Increased oxidative capacity does not protect skeletal muscle fibers from eccentric contraction-induced injury.

Isometric electrical stimulation was delivered to rabbit dorsiflexor muscles at 10 Hz for 1 s on and 1 s off over 30 min, 5 days/wk for 3 wk to induce an increase in muscle oxidative capacity. Stimulation-trained muscles as well as untrained muscles were then subjected to a 30-min eccentric exercise bout to test whether increased oxidative capacity provided a protective effect against muscle injury. Electrical stimulation results in significant training of both the extensor digitorum longus (EDL) and tibialis anterior (TA) muscles, with EDL citrate synthase (CS) activity increasing an average of 67% (P < 0.0001) and TA CS activity increasing by 27% (P < 0.05). For all parameters measured, the magnitude of change was much greater for EDL than for TA muscle. Dorsiflexor fatigability decreased significantly during the 3-wk training period (P < 0.0001), whereas the EDL TA individually showed strong decreasing trends in fatigability after training. TA and EDL capillary density measured histomorphometrically increased from 839 +/- 56 to 1,026 +/- 71 mm-2 (P = 0.07) and from 589 +/- 37 to 792 +/- 66 mm-2 (P < 0.05), respectively. TA and EDL capillary-to-fiber ratio increased from 1.32 +/- 0.10 to 1.55 +/- 0.16 (P > 0.2) and 1.08 +/- 0.07 to 1.36 +/- 0.14 (P > 0.1), respectively. Type 2A fiber type percentage increased after stimulation training by 68% (P < 0.0001) for the EDL and by 32% (P > 0.1) for the TA at the expense of type 2D fibers. Despite the large training effect for the EDL and the modest training effect for the TA, no differences were observed between stimulation-trained and untrained groups for maximum dorsiflexion torque (P > 0.3) or maximum tetanic tension (P > 0.3) after eccentric contraction-induced injury. Additionally, no significant correlation was observed between CS activity and maximum tetanic tension after eccentric contraction-induced injury for either muscle (P > 0.2). Thus we conclude that increasing muscle oxidative capacity by isometric electrical stimulation training did not protect muscle against eccentric contraction-induced injury.

Force transmission in skeletal muscle: from actomyosin to external tendons.

The actual path of force transmission in skeletal muscle from actomyosin interaction to tension at the tendinous insertion site is poorly understood. Within the muscle cell, endo- and exosarcomeric cytoskeletal proteins create series and parallel connections between contractile proteins resulting in a meshwork across which force can be transmitted in practically any direction with respect to the fiber axis. At the surface membrane, connections between the intermediate filament system, dystrophin, and specialized membrane complexes provide the route of force transmission to the extracellular matrix material. Finally, parallel and series connections between muscle fibers allow radial and longitudinal forces to converge on the connective tissue matrix. This complex pathway will certainly be the subject of future studies in muscle biology, biomechanics, and physiology.

Skeletal muscle architecture and fiber-type distribution with the multiple bellies of the mouse extensor digitorum longus muscle.

The purpose of this study was to describe the extent to which architectural and fiber-type characteristics of the four bellies of the mouse extensor digitorum longus (EDL) suggest specialization of the digits, and to mathematically model the functional effects of the structural properties. Six mice were perfused in situ with glutaraldehyde while the lower limb was positioned approximately in the neutral position. After perfusion, lower limbs were removed and placed in glutaraldehyde until the EDL was dissected from the limb and separated into individual muscle bellies corresponding to each digit for architectural determination. The results showed that the muscle belly of digit 5 tended to be different from the muscle bellies of digits 2-4 for many architectural characteristics. Muscle mass, physiological cross-sectional area, muscle length, and fiber length were all significantly greater in digit 5. Proximal tendon length was also significantly longer in digit 5, and distal tendon length, as well as total tendon length, were significantly shorter in digit 5. Sarcomere length was shortest at the proximal end of the muscle and longest, 60-80%, toward the distal end. Fiber type distribution was about 60% FOG, 39% FG with only 1% SO fibers in all muscle bellies. Muscle-tendon modeling illustrated that peak force and maximal shortening velocity were greatest in digit 5. Inclusion of the tendon in the model resulted in a 10% shift of the force-length curve to longer lengths. Assuming muscle structure is matched to function, we speculate that digit 5 of the mouse EDL bears higher loads over a greater excursion during locomotion compared to the remaining digits.

Estrogens in food: the almond mystery.

Studies were made of 36 different nuts, grains, fruits and vegetables commonly used as human foods; each of these was fed to a group of ovariectomized rats for 10 days as a sole diet. The estrogenic activities of the foods were estimated by comparing the uterine weights, uterine fluid volumes and the vaginal cornification indices of each group of rats with those of groups fed other foods. Almonds, cashew nuts, peanuts, oats, corn, wheat and apples all showed estrogenic activity. The original sample of almonds showed the greatest estrogenic activity (p less than 0.01) which was confirmed by repetition of the experiment (p less than 0.01), but subsequent studies of other samples of almonds showed no estrogenic activity. Possible reasons for the disparity of the results with different lots of almonds are discussed.