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J Appl Physiol (June 9, 2005). doi:10.1152/japplphysiol.00481.2005
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Submitted on April 27, 2005
Accepted on June 7, 2005

Differential Serial Sarcomere Number Adaptations in Knee Extensor Muscles of Rats is Contraction Type Dependent

Timothy A Butterfield1, Timothy R Leonard1, and Walter Herzog1*

1 Kinesiology, University of Calgary, Calgary, Alberta, Canada

* To whom correspondence should be addressed. E-mail: walter{at}kin.ucalgary.ca.

Sarcomerogenesis, or the addition of sarcomeres in series within a fiber, has a profound impact on the performance of a muscle by increasing its contractile velocity and power. Sarcomerogenesis may provide a beneficial adaptation to prevent injury when a muscle consistently works at long lengths, accounting for the repeated bout effect. The association between eccentric exercise, sarcomerogenesis and the repeated bout effect has been proposed to depend upon damage, where regeneration allows sarcomeres to work at shorter lengths for a given muscle-tendon unit length. In order to gain additional insight into this phenomenon, we measured fiber dynamics directly in the vastus lateralis (VL) muscle of rats during uphill and downhill walking, and measured serial sarcomere number in the VL and vastus intermedius (VI) following chronic training on either a decline or incline grade. We found that the knee extensor muscles of uphill walking rats undergo repeated active concentric contractions, and therefore suffer no contraction induced injury. Conversely, the knee extensor muscles during downhill walking undergo repeated active eccentric contractions. Serial sarcomere numbers change differently for the uphill and downhill exercise groups, and for the VL and VI muscles. Short muscle length for uphill concentric-biased contractions result in a loss of serial sarcomeres, and long muscle lengths for downhill eccentric-biased contractions result in a gain of serial sarcomeres.




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