Specific force of the rat extraocular muscles, levator and superior rectus, measured in situ

Extraocular muscles are characterized by their faster rates of contraction and their higher resistance to fatigue relative to limb skeletal muscles. A nother often reported characteristic of extraocular muscles is that they ge nerate lower specific forces (sP(o), force per muscle cross-sectional area, kN/m(2)) than limb skeletal muscles. To investigate this perplexing issue, the isometric contractile properties of the levator palpebrae superioris ( levator) and superior rectus muscles of the rat were examined in situ with nerve and blood supply intact. The extraocular muscles were attached to a f orce transducer, and the cranial nerves exposed for direct stimulation. Aft er determination of optimal muscle length (L-o) and stimulation voltage, a full frequency-force relationship was established for each muscle. Maximum isometric tetanic force (P-o) for the levator and superior rectus muscles w as 177 +/- 13 and 280 +/- 10 mN (mean +/- SE), respectively. For the calcul ation of specific force, a number of rat levator and superior rectus muscle s were stored in a 20% nitric acid-based solution to isolate individual mus cle fibers. Muscle fiber lengths (L-f) were expressed as a percentage of ov erall muscle length, allowing a mean L-f to L-o ratio to be used in the est imation of muscle cross-sectional area. Mean L-f:L-o was determined to be 0 .38 for the levator muscle and 0.45 for the superior rectus muscle. The sP( o) for the rat levator and superior rectus muscles measured in situ was 275 and 280 kN/m(2), respectively. These values are within the range of sP(o) values commonly reported for rat skeletal muscles. Furthermore P-o and sP(o ) for the rat levator and superior rectus muscles measured in situ were sig nificantly higher (P< 0.001) than P-o and sP(o) for these muscles measured in vitro. The results indicate that the force output of intact extraocular muscles differs greatly depending on the mode of testing. Although in vitro evaluation of extraocular muscle contractility will continue to reveal imp ortant information about this group of understudied muscles, the lower sP(o ) values of these preparations should be recognized as being significantly less than their true potential. We conclude that extraocular muscles are no t intrinsically weaker than skeletal muscles.