(19) ACUTE MUSCULAR SWELLING AND ECHO INTENSITY ARE PREDICTORS OF ACUTE FATIGUE ONLY UNDER LOW LOADS

Purpose: We investigated the differences in acute effects between high and low load lower body resistance exercise on changes in muscular swelling assessed by ultrasound (US) imaging of the quadriceps. Secondarily, we examined the correlation between changes in US and EMG measures and fatigue, quantified as changes in MVIC force production following exercise. Purpose: We investigated the differences in acute effects between high and low load lower body resistance exercise on changes in muscular swelling assessed by ultrasound (US) imaging of the quadriceps. Secondarily, we examined the correlation between changes in US and EMG measures and fatigue, quantified as changes in MVIC force production following exercise. Methods: Twenty-nine (13M/16F) volunteers who regularly performed resistance training partook in this cross-over study. Participants were assigned to perform 3 sets of unilateral leg extensions with high- (HL; 80% 1RM) or low-load (LL; 30% 1RM) to volitional failure. US images of VL, VM and RF were taken pre- and post-exercise, blinded and analyzed for muscle thickness (MT) and echo intensity (EI). EMG sensors recorded muscular activity of the VL, VM and RF. Twenty-four hrs. later, participants repeated the same protocol with the opposite condition on the other leg. All inferential statistics were run using R 4.4.2. and correlation analysis was run in Excel. All data is presented as mean±95% CI. Results: There were no significant muscle(M) × time(T) × condition(C) × sex(S) or lower-order 3-way interactions for any variable (p=0.081-0.959. For all muscles, there were sig. M×T, C×T, and S×T interactions for MT and S×T and S×M interactions for EI (p=0.0001-0.0460). Post-hoc analysis indicated that when collapsed across S and C, all studied muscles significantly increased in MT as a result of the exercise (VL PRE: 2.4 [2.2-2.5] cm; VL POST: 2.6 [2.5-2.8] cm; VM PRE: 2.7 [2.6-2.9] cm; VM POST: 3.0 [2.8-3.1] cm; RF PRE: 2.5 [2.3-2.6] cm; RF POST: 2.8 [2.7-3.0] cm; p=0.0001-0.005. When collapsed across S and M, MT at PRE was significantly different from POST for both conditions (LL PRE: 2.5 [2.4-2.6] cm; LL POST: 2.9 [2.7-3.0] cm; HL PRE: 2.5 [2.4-2.6] cm; HL POST: 2.8 [2.6-2.9] cm; p’s0.0001), while PRE and POST measures were not sig. diff. between HL and LL (p’s >0.983). When collapsed across C and M, both sexes experienced an increase in EI post-exercise (M PRE: 60.9 [55.5-66.3] au; M POST: 73.8 [68.4-79.2] au; F PRE: 75.8 [70.9-80.6] au; F POST: 81.3 [76.5-86.2] au; p=0.0001-0.001. However, females had higher PRE EI than males (F PRE: 75.8 [70.9-80.6] au; M PRE: 60.9 [55.5-66.3] au; p=0.001) and there was no sig. diff. between sexes at POST (F POST: 81.3 [76.5-86.2] au; M POST: 73.8 [68.4-79.2] au; p=0.253). When collapsed across C and T, male and female VL’s were not sig. diff. while VM and RF were. Linear regression analysis indicated that increases in VM and RF MT and EI and were sig. predictors of MVIC force loss (r’s -0.370 to -0.493, p’s≤.048) in the LL condition. There were no sig. US-derived predictors of MVIC force loss in the 80% condition; however, EMG changes between 1st and last rep of sets 1 and 3 (r’s 0.382-0.393, p’s≤0.041) were sig. correlated with MVIC reduction in HL. Conclusions: Neither acute muscular swelling nor EI was influenced by relative load. Since basal levels of EI are influenced by biological sex, it is difficult to compare effects due to exercise. The differences between male and female EI are driven by the VM and RF, not the VL. Future research should focus on the underlying mechanisms for sex-based differences in muscular echo intensity. Practical Applications: Changes in fatigue are closely associated with muscular swelling and metabolite accumulation under low load conditions, while neuromuscular changes are more predictive at high loads. Acknowledgements: This project was funded by a SURF grant awarded to AK.