Non-myotendinous force transmission in rat extensor digitorum longus muscle.

The extensor digitorum longus muscle (EDL) of the rat hindleg consists of four heads. The heads are named after their insertions on the digits of toes II, III, IV and V. The EDL heads share a proximal tendon and aponeurosis, but have separate distal aponeuroses and tendons. By cutting the distal tendons of selected heads, direct myotendinous force transmission within these heads is prevented. Therefore, force exerted by the muscle would be expected to decrease according to the physiological cross-sectional area disconnected if myotendinous force transmission were the only mechanism of force transmission. The results indicate that EDL force production remained at high levels after acute tenotomy: muscle length-force curves did not alter significantly following cutting of the tendons of heads II and III. Cutting the tendon of head IV as well leaves only head V in its original condition. After tenotomy of head IV, length-force characteristics were altered significantly, but optimum force was maintained at 84% of that of the intact muscle. After separation of head IV from head V intramuscularly for some distance along their interface, the force dropped to much lower levels, with optimum force approaching 50% of that of the intact muscle. The length of active proximal fibres (located within head II) did not remain constant but increased with increasing muscle lengths after tenotomy as well as after partial separation of heads IV and V. The amount of length change decreased after intramuscular separation of the heads, indicating declining reactive forces. It is concluded that force transmission occurred from tenotomized heads to their intact neighbours and vice versa. The magnitude of the force transmitted from head to head was dependent on the degree of integrity of the connective tissue at the interface between heads.