Maximal Force Development Research Articles

SERIES ELASTIC AND CONTRACTILE ELEMENTS IN HEART MUSCLE: CHANGES IN MUSCLE LENGTH.

Isometric force generation, the result of the interaction of an actively shortening contractile element (CE) with a passive series elastic component (SE), has been analyzed in heart muscle using the cat papillary muscle. The stiffness of the SE (Δ load/Δ extension or d p/d l) was shown to be a linear function of developed force of contraction or load ( P). The load-extension curve of the SE was thus exponential in form, the SE being stretched an amount equivalent to 8–10% of initial muscle length during the development of maximum isometric force (600 g/cm2). The load-extension curve of the SE was also obtained from the curve relating initial velocity of shortening to time after stimulation. This latter relation and the linear relation of SE stiffness to force were found to be independent of initial muscle length. These results were interpreted to suggest that increases in muscle length bring about an increase in the number of series elastic components as well as contractile elements arranged in parallel. The series elastic component thus cannot be relegated entirely to external attachments of the muscle (e.g., tendon), but must be closely associated with the contractile element system itself.

Force-velocity relations in mammalian heart muscle.

Force-velocity relations were studied in the cat papillary muscle. As with skeletal muscle, a characteristic relation has been demonstrated between the velocity of shortening (V) and the force developed (Po). Two generalities have been shown to pertain. First, increasing initial muscle length increases the maximal developed force (Po) without a change in the maximal velocity of shortening (Vmax). Secondly, at any one muscle length, changes in frequency of contraction and chemical environment (increased calcium and norepinephrine) increase Vmax with a variable change in Po. Changes in Vmax thus help to characterize an inotropic intervention (altered contractility). Work and power, at any one muscle length, are functions of afterload, with maxima when the load is approximately 40% of isometric tension. With increasing initial muscle length, the work and power at any one afterload as well as the maximal work and power of the muscle are both increased. At constant initial length, positive inotropic interventions (increased frequency, increased calcium, and norepinephrine) increase the work at any one afterload as well as shift the maximal work potential to a higher afterload. Work performance thus depends on muscle length, the prevailing force-velocity curve, and the afterload at which the muscle is operating.