Muscles transduce neural signals into the forces needed for movement. Every muscle is composed of a large population of motor units, each of which generates only a very small force. Virtually all previous work on motor unit properties has relied upon isometric conditions (constant muscle length) to facilitate measurement of these small forces. However, isometric conditions constitute only a small portion of normal motor behavior. Furthermore, data from whole muscles and single muscle fibers have shown that muscle tissue has a wide range of dynamic behaviors. The goal of this proposal is to obtain the first systematic measurements of dynamic motor unit properties. While there are many dynamic properties that could be studied in single units, Specific aim 1 proposes to determine which motor unit properties are actually important in normal movement conditions. For example, most muscle models rely only on the steady-state properties of muscle and thus assume dynamic properties play a minor role in force generation. A new decomposition technique has been developed to test this hypothesis. It has 2 phases: (1) techniques for accurately measuring single motor unit forces in dynamic conditions resembling those in normal movements; and (2) measurements in more controlled conditions that are designed to identify the effect of each mechanical property on the unit force output in those normal movement conditions.
Specific aims 2 &3 focus upon the behavior of motor units as a population of parallel mechanical elements. Since motor units form a heterogeneous population that is activated in order of increasing unit force, the population behavior cannot be predicted from that of any single unit. The hypothesis to be tested is that the population behavior increases the stability of muscle (i.e. its resistance to perturbations). The technique for testing this hypothesis also has 2 phases: (1) measurement of 2 basic motor unit properties that greatly influence stability, the force-velocity and force-length relations; and (20 prediction of population force-velocity-length behavior by use of realistic computer simulations based on these single unit data. These data should provide a foundation for understanding the underlying mechanisms of the functional deficits in diseases affecting both motor units and the control of motor units by the CNS.

Agency
National Institute of Health (NIH)
Institute
National Institute of Arthritis and Musculoskeletal and Skin Diseases (NIAMS)
Type
Research Project (R01)
Project #
1R01AR041531-01A1
Application #
3162000
Study Section
Orthopedics and Musculoskeletal Study Section (ORTH)
Project Start
1993-01-01
Project End
1996-12-31
Budget Start
1993-01-01
Budget End
1993-12-31
Support Year
1
Fiscal Year
1993
Total Cost
Indirect Cost
Name
Northwestern University at Chicago
Department
Type
Schools of Dentistry
DUNS #
005436803
City
Chicago
State
IL
Country
United States
Zip Code
60611
Cui, Lei; Maas, Huub; Perreault, Eric J et al. (2009) In situ estimation of tendon material properties: differences between muscles of the feline hindlimb. J Biomech 42:679-85
Sandercock, Thomas G; Maas, Huub (2009) Force summation between muscles: are muscles independent actuators? Med Sci Sports Exerc 41:184-90
Cui, Lei; Perreault, Eric J; Maas, Huub et al. (2008) Modeling short-range stiffness of feline lower hindlimb muscles. J Biomech 41:1945-52
Maas, Huub; Sandercock, Thomas G (2008) Are skeletal muscles independent actuators? Force transmission from soleus muscle in the cat. J Appl Physiol 104:1557-67
Cui, Lei; Perreault, Eric J; Sandercock, Thomas G (2007) Motor unit composition has little effect on the short-range stiffness of feline medial gastrocnemius muscle. J Appl Physiol 103:796-802
Sandercock, Thomas G (2006) Extra force from asynchronous stimulation of cat soleus muscle results from minimizing the stretch of the common elastic elements. J Neurophysiol 96:1401-5
Perreault, Eric J; Day, Scott J; Hulliger, Manuel et al. (2003) Summation of forces from multiple motor units in the cat soleus muscle. J Neurophysiol 89:738-44
Perreault, Eric J; Heckman, Charles J; Sandercock, Thomas G (2003) Hill muscle model errors during movement are greatest within the physiologically relevant range of motor unit firing rates. J Biomech 36:211-8
Sandercock, T G; Heckman, C J (1997) Force from cat soleus muscle during imposed locomotor-like movements: experimental data versus Hill-type model predictions. J Neurophysiol 77:1538-52
Sandercock, T G; Heckman, C J (1997) Doublet potentiation during eccentric and concentric contractions of cat soleus muscle. J Appl Physiol 82:1219-28

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