The role of muscle is to generate the forces required for movements. Thus, the long term objective of this laboratory is to understand how different muscle properties actually work together to produce the forces required for normal movements. This is an area where little is known because most studies of muscle either characterize its diverse mechanical properties or investigate their molecular mechanisms. The main thesis of this proposal is that a lack of understanding of how muscle properties behave during normal activation is the primary limit to understanding their roles in normal movements. The experiments focus on two of the most fundamental properties of muscle, the length-tension (L-T) and force-velocity (F-V) functions. Norman activation consists of recruitment and rate modulation of motor units, whereas the L-T and F-V functions are typically measured during maximal tetanic stimulation of either whole muscle or single muscle fibers. In Aim 1, the goal is to obtain the first measurements of the L-T and F-V functions during normal recruitment and rate modulation of motor units. An areflexive animal preparation has been developed for this purpose. Preliminary data show that both functions are much steeper at low recruitment and rate levels than would be expected from their tetanic behaviors. Aim 2 seeks to understand how this occurs by measuring the L-T and F-V functions of single motor units at rates that correspond to the physiological range. In Aim 3, the role of the L-T and F-V functions during a variety of dynamic changes in muscle length are investigated. Activation is again supplied by a normal pattern of recruitment and rate modulation in the areflexive preparation. In these conditions, several muscle properties can contribute to force generation, but it is expected that most of the force modulations can be accounted for by the L-T and F-V functions seen during the appropriate level of natural activation. The results of these studies are expected to show that natural activation patterns play a key role in shaping the mechanical output of muscle. This information is important for understanding how muscle is used in motor control in both normal and pathological states.