Single-Fiber MHC Polymorphic Expression Is Unaffected by Sprint Cycle Training ALLEN C. PARCELL, ROBERT D. SAWYER, MICAH J. DRUMMOND, BROCK O’NEIL, NATHAN MILLER, and MANDY T. WOOLSTENHULME Human Performance Research Center, Brigham Young University, Provo, UT ABSTRACT PARCELL, A. C., R. D. SAWYER, M. J. DRUMMOND, B. O’NEIL, N. MILLER, and M. T. WOOLSTENHULME. Single-Fiber MHC Polymorphic Expression Is Unaffected by Sprint Cycle Training. Med. Sci. Sports Exerc., Vol. 37, No. 7, pp. 1133–1137, 2005. Purpose: The present investigation examined single-fiber MHC alterations in response to high-intensity, short-duration, sprint cycle training. Methods: Ten untrained college-age male subjects participated in 8 wk of a progressive sprint cycle training program. Training involved 15-s maximal sprints separated by 5 min of rest beginning with four sprints  2 d in week 1 and increasing to six sprints  3 d at week 8. Muscle samples from the vastus lateralis were obtained before and after training. A 30-s sprint cycle test was used to evaluate performance before and after training. Results: For the 30-s sprint, mean power and total work increased from pre to post. Single-fiber analyses revealed a reduction in the MHC IIx isoform (2.0  1.0 to 0.2  0.1%, pre to post, P  0.05) and an increase in MHC IIa (P = 0.08), whereas there was no change in hybrid fiber composition (total hybrids = 24%). Generally, MHC IIa content increased and MHC IIx decreased (P  0.05) as demonstrated by homogenate analyses of tissue samples. Conclusions: We report that as little as 32 min of high-intensity sprint cycle training over 8 wk is sufficient to improve sprinting performance. This training response is accompanied by an increase in MHC IIa and reduction in MHC IIx content of the vastus lateralis. However, short-duration, high-intensity, sprint cycle training does not cause a reduction in hybrid muscle fiber content. Key Words: HYBRID FIBER, SKELETAL MUSCLE, CYCLING, MYOSIN HEAVY CHAIN, SDS-PAGE T he myosin molecule in human skeletal muscle is composed of two heavy chains and their associated globular protein heads. Myosin heavy chain (MHC) is a primary determinant of the shortening velocity of a muscle fiber. Most human muscle fibers contain only one of three MHC isoforms (I, IIa, IIx); however, single-fiber MHC polymorphic expression is common in untrained, aged, and elite athlete populations (5,6,15,18). Interestingly, the MHC isoform composition of a muscle demonstrates considerable plasticity in response to an exercise-training stimulus (2,24). Several studies have reported a general reduction in the MHC IIx isoform and hybrid fiber content in response to a progressive resistance-training program. Homogenate anal- yses of tissue samples after 19 wk of heavy resistance exercise by Adams and colleagues (2) showed a 12% re- duction in MHC IIx with a concomitant 12% increase in MHC IIa isoforms. Williamson et al. (24,25) have clearly shown that hybrid muscle fiber percentages in the vastus lateralis decline in response to 12 wk of knee-extension resistance training regardless of age or gender. Like resistance training, subjects exposed to high-inten- sity sprint cycle training show an increase in the MHC IIa content in muscle (3). However, some investigators have observed maintenance of the MHC IIx isoform subsequent to the training period (10,12). This finding is unique in that a reduction in the MHC IIx isoform in response to an exercise overload stimulus is a common response that is observed in myosin ATPase histochemical, MHC homoge- nate, and single-fiber MHC isoform analyses (2,9,24). ATPase histochemistry displays the predominant isoform within a cell and is dependent on subjective assessment of a color gradient. Analyses of MHC isoforms from tissue ho- mogenates describe the general distribution of isoforms in the sample. Single-fiber MHC analyses permits the objec- tive assessment of MHC isoform composition of an indi- vidual muscle cell (7). In the aforementioned sprint training studies, it is possible that the observed maintenance of the MHC IIx isoform (IIb ATPase) aspect of the tissue samples is the result of misclassification of MHC IIa/IIx muscle fibers. In addition, it would appear that much of the IIx MHC in human skeletal muscle exists primarily in muscle fibers classified as MHC IIa/IIx hybrids (5,18,25). The Address for correspondence: Allen C. Parcell, Ph.D., Department of Ex- ercise Sciences, Human Performance Research Center, Brigham Young University, 120-E Richards Building, Provo, UT 84602; E-mail: allen_parcell@byu.edu. Submitted for publication November 2004. Accepted for publication March 2005. 0195-9131/05/3707-1133/0 MEDICINE & SCIENCE IN SPORTS & EXERCISE ® Copyright © 2005 by the American College of Sports Medicine DOI: 10.1249/01.mss.0000170123.27209.e1 1133