abstract
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It is known that high-resistance training induces morphological changes in skeletal muscle. Following a resistance training program, increases in maximum torque generating capacity are observed due to both neural adaptations and hypertrophic gains within the trained muscle. Although it has been established that a muscle hypertrophies due to the addition of myofibrillar proteins through increased protein synthesis, the exact mechanism which stimulates the hypertrophic response is unknown.
Previous reports have shown that training in the absence of eccentric contractions generally produces less muscle growth and strength gains, as well as inflicting less damage to the muscle ultrastructure. Likewise, fast eccentric contractions have been shown to increase muscular strength to a greater extent than slow contractions. It has been hypothesized that fast eccentric contractions may maximize muscular damage, thus invoking a greater response of repair mechanisms, including satellite cell recruitment, which would allow an increased addition of contractile proteins to be added to the injured muscle, increasing muscle size and strength to a greater degree.
The effect of fast and slow eccentric training was investigated using a bilateral, within subject model. Twelve men trained one arm fast (3.66 rad/s) and one arm slow (0.52 rad/s) for 8 weeks on an isokinetic training apparatus. Type I muscle fibre size increased with training by an average of 9.3±12.0% (P<0.05, main effect for time). Type II muscle fibres increased more in the subjects' fast trained arm when compared to the slow trained arm according to ATPase histochemical analysis (P<0.05, time x condition interaction). Likewise, whole arm cross-sectional area showed that the fast trained arms had an average increase of 6.8±5.5 % whereas the slow arms only had an average increase CSA of 5.1±5.7% (P=0.065, time x condition interaction). Maximum torque generating capacity was also increased to a greater degree (P<0.05, time x condition interaction) in the fast trained arm with an average of 10.3±16.4 Nm, whereas the slow trained arm increased only 7.3±15.0 Nm, across testing speeds. A decrease in the percentage of type IIx fibres was seen in both arms after training according to both ATPase histochemical staining and MHC gel electrophoresis; however, the percentage of type IIa fibre area increased in the fast trained arms (8.4±8.6%) more significantly (P<0.05, time x condition interaction) than the slow trained arms (1.7±10.9%).
Seven males were trained in a similar manner to determine the extent of muscle damage which was evaluated by both Z-band streaming and force production decrements. After a single exercise bout of fast eccentric training in one arm and slow eccentric training in the other, it was determined that a 1.97±0.74 areas of moderate Z-band streaming per mm^2 of muscle in the fast exercised arm compared to 0.89±0.79 areas of moderate Z-band streaming per mm^2 of muscle in the slow trained arm (P<0.05). In conclusion, training using fast (3.66 rad/s) eccentric contractions causes a greater degree of muscle damage, hypertrophy, and strength gains than does training with slow (0.52 rad/s) eccentric contractions.