Clinical Biomechanics: Kinetics & Muscle Function

Kinetics and Kinetic Chain Study Notes

Kinetics: Forces that cause movement.
Structure of the course content (from the transcript):
- Kinetics, Structure and Architecture, Fibre Types, Excitation–contraction coupling, Muscle Actions, Clinical Biomechanics in Human Locomotion (2023), Force, Power, Movement, Energy, Release.

The Kinetic Chain

The Kinetic Chain concept analyzes how sequential body segments contribute to movement.
Components (as listed in the transcript): Cervical Spine; Thoraco-Lumbar Spine; S-I Joints; Hip Joints; Knee Joints; Foot and Ankle.
Key quotes and ideas:
- It helps analyze human movement patterns.
- Provides rationale for exercise conditioning and rehabilitation programs that emphasize the entire body, even when a target joint or structure is injured (Ellenbecker 2020).
- “Sequentially activated body segments” (Kibler 1994).
- Separate anatomical units work together mechanically as a Kinetic Chain.
Static to dynamic sequencing:
- Static position → Kinetic Chain activation (illustrated by an arrow in the transcript).

The Kinetic Chain in Action

The effective synchronous sequencing of body segments is vital to maximize the efficiency of the kinetic chain (Seroyer).
Conceptual analogy:
- Like a whip, large forces are sequentially built upon one another up the kinetic chain.
- Segments involved: Wrist → Torso → Lower Body → Time (temporal sequencing) → Shoulder → Arm; culminates in a summation of forces.

Kinetic Chain Exercises

Open Kinetic Chain (OKC):
- Distal limb can move (open end of the chain).
- Usually non-weight bearing.
- Often uses a few muscle groups; can isolate specific muscles for strengthening.
Closed Kinetic Chain (CKC):
- Distal limb planted (closed end of the chain).
- Often weight bearing or against something that cannot move/press against.
- Involves multiple muscle groups working together.
- Emphasizes stabilization, proprioception, and balance.

Kinetic Chain: Open vs Closed (summary)

Open vs Closed: A contrast of the two distal-end conditions and the associated neuromuscular demands.

Muscle Architecture and Structure (Myofilaments, Sarcomeres, and Connective Tissues)

Myofilaments:
- Actin (thin filaments)
- Myosin (thick filaments)
Cellular and tissue organization:
- Sarcoplasm
- Myofibril
- Fasciculus
- Endomysium (between fibers)
- Perimysium
- Epimysium (deep fascia)
- Sarcolemma (muscle cell membrane)
- Single muscle fiber
- Nucleus
Tendon, Muscle belly, and Connective-tissue layers:
- Tendon connects muscle to bone.
- Muscle belly contains multiple fasciculi wrapped by connective tissue.
Myofibrils and sarcomeres:
- Myofibrils are composed of subunits called sarcomeres.
- Myofibrils contain the contractile apparatus of the muscle.
Sarcomere (functional unit of force generation):
- From Z Line to Z Line.
- Organization of myofilaments:
- Actin – Thin filaments
- Myosin – Thick filaments
- Contractile proteins and the sliding filament theory underpin muscle contraction.
Titin:
- A large protein in the myofibril (sometimes referred to as a 3rd myofilament).
- Has spring-like characteristics that confer stability and elasticity to the sarcomere.

Myofibrillar and Sarcomere Details

The actin filament consists of actin proteins and regulatory components.
Titin provides elasticity and stabilizes sarcomeric structure during contraction and stretch.
The sarcomere’s organization and the interaction of actin and myosin drive force generation via the sliding filament mechanism.

Kinetic Terms

Kinetics: Analysis of the forces caused by movement.
Mass (m): The amount of matter that makes up an object (Quantity).
Center of mass (CoM or center of gravity): Point at which the body’s mass is evenly distributed.
Momentum (p): Quantity of motion of an object; p = m v.
- p = m v
Force (F): The action of one body on another, represented as a vector and factors in magnitude (the load), location (where the force is applied), direction, duration (how long), frequency (how often), and rate (how quickly applied).

Force Production: Determinants and Concepts

Factors affecting muscle force production:
- Absolute muscle strength (maximum force a muscle can generate).
- Hypertrophy of muscle fibers.
- Neural adaptations: efficiency of recruitment and lower stimulus threshold.
- Joint angle and muscle length-tension relationships.
- Fiber arrangement (pennation) and fiber types: Type I, Type IIa, and Type IIb.
- Force-velocity relationship: contraction velocity vs. force produced.
- Lever arm and mechanical advantage.

Muscle Contractions

Isotonic contractions (muscle changes length):
- Concentric contraction: muscle shortens while generating force greater than the opposing force.
- Positive work: angular momentum at a joint increases during shortening.
- Tendon lengthens through tensile strain during concentric action.
Eccentric contractions: muscle lengthens while generating force, typically slower joint motion and higher force production.
- Negative work: velocity of movement is reduced; higher force is produced with less metabolic energy compared to concentric actions.
- Often associated with shin splints when describing loading of the lower leg.
Isometric contractions: muscle produces force without changing length.
- Force equals the opposing force, no angular momentum around the joint, static joint position is maintained.
Example references in the transcript include: isometric activation of rectus and transversus abdominis for trunk stabilization; quadriceps involvement for stabilization during isometric holds; concentric and eccentric actions described in various leg and hip muscles.

Joint Angle and Force Production

Force production depends on the angle of pull of the muscle and the joint angle at which the action occurs.

Length-Tension Relationship

Maximal force is produced near the muscle’s normal resting length.
Conceptual visualization (from the transcript): a graph showing tension relative to sarcomere length with maximal tension near resting length.

