Cardiovascular responses during isometric exercise following lengthening and shortening contractions

Abstract

The present study investigated the effects of prior lengthening or shortening contractions on cardiovascular responses during isometric exercise. We utilized the history dependence of skeletal muscle, where active 2-s lengthening or shortening before an isometric contraction can increase [residual force enhancement (RFE)] or decrease [force depression (FD)] force production. Matching torque output between RFE and FD conditions yields lower and higher electromyography (EMG) values, respectively. In study 1, heart rate and perceived exertion (PE; Borg10) were measured in 20 participants during 20-s isometric plantar flexion contractions at low (16 ± 4% MVC)-, moderate (50 ± 5% MVC)-, and high (88 ± 7% MVC)-intensity. In study 2, heart rate and blood pressure were measured in 14 participants during 2-min isometric plantar flexion contractions (40% MVC). In both studies, torque output was held constant between FD and RFE conditions resulting in differences in soleus EMG activity ( P < 0.05). In study 1, PE was lower during the RFE condition ( P < 0.01), while increases in heart rate were similar between FD and RFE at low (∆2 ± 8 vs. 3 ± 6 beats/min, P > 0.99) and moderate (∆14 ± 9 vs. 14 ± 9 beats/min, P > 0.99) intensity but smaller during RFE at high intensity (∆35 ± 13 vs. 29 ± 13 beats/min, P = 0.004). In study 2, heart rate responses were smaller in the RFE condition following the initial 20-s period; diastolic blood pressure responses were smaller during the last 80 s. A 2-s active change in muscle length before an isometric contraction can influence heart rate and blood pressure responses; however, these differences appear to be modulated by both intensity and duration of the contraction. NEW & NOTEWORTHY Using the history dependence of isometric force to alter maximal torque production and motor unit activation between residual force enhancement and force depression conditions, we observed that heart rate responses were different between conditions during a subsequent 20-s high-, but not low- or moderate-, intensity isometric contraction. A 2-min moderate-intensity contraction revealed time-dependent effects on heart rate and diastolic blood pressure. Active 2-s shortening and lengthening before an isometric contraction can influence the cardiovascular responses.

Keywords: blood pressure; exercise; heart rate; history dependence of force; isometric.

Fig. 1.

Representative figure of the torque, joint angle, and right soleus muscle activation (EMG) produced during passive movement (column A), preactivation, and active muscle lengthening residual force enhancement (RFE) condition] or shortening [force depressed (FD) condition] (column B), and a 16 s isometric plantar flexion contraction (column C). Of note, participants were required to hold the isometric contraction for an additional 1–2 s to ensure constant torque production throughout the exercise period.

Fig. 2.

Right soleus muscle activation (EMG_RMS) as a percentage of the maximum M-wave amplitude (M-max), indicating maximal motor unit activation, between residual force enhancement (RFE) and force depressed (FD) conditions at low-, moderate-, and high-intensity isometric plantar flexion contractions. The low-, moderate-, and high-intensity contractions were completed at 16 ± 4, 50 ± 5, and 88 ± 7% of maximal voluntary contraction (MVC), respectively. Data were obtained in 20 participants and are expressed as means ± SE.

Fig. 3.

Perceived exertion (PE; A) and the change in heart rate (B) between residual force enhancement (RFE) and force depressed (FD) conditions at low-, moderate-, and high-intensity isometric plantar flexion contractions. The low, moderate, and high intensity contractions were completed at 16 ± 4, 50 ± 5, and 88 ± 7% of maximal voluntary contraction (MVC), respectively. Data were obtained in 20 participants and are expressed as means ± SE. AU, arbitrary units.

Fig. 4.

The change in heart rate (A), systolic (B), and diastolic (C) blood pressure during 2-min of 40% plantar flexion isometric contraction in the residual force enhancement (RFE) and force depressed (FD) conditions. Data were obtained in 14 participants and are expressed as means ± SE.