Neuromuscular Training and Kinetic Chains

Overview

• Lecture covers exercise prescription concepts: speed of contraction, neurologic adaptations, kinetic chain, and reps/sets programming.
• Focus on applying these principles to ensure safe and effective rehabilitation and improve performance outcomes.
• Recaps earlier discussion on contraction modes, exercise modes, and muscle strengthening.

Force-Velocity Relationship: Isokinetic vs Isotonic

• Isokinetic exercise: Velocity is fixed by a machine, while torque output varies depending on contraction type and speed.

  • At zero velocity (the center of the force-velocity curve): isometric contraction, where all force is applied to a non-moving system, typically producing higher torque than fast concentric contractions.
  • Fast concentric isokinetic contractions: Torque is reduced because more force is needed to keep up with the machine's speed. Slower concentric speed allows for more torque.
  • Isometric contractions: All force is directed into an unmoving system, producing greater torque.
  • Eccentric isokinetic: Slower speed allows higher torque compared to concentric or isometric; even faster eccentric speed increases torque further.

• Isotonic exercise (e.g., squat): The external load is constant while speed varies by torque produced.

  • When standing motionless with a load, the contraction is isometric—torque output matches the load, resulting in no movement.
  • During eccentric lowering, produced torque is less than the load, and the less torque generated, the quicker the descent.
  • For concentric movement (rising), produced torque exceeds the load. Generating more torque increases speed and can even produce explosive movements like jumps.

Practical Speed Guidance and Power

• In isotonic training, recommend a slow eccentric lowering phase paired with a faster concentric rise.
• Power training involves producing force rapidly; uses faster concentric actions with lighter to moderate loads (20–50% 1RM) to allow acceleration.
• Rate of force development is critical—not just overall strength. Functional tasks often require muscles to generate torque in less than 300 ms, particularly in athletics and quick-reacting situations.

Rate of Force Production and Function

• Fast torque generation is essential for preventing falls, such as when tripping over something during daily activities.
• Gradually advancing exercise speed is necessary to restore the ability to produce rapid force, following the SAID principle (Specific Adaptations to Imposed Demands).
• Both athletic and daily functional activities rely on rapid muscle activation and force production.

Neurologic Adaptations

• Size principle: Motor units are recruited in order—type I (slow-twitch) fibers first, followed by type IIa and IIb (fast-twitch) as force requirements rise.
• Type II fibers are responsible for strength and are engaged with higher loads, eccentric contractions, or blood flow restriction training—even with lighter loads.
• Early strength improvements (first 6–8 weeks, sometimes 8–12+ weeks following injury) are primarily due to neural adaptations rather than muscle hypertrophy.

Neural Summation Mechanisms

• Spatial summation: Increased recruitment of additional motor units with progressive exercise stimulus. The central nervous system activates more motor units to meet rising force demands.
• Temporal summation: Increased firing frequency of activated motor units, further boosting force output.
• Eccentric contractions can modulate the activity of muscle spindles and Golgi tendon organs, helping to recruit type II fibers effectively even at lower loads.
• Most initial strength gains during early rehab or training result from these neural mechanisms, not from an actual increase in muscle size.

Eccentric vs Concentric Emphasis

• Muscles trained eccentrically can produce rapid tension needed for high-demand actions like plyometric jumps.
• Overemphasizing concentric training without adequate eccentric preparation may lead to muscle failure when eccentric force is needed, or even muscle inhibition.
• Early rehab should prioritize neurologic and eccentric adaptations; muscle hypertrophy can be addressed after neural improvements are established.

Kinetic Chain Concepts

• The kinetic chain refers to the connection from the most distal limb segment to the axial skeleton, including all bones, joints, muscles, and connective tissues.
  • Upper limb chain: Fingers → hand → wrist → forearm/elbow → humerus/shoulder → scapula → thoracic spine.
  • Lower limb chain: Toes → foot/ankle → lower leg/knee → femur/hip/pelvis → lumbar spine.
• Closed Kinetic Chain (CKC): Distal segment is fixed; the rest of the limb or body moves over/around it.
  • Advantages: Promotes joint compression/congruency, increases neuromuscular activity, encourages co-contraction of opposing muscle groups, and supports functional movement patterns.
  • Drawbacks: Compensation is easy—strong muscles may mask weak ones, making side-to-side compensation likely.
• Open Kinetic Chain (OKC): Distal segment moves; proximal remains fixed.
  • Typically less joint compression; allows isolated muscle contraction and better identification/strengthening of weak points.
  • Compensation is difficult since the target muscle must engage.
• Best rehabilitation programs use both OKC (for isolating and strengthening weak segments) and CKC (for functional, integrated movement).

