On methods of evaluation of lower extremity eccentric muscle performance and loading

This investigation has developed and evaluated methods for measuring eccentric muscle function and loading of the human lower extremity, including isokinetic tests of knee extensor muscle performance and a comprehensive in vivo method for conditions of functional loading. For measurement of knee extensor muscle performance, a protocol of 100 repetitive maximal actions at 90° was applied using a Biodex isokinetic dynamometer. The test-retest reliability was studied in sixteen healthy volunteers. Intra Class Correlation coefficients (ICCs) for eccentric knee extensor endurance indicated high reliability for total work (Joules) and absolute endurance (ICC, 0.92 and 0.93) but not for relative endurance (ICC, 0.71). The validity of this test with reference to treadmill running capacity was studied in nine male elite middle-distance runners.

Eccentric total work was significantly related to submaximal oxygen uptake (ml kg~0 75 min~') at all three running velocities investigated (12, 14, and 15 km-h~~). The R2-values indicated that 48-59% of the variability in submaximal oxygen uptake could be explained by the level of eccentric total work. Further, the isokinetic test was used for describing knee extensor endurance in seventeen professional ballet dancers, a group exposed to daily repetitive eccentric loading, in relation to a control group of moderately active subjects. Eccentric total work/kg was 31% greater for the dancers compared with the controls (p<0.001). The eccentric/concentric total work ratio was greater in the dancers (x =2.07) compared with the controls (x =1.74) (p<0.001).

The isokinetic technique used quantifies isolated muscle performance. When studying functional loading, the implementation of a comprehensive approach may be more appropriate. A new experimental method integrating four different measurement systems was developed and tested in seven healthy volunteers. The surgical procedure for implantation of two tibia force transducers in local anesthesia was uncomplicated. The transducers consisted of straingauges mounted on a surgical staple and was designed to measure local bone deformation. For the pilot study, two conditions of forefoot or heel landing after a standardized jump were studied. The local peak tibia deformation occurred at 20-42 ms (median) after ground contact and was up to eight times higher during stance phase loading compared to standing still on one leg.

Systematic differences in ground reaction force and muscular activation patterns were observed for the different conditions of foot strike. With this experimental method, the effect of moderate duration exercise on muscle activation characteristics, local tibia deformation, ground reaction forces, and lower extremity kinematics during standardized jumping and landing was studied in five volunteers. The peak vertical ground reaction force and tibia force transducer amplitude occurred earlier with respect to foot strike after 30 minutes of ergometer cycling at a perceived heavy exertion level, the median changes were -22 ms (p=0.04) and -12 ms (p=0.04), respectively. This shift in timing of peak loads was not accompanied by a similar shift in time of peak lower extremity muscle activation.

In summary, measurement of isokinetic eccentric knee extensor endurance may be utilized in clinical evaluation and research if issues of reliability and validity are considered. The presented comprehensive experimental in vivo method for simultaneous measurement of muscle activity, ground reaction forces, local bone deformation, and joint motion may be of interest for further studies of loading conditions associated with the etiology of lower extremity overuse injuries.