Innervation ratio is an important determinant of force in normal and reinnervated rat tibialis anterior muscles.

1. The technique of glycogen depletion and periodic acid-Schiff (PAS) staining, which identifies glycogen-free muscle fibers, was used to directly count the number (N) and measure the cross-sectional area (CSA) of muscle fibers in single motor units (MUs) from normal and reinnervated tibialis anterior (TA) muscles. Indirect estimates, derived from the proportions of muscle fiber types to MU types, were also made, and force per unit area (or specific force, SF) was calculated. Previous results using direct and indirect approaches have been contradictory. To shed more light on this issue, the relative contributions of N, mean fiber area (A), and SF to muscle-unit force were determined by the use of both methods. 2. TA muscles were examined in experimental rats 3.5-10 mo after cutting and resuturing the common peroneal nerve in one hindlimb and in muscles in age-matched control rats. Ventral roots were dissected to isolate and characterize single MUs according to contraction speed, sag, and fatigability. One unit per muscle was selected for repetitive tetanic stimulation designed to deplete muscle fiber glycogen stores. Muscles were removed for identification of the unit with the PAS reaction and histochemical fiber typing by the use of modified standard techniques. 3. In the total population of MUs sampled, isometric tetanic force ranged from 5 to 441 mN in normal muscles and from 5 to 498 mN in reinnervated muscles, and the mean values were not significantly different. In the smaller sample of glycogen-depleted units from normal muscle, for a force range of 14-217 mN, N varied from 57 to 202, and A varied from 1,135 +/- 45 to 6,706 +/- 172 (SE) microns2. Within each unit the variation in fiber area is broad. After reinnervation, for a force range of 30-278 mN, N varied from 70 to 374, and A varied from 1,694 +/- 81 to 5,425 +/- 93 microns2. Mean fiber number was 153 +/- 18 in reinnervated muscle, which is significantly higher (P less than 0.01) than the normal value of 121 +/- 9. 4. The contribution of N and A to MU tetanic force was assessed by plotting each factor as a function of force on a log-log scale. N accounts for 39% and A for 49% of the variation in force in normal muscle. The contributions are changed after reinnervation where N, accounting for 65% of force, appears to compensate for the reduced range in A, which accounts for only 19% of the variation in force.(ABSTRACT TRUNCATED AT 400 WORDS)