Examples: Open vs Closed Kinetic Chain

• Quadriceps OKC: Seated knee extension (full-arc quad) where the lower leg moves while the thigh remains stationary.
• Quadriceps CKC: Sit-to-stand movement; feet remain fixed while the rest of the body moves, engaging ankle, knee, hip, and spinal muscles.
• Shoulder in quadruped (hands and knees): This position is CKC for the shoulder due to weight-bearing, increasing joint congruency and stabilizing co-contraction around the joint.

Reps, Sets, Loads, and Rest

• Recommended loading parameters:
  • Endurance: <60% 1RM
  • Hypertrophy: 60–80% 1RM
  • Strength: 80–100% 1RM
  • Power: 20–50% 1RM (emphasizing acceleration)
• In early rehab, begin with lower-than-maximum loads since early strength gains are primarily neurologic.
• Utilize eccentric-focused work and blood flow restriction techniques (when appropriate and not contraindicated) to recruit type II fibers with relatively light loads.
• Sets: 2–5 per movement pattern, which can be distributed across several exercises for a given pattern.
• Rest intervals: Traditionally, 2–3 minutes for strength and hypertrophy work; endurance—use rest based on individual conditioning and injury.
• Human movement patterns: Upper body push/pull, lower body squat/hinge/deadlift, split pelvis, carrying activities, twisting. Programming should include a variety of exercises within these foundational patterns for comprehensive development.

1RM Estimation and Rep Mapping

• Avoid testing true 1RM in injured or rehabilitating patients. Instead, estimate 1RM by finding the maximal load the patient can perform for 10 repetitions.
  • If the 10th rep is the absolute last possible, the weight used approximates 75% of calculated 1RM.
  • If more than 10 reps are possible, adjust downwards for a more accurate estimation.
• Adjust loads and rep schemes based on the estimated 1RM and the specific adaptation targeted (endurance, hypertrophy, strength, power).

Repetition Ranges and %1RM Table

Key Terms & Definitions

• Force-velocity curve: Relationship showing how muscle force or torque production changes with contractions at different speeds.
• Isokinetic: Movement at a constant speed, controlled by a machine; used to assess torque at various contraction velocities.
• Isotonic: Movement against a constant external load; the speed of movement depends on how much force is produced.
• SAID principle: Body makes specific adaptations to the specific demands placed on it through exercise.
• Size principle: Muscle fibers are recruited from type I to type II as intensity or force requirements increase.
• Spatial summation: Increasing the number of activated motor units.
• Temporal summation: Increasing the rate at which individual motor units are activated.
• Closed Kinetic Chain (CKC): Distal limb segment is fixed; enhances joint stability and functional muscle coordination.
• Open Kinetic Chain (OKC): Distal segment moves freely; used to isolate weak muscles.
• Blood flow restriction: Training technique restricting blood supply to engage type II fibers with low loads.

Action Items / Next Steps

• Start rehabilitation with loads below patient capacity, using careful control and an emphasis on slow eccentrics for neuromuscular adaptation.
• Gradually increase speed as the patient progresses, ensuring the rate of force development is appropriate for functional tasks and injury prevention.
• Incorporate both functional CKC movements and OKC exercises targeting isolated muscle weakness in every program.
• Use 10-rep maximum efforts to estimate 1RM, setting loads and reps according to the specific training goal (endurance, hypertrophy, strength, power).
• Prescribe 2–5 sets per movement pattern, using a mix of exercises and considering patient conditioning when planning rest intervals.
• When trained and appropriate, consider blood flow restriction early in rehab to accelerate strength improvements.
• Always "undercook" (underdose) at the start, since you cannot undo excessive load or volume—progress as tolerated and as adaptations occur.