Henneman’s Size Principle dictates that the central nervous system recruits motor units sequentially from smallest (slow-twitch) to largest (fast-twitch) based on mechanical demand. Physiotherapy assists in managing movement dysfunction by manipulating this neurological switch, helping to optimize movement. The biomechanical root cause of many acute muscle strains is a "Kinetic Compensation Trap," where a load exceeds the capacity of the slow-twitch stabilizers, forcing the brain to violently recruit easily fatigued fast-twitch prime movers to protect the joint.
The Patient’s Story / Toronto Context
If you work out at a boutique fitness studio in King West or lift weights in a Liberty Village gym, you probably have a favorite style of training. Some Toronto professionals swear by high-repetition Pilates classes, while others dedicate themselves exclusively to heavy barbell deadlifts.
Often, a patient arrives at Rehab Mechanics deeply frustrated because they feel strong in their specific class, but they get injured doing something completely different. The Pilates enthusiast pulls a hamstring sprinting for the King streetcar, or the heavy powerlifter experiences agonizing lower back spasms after merely standing at a concert for two hours.
They ask us, "If I work out five days a week, why does my body feel so unprepared for real life?"
The answer lies in how your brain chooses to spend energy. Your muscles do not have a mind of their own; they are controlled by a strict neurological rule called Henneman’s Size Principle. Your brain is incredibly efficient—and slightly lazy. It will never use a massive, fuel-hungry power muscle if a tiny, efficient endurance muscle can do the job. By understanding this neurological switch, specialized physical therapy can decode exactly why your current workout routine is leaving gaping holes in your structural armor, and how to program your movement to ensure every fiber is fully awakened.
Structural / Biomechanical Analysis
To understand why you get injured when stepping outside your normal routine, we must perform a biomechanical analysis of how the brain talks to muscle fibers under varying degrees of stress.
The Biological Law: Henneman’s Size Principle
The brain communicates with muscles via motor neurons. One neuron controls a specific bundle of muscle fibers (a motor unit).
The Sequential Switch: In 1957, Dr. Elwood Henneman discovered that the brain always recruits these motor units in a specific order: from smallest to largest.
Small to Large: When you pick up a light object or hold your posture, the brain only turns on the small, highly efficient Type I (slow-twitch) endurance fibers. It leaves the massive Type II (fast-twitch) power fibers completely turned off to save energy.
The Effort Threshold
How do you turn on the big engines? You have to force the brain to hit the "Effort Threshold."
Heavy Load or High Speed: The brain will only flip the switch to recruit your fast-twitch (Type II) fibers if the mechanical load is extremely heavy (e.g., a 1-rep max deadlift) or if the movement requires explosive velocity (e.g., jumping or sprinting).
The High-Rep Illusion: If you only ever lift 10-pound dumbbells for 30 repetitions, you will never recruit your highest-threshold fast-twitch fibers. They will remain dormant, eventually wasting away from disuse.
The Kinetic Compensation Trap
This neurological rule explains exactly how acute injuries happen.
The Breakdown: Imagine you are carrying a heavy box of groceries up the stairs to your apartment. Your slow-twitch stabilizers in your core try to do the work, but the box is too heavy.
The Violent Rescue: Following Henneman's Principle, the brain panics and instantly recruits the massive, fast-twitch prime movers in your lower back (the erector spinae) to save your spine from collapsing. Because these fast-twitch muscles have terrible endurance and were not designed for stabilization, they violently lock into a painful, protective muscle spasm.
Clinical Red Flags
We meticulously assess your neuromuscular recruitment patterns to identify if a failure in the Size Principle is driving your pain:
The "Shakes" Under Light Load: A limb violently trembles or shakes when holding a very light weight in a static position, indicating the small slow-twitch stabilizers are completely fatigued and the brain is frantically trying to cycle fast-twitch units to help.
Velocity-Dependent Pain: The patient can slowly lower themselves into a chair without pain, but dropping quickly or jumping triggers a sharp, biting pain, proving the high-threshold fast-twitch fibers are damaged or inhibited.
Synergistic Dominance: When asked to gently activate the glute (a slow-twitch stabilizer task), the hamstrings instantly cramp, proving the brain's recruitment software is entirely bypassed and glitching.
Loss of Explosive Power: An absolute inability to generate speed from a dead stop, commonly seen in older adults suffering from targeted fast-twitch atrophy (Sarcopenia).
