Elwood Henneman (December 12, 1914 – April 1, 1996) was a prominent American neurophysiologist recognized for his groundbreaking work on the organization and function of the motor system. He is best known for formulating "Henneman's Size Principle," a fundamental concept describing the orderly recruitment of motor units in skeletal muscle.
Biography and Education
Born in Lake City, Minnesota, Henneman pursued his undergraduate studies at the University of Minnesota, where he also earned his medical degree in 1939. Following his medical training, he completed an internship and residency in neurosurgery at Massachusetts General Hospital. His early career was interrupted by World War II, during which he served in the U.S. Army Medical Corps.
After the war, Henneman undertook a research fellowship at Yale University, working with the renowned physiologist John Fulton, which solidified his commitment to neurophysiological research.
Career and Research
In 1948, Elwood Henneman joined the faculty of Harvard Medical School in the Department of Physiology. He remained at Harvard for the entirety of his distinguished academic career, eventually becoming a Professor of Physiology. He retired as Professor of Physiology, Emeritus, in 1985.
Henneman's research focused on understanding the mechanisms by which the central nervous system controls muscle movement. His meticulous investigations involved studying the properties of individual motor neurons and their corresponding muscle fibers (motor units) in cats. His work revolutionized the understanding of motor unit recruitment.
Henneman's Size Principle
Henneman's most significant contribution, the Size Principle of Motor Unit Recruitment (often simply "Henneman's Size Principle"), was formulated in the late 1950s and early 1960s. This principle states that motor units are recruited in an orderly, fixed sequence from smallest to largest, irrespective of the type of contraction (concentric, eccentric, or isometric).
Key aspects of the principle include:
- Smallest Motor Units First: At low levels of force requirement, the smallest motor units, which are typically composed of slow-twitch, fatigue-resistant muscle fibers, are activated first. These units produce minimal force but are highly efficient for sustained, low-intensity activities.
- Progressive Recruitment: As the demand for force increases, progressively larger motor units are recruited. These larger units consist of fast-twitch, more powerful, but more fatigable muscle fibers.
- Graded Contraction: This orderly recruitment allows for a smooth, finely graded increase in muscle force, optimizing energy expenditure by engaging only the necessary muscle fibers for a given task.
Henneman proposed that this orderly recruitment is due to the inherent electrophysiological properties of the motor neurons themselves; smaller motor neurons have higher input resistance and thus reach their firing threshold with smaller synaptic inputs compared to larger motor neurons.
Legacy and Impact
Henneman's Size Principle became a cornerstone of modern neurophysiology and motor control. It provided a clear, testable mechanism for how the nervous system regulates muscle contraction and force generation. His work fundamentally changed how scientists and clinicians understand movement, muscle fatigue, and rehabilitation. The principle is a standard topic in virtually all textbooks covering neurophysiology, motor learning, and exercise science.
Henneman's meticulous experimental approach and insightful interpretations left a lasting impact on the field, guiding subsequent research into spinal cord circuitry, motor neuron excitability, and the neural control of movement.
Selected Publications
- Henneman, E., & Olson, C. B. (1965). Relations between structure and function in the design of skeletal muscles. Journal of Neurophysiology, 28(3), 581-598.
- Henneman, E., Somjen, G., & Carpenter, D. O. (1965). Excitability and inhibitability of motoneurones of different sizes. Journal of Neurophysiology, 28(3), 599-620.
- Henneman, E., Somjen, G., & Carpenter, D. O. (1965). Functional significance of cell size in spinal motoneurons. Journal of Neurophysiology, 28(3), 560-580.
- Henneman, E. (1957). Relation between size of motoneurons and their axonal conduction velocity. American Journal of Physiology, 190(3), 582.