Eccentric Emphasis in Rehab

Overview

Lecture by Daniel Bodkin on exercise prescription (Part 1), covering modes of muscle contraction, modes of exercise, and foundational concepts in muscle strengthening.

Modes of Muscle Contraction

• Three main types:
  • Isometric: Muscle contracts with no change in length or joint angle.
  • Concentric: Muscle contracts while shortening, movement follows joint angle direction.
  • Eccentric: Muscle contracts while lengthening, movement is opposite the joint angle change.
• Injury mechanisms:
  • Most injuries occur during eccentric contractions; common examples include lateral ankle sprains, rotator cuff tears, hamstring strains, and knee injuries.
• Car analogy:
  • Isometric = suspension/frame (stability)
  • Concentric = engine (acceleration)
  • Eccentric = brakes (deceleration)
• Understanding injury mechanisms and contraction mode is key for effective intervention.

Isometric Contractions

• Provide joint stability; can be primary (holding a position, e.g., carrying or pushing) or secondary (stabilizing one segment to allow free movement in another).
• Used for:
  • Early neuromuscular re-education
  • Muscle strengthening
  • Tissue loading without movement
• Angle-specific carryover: effect spans approximately 10 degrees on either side of the trained joint angle.
• Patient responsibility: generate force only; no need to control speed or movement direction.
• Examples:
  • Wall sit: Increasing knee flexion raises load; promotes sustained muscle activation and time under tension.
  • Pallof press: Isometric hold in lower extremity and spine while upper limb moves, resisting torsion.

Concentric vs. Eccentric Contractions

• Concentric: Used for acceleration and overcoming gravity; actin and myosin slide together to shorten muscle.
• Eccentric: Used for deceleration and force absorption; muscle lengthens as it resists load, utilizing elastic components (titin and tendon) for additional passive force.
• Example: Sit-to-stand = concentric when rising, eccentric when lowering to a seat.
• Eccentric contraction features:
  • Alters Golgi tendon organ and muscle spindle activity, resulting in distinct myoelectric and neural control.
  • Preferential recruitment of fast-twitch fibers even at low levels of force.
  • Requires 2–3 times less EMG activity versus concentric at same load; more efficient at leveraging the stretch–shortening cycle.
  • Requires 70–75% less oxygen but can create 30% more tension.
  • Associated with greater delayed onset muscle soreness (DOMS).

Eccentric Demand in Functional Tasks

• Example (Sprinting): Peak activity in glute, hamstring, and quadriceps occurs during hip flexion before foot contact, as muscles lengthen eccentrically—this is the phase where injuries are likely.
• Example (Pitching): Rotator cuff and shoulder muscle activity peaks during the deceleration phase, reflecting the high eccentric demand.
• Many functional activities produce high eccentric control requirements, increasing injury risk during these phases.

Modes of Exercise

• Four main types:
  • Passive Exercise (PROM): No patient effort; movement performed by an external force. Used post-operatively, for acute injuries, and to regain range of motion or reduce swelling/pain. Example: Wand-assisted shoulder elevation using the non-involved arm.
  • Isometric Exercise: No limb movement; engages force against fixed resistance. Used for neuromuscular re-education, strength, and tissue loading. Carryover is specific to about 10 degrees on either side of the trained angle.
  • Isokinetic Exercise: Constant movement speed with accommodating resistance; requires specialized equipment. Patient must control force and movement direction. Ideal for neuromuscular control, early eccentric/deceleration training, strength, and tendon/tissue loading. Maximizes muscle challenge across whole range of motion and removes sticking points.
  • Isotonic Exercise: Constant external resistance; patient controls speed and direction. Used for submaximal or maximal strength, endurance, power, and neuromuscular control (with biofeedback). Muscle is maximally loaded primarily at mid-range, underloaded in strong ranges, and overloaded at weak points (sticking points).

Isotonic Loading Examples and Progression

• Dumbbell squat (weights at sides): Adds resistance, but pulls torso forward, limiting trunk engagement.
• Front squat (weights held at shoulders): Promotes upright position, increases engagement of anterior chain and upper back.
• Goblet squat: Heavy dumbbell held in front; progression when bilateral hold is limiting; challenges postural control.
• Unilateral front load: Weight held on one side; increases core and hip activation to resist lateral bending.
• Dynamic surfaces (Airex pad, tiltboard, BOSU): Instability increases difficulty; can be paired with any of the above squat variations to increase neuromuscular challenge.

Force–ROM Concepts

• Decision of exercise type is influenced by practicality, equipment, and desired neuromuscular adaptations.

Muscle Strengthening Principles

• SED Principle (Specific Adaptations to Imposed Demands): Exercise program must mimic the activity’s demands and adapt as a patient progresses through rehab phases (early phases focus on neuromuscular control, later phases focus on strength and endurance).
• Overload Principle: Continued adaptation requires progressive increase in training stimulus.

Key Overload Variables

• Resistance/Load: Gradually increase weight or effort as strength improves.
• Repetitions: Progress from, for example, 10 to 12 or more reps as capacity increases.
• Tempo/Rate: Manipulate speed of exercise (slow down for more time under tension or speed up as strength improves).
• Duration: Increase total duration or number of sets as tolerated.
• Exercise Difficulty: Use more challenging but goal-consistent variations at similar load levels.
• Stability/Base of support: Alter stance or surface (e.g., unstable or narrow base) to increase neuromuscular demand.

Muscle Fiber Types

• Type I fibers are recruited at low force, used for endurance; Type II recruited for higher forces and faster, powerful movements, but fatigue faster.

Knowledge Check

• Question: An ankle inversion sprain demonstrates muscle failure during which contraction type?
• Answer: Eccentric. Peroneal muscles fail to contract eccentrically fast or strong enough, causing lateral ligament injury.

Key Terms & Definitions

• Isometric contraction: Muscle activation without length or joint angle change; stabilizes joints.
• Concentric contraction: Muscle shortens to produce force; accelerates movement or overcomes gravity.
• Eccentric contraction: Muscle lengthens while producing force; decelerates and absorbs load.
• PROM (Passive Range of Motion): External movement without patient muscle activation in the limb being moved.
• Isokinetic exercise: Constant movement speed, accommodating resistance, requires special machines.
• Isotonic exercise: Constant external resistance with patient-controlled movement speed/direction.
• SED principle: Adaptations are specific to the demands placed by training or therapy activities.
• Overload principle: Progress requires increasing the exercise challenge over time.
• DOMS: Delayed onset muscle soreness, most noticeable after high eccentric loading.

Action Items / Next Steps

• Prioritize eccentric-focused interventions due to high injury prevalence during deceleration/lengthening movements.
• Select exercise modes appropriate for rehab stage: start with PROM in acute phase, progress to isometrics for neuromuscular training, and advance to isotonic or isokinetic for strength/power.
• Apply overload systematically by adjusting load, reps, tempo, duration, exercise variation, and stability requirements.
• Program according to the SED principle, progressing from motor control to muscular strength and endurance.