Primary Source Proof (PubMed / NIH)
Clinical neurophysiology heavily relies on Henneman’s Size Principle to explain motor control, confirming that varying load intensity and movement velocity are absolutely required to fully recruit and rehabilitate the entire spectrum of skeletal muscle fibers.
Review the Clinical Evidence on PubMed: Henneman's Size Principle and Motor Unit Recruitment (National Institutes of Health)
Review the Clinical Evidence on PubMed: Adaptations of Motor Unit Activity to Resistance Training (National Institutes of Health)
Review the Clinical Evidence on PubMed: Age-Related Changes in Motor Unit Recruitment and Sarcopenia (National Institutes of Health)
The Rehab Mechanics Corrective Protocol
We do not just prescribe 3 sets of 10. We use Henneman’s Principle to strategically manipulate your nervous system, forcing it to recruit the exact fibers you are missing.
Phase 1 — Load Modification (Isolating the Base): If you are trapped in a fast-twitch spasm, we must quiet the brain. We use very low-load, high-time-under-tension isometric holds to specifically target and re-awaken the tiny, slow-twitch stabilizers without triggering the massive prime movers.
Phase 2 — Pelvic Fortification (The Endurance Threshold): We build the slow-twitch foundation. Utilizing exercises like bird-dogs and extended planks, we train the core to handle prolonged stress so the brain never has to panic and recruit the lower back for stability.
Phase 3 — Gait Retraining / Mechanics Correction (Velocity Integration): We bridge the gap. We transition the patient to movement where the load is light, but the intent is incredibly fast. This neurological intent to move quickly safely tricks the brain into flipping the switch and recruiting the dormant fast-twitch fibers.
Phase 4 — Return-to-Activity Strategy (Maximal Recruitment): To guarantee full recovery, we must hit the absolute top of the Size Principle. We introduce heavy compound lifting and plyometrics, proving to the central nervous system that both the endurance and power engines are fully integrated, online, and ready for real-world chaos.
Related Conditions We Treat
Acute Muscle Strains and Spasms
Overtraining Syndrome / CNS Fatigue
Gluteal Amnesia (Dead Butt Syndrome)
Age-Related Sarcopenia
Chronic Lower Back Pain
Myofascial Pain Syndrome
Related Blogs
Muscle Fiber Typing 101: The Cellular Machinery of Human Movement (Part 1 of 5)
The Slow-Twitch Engine: Postural Stability, Endurance, and Injury Resilience (Part 2 of 5)
The Fast-Twitch Dynamo: Power, Sarcopenia, and the Mechanics of Aging (Part 3 of 5)
High Load, Low Reps: The Science of True Strength and Neuromuscular Drive
Services Used in Treatment
Neuromuscular Re-Education
Biomechanical Movement Assessments
Strengthening Programs
Gait Retraining
Manual Therapy
Soft Tissue Release
Shockwave Therapy
Custom Orthotics
FAQ Section
Can physiotherapy assist in fixing muscle spasms caused by lifting heavy objects? Yes. Physiotherapy supports recovery by analyzing your movement software to see why your stabilizing muscles failed, helping to optimize movement and prevent your prime movers from going into a protective spasm.
What is Henneman's Size Principle? It is a biological law stating that your brain uses small, endurance muscle fibers first, and only uses large, power fibers when lifting something very heavy. We use this law to assist in managing your specific rehabilitation loads.
Why do I get injured when I try a new workout class? Your body may be strong in one fiber type but completely untrained in another. We help address contributing factors by exposing your nervous system to a variety of loads and speeds to eliminate these weak blind spots.
If I do 50 reps with a light weight, will I build power? No. Based on the Size Principle, the brain will not recruit the largest fast-twitch power fibers unless the weight is incredibly heavy or moving extremely fast. We optimize movement by prescribing the correct weight for your specific goals.
Why does my leg shake when I do slow exercises? Shaking happens when your slow-twitch endurance fibers fatigue, forcing your brain to clumsily cycle fast-twitch fibers on and off to help. We utilize strengthening programs to build this missing endurance.
How do you train the brain to use the right muscle? We use targeted neuromuscular re-education. By carefully controlling the weight, speed, and your physical position, we force your brain to send electrical signals only to the exact muscle fiber type we want to heal.
Does custom orthotics change how my muscles fire? Yes. If your foot collapses, it changes the mechanical load on your leg. Custom orthotics can help optimize movement by providing a stable base, allowing the correct muscles to fire sequentially.
