Author(s): DAY, E., HUDGINS, J., PALLONE, R., SOPEÑA, B., CONRAD, B., SUMNER, J., Institution: BROOKS SPORTS, Country: UNITED STATES, Abstract-ID: 2617

INTRODUCTION:
Mechanical work is commonly used in biomechanics to explain changes in whole-body metabolic cost [1]. In a fatiguing run, stance phase joint work shifts from the ankle to knee, hypothesized to increase metabolic cost due to higher energy cost of contracting the quadriceps [2]. However, joint work does not reflect internal muscle contraction dynamics (e.g. muscle activation, contractile vs. series-elastic contributions, fiber lengths, etc.) that influence the local metabolic stress (LMS) of the muscles performing work. Therefore, the purpose of this study was to determine if changes in mechanical joint work and LMS of the muscles at the knee are correlated during a fatiguing run.
METHODS:
Ten recreational runners completed a 45 min treadmill run at 88% of their estimated maximal metabolic steady-state speed. Stance phase sagittal plane total knee joint work (Wtot = W+ + |W-|) was calculated from biomechanical data . Muscle oxygen saturation (%SmO2) slope of the rectus femoris (RF), representing LMS, was assessed via near-infrared spectroscopy. To account for initial exercise muscle oxygenation dynamics, data from 30 -100% of run duration were analyzed. Rating of perceived exertion (RPE; Borg scale) was collected at the start and end of run.
RESULTS:
RPE rose from 9 ± 2 to 16 ± 2, indicating a perceived effort of ‘very hard’ at the end. There was a moderate but not significant correlation between %SmO2 slope and percent change in total knee joint work (r = 0.362, p = 0.33). Post-hoc review of individual runner data showed that the group was evenly split between runners with a negative or positive %SmO2 slope. Of the 5 runners with a negative %SmO2 slope, 4 had a decrease in joint work (-2.5% average); and of the 5 runners with a positive %SmO2 slope, all had an increase in joint work (+6.6% average).
CONCLUSION:
A negative %SmO2 slope, indicating increased LMS, was not strongly correlated with the percent change in joint work performed at the knee, suggesting no strong linear relationship between the LMS of the RF and stance phase knee joint work. This suggests that changes in knee joint work should be interpreted with caution when drawing conclusions to the quadriceps metabolic demand. While joint work and %SmO2 slope were not significantly correlated, this suggests that more exploration into the relationship between muscles’ metabolic environment and mechanical work is warranted. These findings challenge the assumption that changes in knee joint work directly reflect quadriceps metabolic demand during fatigue; and suggest that muscle metabolic stress reflects a complex, integrated response – highlighting the need for interdisciplinary approaches.

References
1. Riddick & Kuo. Sci Reports 2022; 12:645.
2. Sanno M., Willwacher, S., Epro, G., Bruggemann, G-P. Med Sci Sport Exerc. 2018. 2507-2517.