The Length-Tension Graph (Conceptual)

Sarcomere length (um) on the x-axis; tension (% of maximum) on the y-axis.
Maximum force occurs near the resting length of the sarcomere.

Pennation and Muscle Architecture

Pennation: muscle fibers attach obliquely (angle of pennation, θ).
Line of pull: from tendon to tendon; orientation can be longitudinal (0° pennation) or oblique (>0°).
Effects of pennation:
- If θ = 0°, all force produced by the muscle is transmitted across the tendon and joint.
- If θ > 0°, there is a decrease in the component of force transmitted along the tendon due to the cosine of the pennation angle.
Cosine relationship:
- The fraction of muscle force transmitted along the tendon is given by the cosine of the pennation angle:
- \cos(\theta)
Compensatory factors:
- Pennate muscles can offset reduced force along the line of pull by having a greater number of muscle fibers.
- Pennation tends to yield shorter muscle fibers but a higher total number of fibers, increasing cross-sectional area and potential force when fully activated.
Summary: Pennate muscles can generate large forces due to high fiber volume, despite lower force transmission along the tendon per fiber.

Muscle Shape and Functional Implications

Muscle shape and cross-section affect performance:
- Large cross-section muscles (e.g., gluteus maximus) can produce greater force but may have less range of motion.
- Long tendons (e.g., tendons to fingers and toes) facilitate rapid motion over a distance.

Force-Velocity Relationship

The relationship between contraction velocity and force production:
- Slower contraction velocity allows greater formation of myosin–actin cross-bridges, enabling higher force.

Levers in Human Movement

Levers: simple machines that modify mechanical advantage (MA).
Key terms:
- Axis: Fulcrum, joint, contact point, or pivot.
- Force: Effort, typically supplied by a muscle.
- Resistance: Load or the weight the lever system moves.
- Lever arm: The bone on which the muscle contracts.
- ARF: Remembering which part is in the middle (Axis-Resistance-Force): A mnemonic for lever positioning.
Mechanical Advantage (MA):
- MA = (Effort Arm) / (Load Arm) = \frac{L{EA}}{L{LA}}.
Classifications of levers:
- First Class Lever: Fulcrum between force and resistance (axis between effort and load).
- Example: Teeter-totter; Splenius extending the head across the atlanto-occipital joint.
- Second Class Lever: Load between fulcrum and force.
- Example: Wheelbarrow; Gastrocnemius into plantarflexion (standing on toes).
- Characteristic: Typically provides a large MA.
- Visual cue: “Load in the middle.”
- Third Class Lever: Force (effort) between the fulcrum and the resistance.
- Example: Most body lever systems (e.g., using a shovel).
Practical configurations and naming:
- Axis = Fulcrum or joint; Force = Effort (muscle); Resistance = Load (external or body weight).
- The lever arm is the bone segment where the muscle attaches.
- MA is higher when the effort arm is longer relative to the load arm, generally making two-class levers more mechanically advantageous.
Illustrative configurations (from the transcript):
- Second Class Levers: Wheel barrow; Lemon squeezer; Bottle opener; Nut cracker.
- Third Class Levers: Illustrated by the arrangement of Effort, Load, and Fulcrum (common in the human body).

Practical Implications and Synthesis

The kinetic chain concept emphasizes coordinated, sequential activation across body segments to maximize movement efficiency and rehabilitation outcomes.
Understanding open vs closed kinetic chain dynamics informs rehabilitation, conditioning, and sport-specific training design.
Knowledge of muscle architecture (pennation, fiber type distribution, muscle shape) guides expectations for force production and speed, and explains why certain muscles are optimized for strength versus rapid movement.
The lever framework helps analyze how joint mechanics and muscle insertions affect the capacity to generate force and perform work in various activities (e.g., jumping, running, lifting).

References and Foundational Statements (from the transcript)

Ellenbecker 2020: Kinetic chain analysis provides rationale for conditioning and rehab that emphasize the entire body, even when a specific joint is injured.
Kibler 1994: The body’s segments are sequentially activated.
Seroyer: The effective synchronous sequencing of body segments is vital for kinetic chain efficiency.
Additional context: Figures and diagrams referenced in the transcript (e.g., at heel strike in walking vs running) illustrate eccentric and concentric activity in muscles like tibialis anterior and triceps surae, Achilles tendon elongation, and dorsiflexion moments generated by ground reaction forces.

Summary of Key Formulas and Concepts

Momentum: p = m v
Mechanical Advantage: MA = \frac{\text{Effort Arm}}{\text{Load Arm}} = \frac{L{EA}}{L{LA}}
Force transmission along tendon with pennation: fraction along tendon = \cos(\theta), where θ is the pennation angle.
Lever classifications and their qualitative MA implications (First Class: axis between force and load; Second Class: load between axis and force; Third Class: force between axis and load).
Length–tension concept: maximal force near resting length (conceptual graph with sarcomere length on the x-axis, tension on the y-axis).

Notable Examples Mentioned in the Transcript

First Class Lever example: Splenius extends the head across the atlanto-occipital joint.
Second Class Lever examples: Wheelbarrow; Gastrocnemius into plantarflexion (standing on tiptoes).
Third Class Lever examples: Common in the body; shovel use as a general analogy.

Key Terminology to Remember

Kinetic Chain; Open Kinetic Chain; Closed Kinetic Chain; Myofilaments; Actin; Myosin; Sarcomere; Z Line; Titin; Epimysium; Perimysium; Endomysium; Sarcolemma; Fasciculus; Myofibril; CoM (center of mass); p (momentum); MA (mechanical advantage); θ (pennation angle); Type I, Type IIa, Type IIb fibers; Isometric, Isotonic (Concentric and Eccentric) contractions; Length–Tension; Force–Velocity; Lever arms; Axis/Fulcrum; Load/Resistance; Effort.