How long does it take to fix my muscle recruitment patterns? While neuromuscular timing and coordination (the software) can improve in just a few sessions, physically thickening the muscle fibers (the hardware) typically requires 6 to 8 weeks of targeted resistance.
How Physiotherapy Helps
Reducing tissue irritation by terminating the chronic spasm of overloaded fast-twitch muscles
Correcting pelvic drop to ensure slow-twitch core stabilizers carry the load during walking
Improving cadence and running efficiency by properly balancing fiber recruitment
Strengthening stabilizers in the deep spine using high-time-under-tension protocols
Reducing mechanical overload on joints caused by the Kinetic Compensation Trap
Improving foot mechanics to safely anchor the body during both slow and explosive movements
CTA — Contact Us Today
Contact Us Today — All you have to lose is the pain Stop guessing which muscles you are actually training. Book a comprehensive neuromuscular and biomechanical assessment with our clinical team today. Located inside the Prime Medical Centre at 68 Abell Street, Queen West. Email: info@rehabmechanics.com | Phone: (416) 533-3900
About the Author
Mr. Sanjay Attwala (B.Sc., M.Sc., RPT) is a Registered Physiotherapist, clinical director, and the founder of Rehab Mechanics in Toronto. With over 15 years of registered clinical practice and a deep specialization in complex musculoskeletal rehabilitation, Sanjay synthesizes rigorous international academic training with advanced evidence-based therapeutics to guide his clinical practice and patient education initiatives.
Academic Background & Credentials
Master of Science (M.Sc.) in Physiotherapy – University of Keele, United Kingdom (2010).
Bachelor of Science (B.Sc.) – University of Waterloo, Ontario, Canada.
Registered Physiotherapist (RPT) – Regulated health professional in excellent standing with the College of Physiotherapists of Ontario (CPO).
Corporate Entity – Operating officially under the S. Attwala Physiotherapy Professional Corporation with a DBA of Rehab Mechanics.
Clinical Expertise & Philosophy
Sanjay’s clinical approach rejects passive symptom management in favor of identifying underlying biomechanical root causes. His diverse expertise spans advanced manual therapies, personalized corrective exercise prescription, and modern physical modalities. At the Rehab Mechanics Toronto Queen West clinic, he routinely diagnoses and treats complex conditions including:
Spinal & Discogenic Pathology – Cervical, thoracic, and lumbar disc injuries, sciatica, and sacroiliac joint (SIJ) dysfunction.
Upper & Lower Extremity Injuries – Rotator cuff tears, frozen shoulder, tennis/golfer’s elbow, carpal tunnel syndrome, and complex ankle/foot pathologies.
Perinatal & Pelvic Health Rehabilitation – Specialized assessment and rehabilitation protocols tailored specifically for women during pregnancy and the post-partum period, addressing pelvic girdle pain, diastasis recti, and core stabilization.
Specialized Rehabilitation – Pelvic health therapy, TMJ dysfunction, post-surgical rehabilitation (including Total Hip and Total Knee Replacements), and custom orthotics dispensing.
Shockwave Therapy: with advanced cutting edge technological devices to suit your needs.
Interdisciplinary Practice & Patient Care
Sanjay practices an integrated model of healthcare, working closely alongside medical doctors inside the Prime Medical Centre on Abell Street to streamline patient recovery pathways. He maintains a human-centric, communication-first clinical framework, ensuring that care remains fully customized rather than automated. His clinical caseload encompasses a broad operational spectrum under Ontario's regulatory frameworks, including:
Motor Vehicle Accident (MVA) Claims – Rehabilitation navigating Ontario’s statutory accident benefits schedule.
Workplace Safety and Insurance Board (WSIB) – Occupational injury management and return-to-work screening.
Extended Health Care (EHC) & Private Practice – Multi-tier insurance coordination and long-term athletic development plans.
Commitment to Research & Community
Outside of his clinical caseload at Rehab Mechanics and his additional practice affiliations in Etobicoke, Sanjay is an active health writer and community educator. He translates contemporary peer-reviewed medical research into accessible, actionable guidance on his professional blog. As a dedicated father and husband, he mirrors his professional advice in his personal life, focusing on structural mobility, cross-training, and longevity to help his family and his community thrive. Naturally he takes he a keen interest in rehabilitation for women who are pregnant and post-partum.
Disclaimer:
The information provided on this blog is intended for educational and informational purposes only. It does not constitute medical advice, diagnosis, or a treatment plan. Always seek the direct advice of a Registered Physiotherapist, physician, or other qualified health provider regarding any medical condition or physical rehabilitation routine.
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