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1 Departments of Orthopaedics, Medicine, and Bioengineering, Biomedical Sciences Graduate Group, University of California and Veterans Affairs Medical Centers, San Diego, California 92161; and 2 Department of Hand Surgery, Göteborg University, Göteborg, Sweden S-412
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 resulted 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 and 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.
muscle metabolism; fiber type; muscle mechanics
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