DYNAMIC KNEE-EXTENSOR AND CYCLE EXERCISE - FUNCTIONAL MRI OF MUSCULAR-ACTIVITY

Repeated studies using human dynamic knee-extensor exercise have repor ted high mass specific blood flows. These studies suggest that the hig h perfusion-to-muscle mass ratio can approach 400 ml(-1).min.100 g(-1) in the human quadriceps. However, in these studies mass specific bloo d flows were calculated based on the assumption that the quadriceps ar e the only muscles involved in the knee-extensor exercise, which is di fficult to verify in an in vivo human model. Previous validations of t his assumption have been performed using electromyography (EMG) and as sessments of strain gauge tracings, but neither has been able to compl etely assess the involvement of all thigh muscles in this exercise. To address this issue four subjects exercised at 90% of their work rate maximum for 2.0-2.5 minutes (45-100 watts) and then a transverse secti on of the thigh (20cm proximal to the knee) was studied using proton ( H-1) transverse relaxation time (T-2) weighted magnetic resonance (MR) imaging to distinguish active from non-active muscles by the increase d signal intensity (SI). On a separate occasion, measurements followin g 2.0-2.5 minutes of conventional two legged cycle ergometry at 90% of maximum work rate (150-400 watts) were made in the same subjects to c ontrast this traditional ''whole leg'' exercise with the unique muscle recruitment in dynamic knee-extension, Following knee-extensor exerci se there was a clearly visible change in SI and a significant increase in T-2 only in the four muscles of the quadriceps (P<0.05). After bic ycle exercise SI changes and T-2 revealed a varied muscle use across a ll muscles. From these MR data it can be concluded that unlike cycle e xercise, in which all muscles are recruited to varying extents, single leg knee-extensor exercise is limited to the four muscles of the quad riceps. Thus, the common practice of normalizing blood flow and metabo lic data to the quadriceps muscle mass in human knee-extensor exercise studies appears appropriate.