Cardiovascular and Muscular Consequences of Work-Matched Interval-Type of Concentric and Eccentric Pedaling Exercise on a Soft Robot

Abstract

Eccentric types of endurance exercise are an acknowledged alternative to conventional concentric types of exercise rehabilitation for the cardiac patient, because they reduce cardiorespiratory strain due to a lower metabolic cost of producing an equivalent mechanical output. The former contention has not been tested in a power- and work-matched situation of interval-type exercise under identical conditions because concentric and eccentric types of exercise pose specific demands on the exercise machinery, which are not fulfilled in current practice. Here we tested cardiovascular and muscular consequences of work-matched interval-type of leg exercise (target workload of 15 sets of 1-min bipedal cycles of knee extension and flexion at 30 rpm with 17% of maximal concentric power) on a soft robotic device in healthy subjects by concomitantly monitoring respiration, blood glucose and lactate, and power during exercise and recovery. We hypothesized that interval-type of eccentric exercise lowers strain on glucose-related aerobic metabolism compared to work-matched concentric exercise, and reduces cardiorespiratory strain to levels being acceptable for the cardiac patient. Eight physically active male subjects (24.0 years, 74.7 kg, 3.4 L O2 min^-1), which power and endurance performance was extensively characterized, completed the study, finalizing 12 sets on average. Average performance was similar during concentric and eccentric exercise (p = 0.75) but lower than during constant load endurance exercise on a cycle ergometer at 75% of peak aerobic power output (126 vs. 188 Watt) that is recommended for improving endurance capacity. Peak oxygen uptake (-17%), peak ventilation (-23%), peak cardiac output (-16%), and blood lactate (-37%) during soft robotic exercise were lower during eccentric than concentric exercise. Glucose was 8% increased after eccentric exercise when peak RER was 12% lower than during concentric exercise. Muscle power and RFD were similarly reduced after eccentric and concentric exercise. The results highlight that the deployed interval-type of eccentric leg exercise reduces metabolic strain of the cardiovasculature and muscle compared to concentric exercise, to recommended levels for cardio-rehabilitation (i.e., 50-70% of peak heart rate). Increases in blood glucose concentration indicate that resistance to contraction-induced glucose uptake after the deployed eccentric protocol is unrelated to muscle fatigue.

Keywords: concentric; eccentric; feedback; lactate; muscle; power; respiration; robot.

Figure 1

Sketch summarizing the study protocol. Subjects conducted a number of tests over 5 weeks with ample time to allow recovery from the individual types of exercise. In the first weeks this included a familiarization to the equipment. In the second week subjects performed a ramp test (to determine maximal cardiovascular performance) and an endurance exercise at constant load (to determine the time-to-exhaustion). In the third week the subjects carried out power tests on the soft robot or force plate to determine maximal force and power of single knee extensions. In the fourth and fifth week, subjects carried out a session of interval-type concentric exercise (week 4), and eccentric exercise (week 5), on the soft robot with the same overall work output for both legs. Thereby force and blood serum measurements were made before and after the exercise sessions (arrows). Below, scheme summarizing the 15 sets of 1-min exercise, during which 30 contractions (work cycles) were performed with each leg, and the subsequent periods of recovery.

Figure 2

Work diagrams of the interval-types of concentric and eccentric exercise. Line graphs visualizing the sampled mechanical characteristics during 30 work cycles of each leg within one set of the concentric (A) and eccentric (B) interval exercise on the soft robot. Red and blue lines symbolize the measured parameters for the left (blue) and right leg (red). The length of a work cycle is indicated with a swung bracket (top left in panel A). The zones corresponding to the phases where positive and negative work is developed are indicated. Values for power, force, velocity and arc length (arc_len) are shown. Note a considerable portion of the power is produced points in the negative direction for the concentric protocol, and the positive direction for the eccentric protocol, respectively. (C,D) Line graphs visualizing the average of normalized force output in different sets of the interval-type of concentric (C) and eccentric exercise (D). The direction in which work is performed is indicated with arrowheads. The inspection of the force: length relationships in (C,D) of the same representative subject reveals the average contribution of positive and negative force production at different arc length, and differences between sets, to the work cycle within an exercise protocol. For instance, (C) shows that force transits from nearly constant and higher levels in the extension phase to lower and equally constant values at the largest arc length (when legs become fully extended) in the flexion phase during the concentric interval exercise. In consequence, eccentric force is developed during the concentric protocol of interval exercise. Note that in the last cycle of this protocol the force developed during, both, the eccentric and concentric phase is higher for the displayed subject. Conversely, in (D) showing the work cycle during the eccentric interval exercise, one can recognize that the developed forces are higher during the flexion compared to the extension phase. Thereby the forces developed in both phases are less regular and more distinct than for the concentric exercise.

Figure 3

Reduction in mechanical output after interval exercise on the soft robot. Box Whisker plots visualizing the median + standard error (box and central line) and minima/maxima (whisker) of the post- vs. pre- exercise differences in Ppeak and the RFD for the concentric and eccentric protocol, respectively. Power values were assessed by the two-legged reactive power test, real power test, negative power test, CMJ, SJ, and the one-legged multiple jumps. RFD was determined for the reactive power test, real power test and negative power test. $: p < 0.05 vs. pre (see Table 1). Repeated ANOVA for the repeated factors of time point relative to the exercise on the soft robot (i.e., pre, post) and exercise protocol (i.e., concentric, eccentric) with post-hoc test for least significant difference.

Figure 4

Respiration during interval exercise. Scatter plot of average heart beat (A,D), oxygen uptake (B,E), and respiration exchange ratio (RER) (C,F), over all subjects in function of time over all performed sets of interval-type concentric (A–C) and eccentric (D–F) exercise. Stippled lines in (A,D) indicate upper and lower limits of the interval zone being recommended for the cardiac patient (i.e., 50–70% of peak heart rate). Note the periodic influence of the intervals of exercise and rest.

Figure 5

Cardiovascular differences between interval-type concentric and eccentric exercise. Box Whisker plots of the percentage differences between the eccentric vs. the concentric protocol for cardiovascular parameters. $: p < 0.05 for eccentric vs. concentric exercise. Repeated ANOVA for the repeated factor of the exercise protocol performed (i.e., concentric, eccentric) with post-hoc test for least significant difference.

Figure 6

Differences in alterations of serum metabolites between the exercise protocols. (A) Line graph showing the mean + SE of blood lactate concentration during the course of matched concentric and eccentric type of interval exercise on the soft robot. (B,C) Box Whisker plots showing the baseline-differences in blood lactate (B) and glucose (C) concentration immediately and 8 after finishing concentric and eccentric interval exercise on the soft robot or endurance exercise at constant load. ^*p < 0.05 for the indicated comparison. Repeated ANOVA for the repeated factor of the exercise being performed (constant, eccentric, concentric) with post-hoc test for least significant difference.