Role of absolute muscle strength in determining the blood pressure response to static and dynamic knee extension

Relative contraction intensity is considered the primary factor determining the hemodynamic response to exercise, though premenopausal women still commonly present with lower blood pressure (BP) and heart rate responses. Our group has shown that within‐ and between‐sex differences in BP responses during static handgrip are abolished when controlling for maximal voluntary effort (MVE). Whether these findings apply to static contractions in the lower limb, dynamic contractions, or are affected by differences in contraction intensity remain untested. 27 young healthy men and women (23±3 years [mean ±SD]; 10 women) performed static and dynamic single leg knee extensor exercise on 2 separate visits, with one modality being tested during a single visit. Both contraction types were performed at 3 separate intensities, 10% MVC, 30% MVC, and 25 nM as an absolute intensity, with 20 minutes of recovery between bouts. All static contractions were held for 2 minute bouts, whereas the dynamic tasks were performed for 3 minutes. Dynamic contractions were performed at a 1:2 second work‐to‐rest ratio at a velocity of 60□/second. The intensities were tested in order from lowest to highest. At baseline and during exercise, continuous BP and heart rate were measured using finger photoplethysmography and single‐lead electrocardiography, respectively. MVC was measured from the left knee extensors on a dynamometer (HUMAC‐norm) at 80□ of knee flexion, and voluntary activation was assessed using the interpolated twitch technique, with ≥90% voluntary activation of the knee extensors set as the acceptable activation threshold. To compare those with HIGH and LOW muscle strength, we split males and females separately by the median MVC. BP and heart rate responses were assessed as the difference from baseline to the last minute of exercise. The groups were matched for baseline characteristics aside from the HIGH group having a higher body mass (78±13 vs. 67±9 kg, p=0.02). By design, knee extensor MVC was different between groups (169±57 vs. 114±38 Nm, p=0.009). During the second minute of the 10% static contractions, diastolic BP responses were larger in HIGH compared to LOW participants (Δ10±4 vs. 6±3 mmHg, p=0.01). The changes in systolic BP (Δ16±9 vs. 10±7 mmHg, p=0.06) and heart rate (Δ11±8 vs 6±4 beats/minute, p=0.07) were also trending towards being larger in HIGH participants. In contrast, the changes in BP and heart rate were not statistically different during the 30% static MVC contraction or the absolute intensity contraction between groups (all p=0.17). During 10% dynamic contractions, systolic BP responses were trending towards being larger in HIGH participants (Δ8±6 vs. 4±4 mmHg, p=0.06), while heart rate responses were greater during 30% dynamic contractions in HIGH participants (Δ14±5 vs. 9±6 beats/minute, p=0.02). In conclusion, our data suggest that absolute torque affects BP and heart rate responses during lower limb static and dynamic knee extension exercise. The mechanisms responsible for how absolute torque influences BP and heart rate warrants future investigation. Support or Funding Information Natural Sciences and Engineering Research Council of Canada (NSERC) Discovery Grant; Canada Foundation for Innovation

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