-packed positions describe joint congruence; open-packed often allows more mobility, while close-packed provides maximal stability and joint contact.

Concave-on-convex vs convex-on-concave arthrokinematics describe how joint surfaces glide and roll relative to each other when a segment moves. Correct interpretation helps predict motion direction and joint loading.

Open vs Closed kinetic chain movements describe whether distal segments are free (open) or fixed (closed) during a movement, influencing muscle activation patterns and joint loading.

Identify concave vs convex bones in a given joint to determine arthrokinematic directions. Hypermobility and hypomobility describe joints with excessive or limited motion, respectively.

Dislocation is a complete loss of joint contact, while subluxation is a partial loss of contact. These disruptions affect joint stability and require different management.

Screw-home mechanism describes a last-0term knee rotation during terminal extension in open kinetic chain and its variants in closed-chain movements, contributing to joint stability.

Arthritis types include osteoarthritis, rheumatoid arthritis, and inflammatory arthritides, each with distinct etiologies and joint involvement patterns.

Intervertebral discs consist of the annulus fibrosus surrounding the nucleus pulposus; herniation involves posterior or posterolateral displacement of disc material affecting neural structures.

🧠 Nervous System & Motor Control

CNS vs PNS: The Central Nervous System includes the brain and spinal cord, while the Peripheral Nervous System comprises nerves that convey motor and sensory information to the body. These systems coordinate movement and reflexes.

Motor areas of the brain include the primary motor cortex and associated planning regions (premotor and supplementary motor areas). They initiate and modulate voluntary movements.

Spinal nerves exit the spinal cord to transmit motor commands and sensory information to tissues and organs.

Parts of a neuron include the cell body, dendrites, and axon; communication occurs via electrical and chemical signals at synapses.

🏋️ Muscle Physiology & Contraction

Physiology of a muscle contraction begins at the motor end plate, with an action potential triggering calcium release, enabling cross-bridge cycling as actin slides past myosin.

Sarcomere structure consists of Z-lines, I-band, A-band, and H-zone; shortening of the sarcomere underlies muscle contraction according to the sliding filament theory.

Muscle fiber types: Type I (slow-twitch, fatigue resistant) and Type II (fast-twitch; IIa and IIx) differ in contraction speed and fatigue profile. These differences influence performance and training adaptations.

Motor unit comprises a motor neuron and all the muscle fibers it innervates; precise control uses small units, while greater force recruitment uses many units.

Gradation of strength occurs via motor unit recruitment following the size principle: smaller units activate first, followed by larger units as needed.

Muscle properties include extensibility, elasticity, excitability, and contractility. These properties determine how muscles respond to stretch and load.

Muscle actions are categorized as concentric (shortening), eccentric (lengthening), or isometric (static). Each has distinct functional and performance implications.

Roles of muscles: prime movers (agonists), antagonists, synergists, stabilizers, and stabilizers that fix joints during movement. Understanding these roles helps analyze movement efficiency and injury risk.

Muscle strain, tendonitis, sprain describe injuries to muscle, tendon, and ligament tissues, often from overuse or acute overload.

Length-tension relationship describes how force production varies with sarcomere length; there is an optimal length for maximal force. Excessive shortening or lengthening reduces force output.

Force-velocity relationship shows that concentric force decreases with higher shortening velocity, whereas eccentric actions can generate higher forces at a given velocity.

Stretch-shortening cycle leverages stored elastic energy and stretch reflexes to augment subsequent force production during explosive movements.

Henneman's size principle explains hierarchical motor unit recruitment based on neuron size and thresholds, shaping how muscles produce graded force.

GTO and muscle spindle provide proprioceptive feedback; they regulate tension and length and influence reflex activity and motor patterns.

PNF stretching techniques use patterns and sensory cues to facilitate flexibility, engaging GTO and muscle spindle pathways.

Proprioception is the body's sense of position and movement, crucial for balance, coordination, and safe movement execution.

Active & passive insufficiency describe limitations in multi-joint muscles: active insufficiency occurs when a muscle cannot shorten enough to produce desired joint movement; passive insufficiency occurs when a muscle cannot stretch enough to allow full range of motion at its antagonist joints.

Muscle strain vs tendonitis differentiate tissue injury mechanisms and treatment considerations.

Delayed onset muscle soreness (DOMS) reflects microtrauma and inflammatory processes after unfamiliar or intense exercise.

EMG (electromyography) provides insight into the timing and level of muscle activation and is often used to interpret muscle function and neuromuscular control.

Biarticular muscles cross two joints and can transfer energy across segments. Examples include the hamstrings, rectus femoris, gastrocnemius, soleus, biceps brachii, triceps brachii, flexor digitorum superficialis, and extensor digitorum.

Origin, insertion, action for key muscles:

• All hamstrings
• Rectus femoris
• Gastrocnemius
• Soleus
• Biceps brachii
• Triceps brachii
• Flexor digitorum superficialis
• Extensor digitorum

🧭 Biomechanical Concepts & Stability

Mass vs weight: weight is the force due to gravity acting on a mass; mass remains constant while weight changes with the gravitational field.

Center of gravity (CG) is the point where the body's weight is concentrated and can be used to predict balance and tipping tendencies. The CG can be estimated by balancing weight distribution across body segments; simple approximations use segment masses and their positions, e.g., $CG = (m_1 x_1 + m_2 x_2 + ...)/(m_1 + m_2 + ...)$.

Base of support (BOS) is the contact area between the body and support surface; increasing BOS or lowering the CG enhances stability. A narrow BOS can improve mobility but may reduce stability.

Center of gravity, base of support, and stability interplay to explain how posture, movement, and external loads influence balance and fall risk. Strategies to increase stability include widening the stance, lowering the CG, and aligning the COM within the BOS.

🧠 Practical Takeaways for Exam Prep

Be comfortable differentiating terms such as osteokinematics vs arthrokinematics, and open-packed vs close-packed joint positions. Know the major joint types and how arthrokinematic rules apply to concave-on-convex and convex-on-concave scenarios. Practice identifying planes of motion, axes, and open vs closed kinetic chain contexts for common movements.

Understand the basics of the nervous system’s role in motor control, from motor units and recruitment strategies to proprioceptive feedback and reflex pathways. When thinking about muscles, focus on function, exercise implications, and common injury mechanisms (strain, tendonitis, sprain).

Remember that this course emphasizes conceptual understanding over calculations; be ready to explain mechanisms, not perform complex math. Refer to figures and tables in your study materials for visual reinforcement of these concepts.