Research Interests

Neural control of muscle contraction; muscle contraction kinetics; muscle power output and efficiency; relations between muscle ultrastructure and performance

Neural events are converted into behavior through effectors, of which muscles are by far the most important.  The properties of muscles--their speed, power, efficiency and endurance--pose major limitations on the performance of organisms.  The research in our laboratory is oriented toward characterizing the mechanical performance of muscle and the underlying reasons for the diversity in performance found in muscles from different sources.  We have been particularly interested in very fast muscles and in muscles with the capacity for high mechanical power output.

The major part of our research deals with mechanical power output and efficiency.  The most important functional capacity of muscle is its ability to shorten against a load and thus to do work.  Yet work output by muscles has been poorly characterized, largely for lack of an appropriate procedure for quantifying mechanical work output under conditions that approximate those of normal muscle use.  We have developed an experimental approach for measuring the work output of muscles during cyclic contraction.  This approach, in which work output is determined by evaluating the integral of muscle force times shortening distance over a full shortening-lengthening cycle, has come to be called the work loop method.  We are using the work loop method to investigate the parameters that control and limit muscle power output.  Part  of this study involves characterizing the effects of those factors which determine the force, and hence work output, throughout a shortening-lengthening cycle. We are also combining the work loop approach with measurements of muscle metabolism in order to investigate muscle efficiency and changes therein under different operating conditions. 

There are correlations between muscle ultrastructure and contractile performance.  For example, muscle fibers that give short twitches tend to have thin myofibrils and abundant sarcoplasmic reticulum (sarcoplasmic reticulum is the internal tubular system within muscle fibers that is involved in calcium regulation).  One of the facets of our research is the quantification of relationships between muscle ultrastructure and contraction kinetics.  We are seeking rules that will allow us to predict accurately a muscle’s performance from its ultrastructure, or ultrastructure from measured performance.  Such rules should help us to identify the rate-limiting processes in muscle performance.

Recent Papers

Josephson, R.K. (1993).  Contraction dynamics and power output of skeletal muscle.  Annual Reviews of Physiology 55, 527-546.

Josephson, R.K. and Edman, K.A.P. (1998).  Changes in the maximum speed of shortening of frog muscle fibres early in a tetanic contraction and during relaxation.  Journal of  Physiology 507, 511-525.

Josephson, R.K. and Stokes, D.R. (1999).  The force-velocity properties of a crustacean muscle during lengthening.  Journal of Experimental Biology 202, 593-607.

Josephson, R.K. and Stokes, D.R. (1999).  Work-dependent deactivation of a crustacean muscle.  Journal of Experimental Biology 202, 2551-2565.