Muscle use during dynamic knee extension: implication for perfusion and metabolism

Dynamic one-legged knee extension (DKE) iscommonly used to examine physiological responses to "aerobic"exercise. Muscle blood flow during DKE is often expressed relative toquadriceps femoris muscle mass irrespective of work rate.This is contrary to the notion that increased force is achieved byrecruitment in large muscles. The purpose of this study, therefore, wasto determine muscle use during DKE. Six subjects had magnetic resonance images taken of their quadriceps femoris before and after 4 min of DKEat 20 and 40 W. Muscle use was determined by shifts in T2. Thecross-sectional area of quadriceps femoris that had an elevated T2 was16 ± 1% (mean ± SE) preexercise, and 54 ± 5 and 94 ± 4% after 20- and 40-W DKE, respectively. Volume of quadriceps femorisincreased 11.4 ± 0.2% (P = 0.006), from 2,230 ± 233 cm³before exercise to 2,473 ± 232 cm³ after 40-W DKE. Extrapolationof these data indicates that 1,301 ± 111 cm³ of quadriceps femoris wereengaged during 20-W DKE compared with 2,292 ± 154 cm³ during 40-W DKE. By usingmuscle blood flow data for submaximal DKE at 20 W P. Andersenand B. Saltin. J. Physiol. (Lond.)366: 233-249, 1985; and L. B. Rowell, B. Saltin, B. Kiens, and N. J. Christensen. Am. J. Physiol. 251 (Heart Circ.Physiol. 20): H1038-H1044, 1986] andestimating muscle use in those studies from our data (total muscle mass × 0.54), extrapolated blood flow to active muscle (263 and 278 ml · min¹ · 100 g¹, respectively) iscomparable to that obtained during peak aerobic DKE when expressedrelative to total muscle mass (243 and 250 ml · min¹ · 100 g¹,respectively). These findings indicate that increasedpower during aerobic DKE is achieved by recruitment.Additionally, they suggest that blood flow to the active quadricepsfemoris muscle does not increase with increases in submaximal work ratebut instead is maximal to support aerobic metabolism. Thus increases inmuscle blood flow are directed to newly recruited muscle, not toincreased perfusion of muscle already engaged.