Aging causes reductions in the ability to perform tasks of daily living. Beginning after the third decade of life, people normally experience progressive reductions, expressed per decade, in muscle mass (8%–10%) (1), strength (8%–10%) (2), maximal power (11%) (3), and maximal aerobic power (10%) (4). Part of these declines can be offset by exercise training, and there is a continued need to identify the most effective and efficient exercise regimens that can be adapted to older individuals (5,6) with minimal time commitment (7). These declines are at least partly due to an inadequate exercise stimulus because walking or other similar low-intensity activities performed by older adults elicit a low recruitment of muscle fiber number in general and in particular very low recruitment of the high-threshold fast-twitch muscle fibers and motor units (8). It has been shown that heavy resistance training can stimulate hypertrophy in older adults (9–11), but these slow moving exercises do not necessarily increase maximal power (9) or functional tests of living (FTL) that are performed at higher angular velocities (3). Training at fast velocities of contraction results in superior hypertrophy of fast-twitch muscle fibers versus slow velocity training and greater improvements in maximal neuromuscular power (Pmax) (12). As discussed, cycling with 4 s of maximal effort while accelerating an inertial load cycle ergometer flywheel from 0 to >160 rpm not only elicits maximal power but also demands high force at the beginning and maximal velocity at the end, with peak power identified (13).
In healthy sedentary adults, maximal aerobic capacity (V˙O2max) declines 10% per decade after 25–30 yr of age (14) and arterial stiffening occurs (15). High-intensity interval training can elicit similar aerobic (16) and arterial (17,18) improvements as continuous moderate-intensity endurance exercise albeit in a fraction of the time. Therefore, the purpose of the present study is to investigate the effect of power cycling (PC) training on changes in skeletal muscle mass, Pmax, aerobic power generation, and vascular function in older adults (i.e., 50–68 yr) as well as its translation to functional tests of daily living.
METHODS
Overall design
PC training was performed 3 times per week for 8 wk, and participants were studied immediately before and after this period. We devised a 15-min training program to improve both anaerobic and aerobic power over an 8-wk period. The cycling action was chosen as it activates a large muscle mass and is safe to perform at maximal effort without eccentric contractions. Second, we chose to load the cycling flywheel using only inertial loading because it is smooth during a maximal effort and it safely accommodates the work put into it. The person accelerates from 0 to >160 rpm, their approximate maximal velocity, and the PC exposes the neuromuscular system to the full range of the torque versus velocity relationship. Thus, each sprint was approximately 4-s long, which stressed heavily the ATP-PC system during exercise and the oxidative pathways during recovery to resynthesize ATP-PC. By progressively decreasing the recovery period between sprints from 56 to 26 s, over the 8 wk, the rates of oxygen uptake were systematically increased over the 8 wk from 45% to 75% peak oxygen consumption (V˙O2peak) (unpublished observation). This study was registered to ClinicalTrials.gov ID no. NCT03258346.
Participants
The participants were 39 men and women between the ages of 50 and 68 yr with a body mass index of <30 kg·m−2. They were free of overt heart disease, hypertension, or knee/hip joint problems. They were not engaged in systematic physical training. All subjects completed an informed consent for this study that was approved by the Institutional Review Board of the University of Texas at Austin.
Baseline and posttraining testing
Participants who passed the initial medical screening for the study involving a physical exam, a blood lipid analysis, and an exercise stress test underwent several measures of baseline physical fitness and skeletal muscle mass and strength. These tests were performed before initiation of the training protocol and after 8 wk of exercise training.
Thigh muscle volume
Thigh muscle volume (TMV) with MRI scanning was conducted using a Siemens Skyra 3T MRI scanner. The scan was a 3D two-point Dixon Volumetric Interpolated Breath-hold Exam (VIBE). Data were postprocessed using MATLAB Dixon software written by Dr. Jeff Luci to analyze the thigh musculature of both legs to quantify muscle volume and intermuscular fat volume before and after training. The head of the femur and the patellar surface were used as anatomical landmarks, and the custom analysis software automatically segmented out the middle third of the femur length for volumetric analysis of the quadriceps, hamstring, and other musculature.
Pmax
Pmax, both during pre- and posttesting as well as during training, was measured while cycling using the inertial load ergometer (ILE) and recorded on the laboratory computer as generally described previously (13). Participants performed four maximal 4-s sprints with 2 min rest, after a 5-min controlled warm-up on a cycle ergometer (Lode, Groningen, Netherlands). The average Pmax of the four sprints was recorded.
Total body lean mass
Participants’ whole body composition was assessed by dual-energy x-ray absorptiometry, and total body lean mass (TBLM) was recorded pre- and posttraining.
FTLChair and FTLRamp
Participants performing the FTLChair were asked to rise from a chair 10 times as fast as possible with their arms folded across their chest (19). This procedure was repeated after 2 min of recovery, and the average of the two trials was recorded. In the ramp protocol, participants were timed as they walked as quickly as possible up a 20-m ramp at a 15% incline (19) with the average of two trials recorded.
V˙O2peak
Participants completed a continuous, incremental V˙O2peak test on a cycle ergometer (Lode) with 1-min stages. The total length of the test was 6–12 min, with cessation determined by volitional fatigue (inability to maintain cycling cadence above 60 rpm) and criterion measures for determining achievement of a V˙O2peak by an RER >1.10, a plateau in V˙O2 (change less than 2 mL·kg−1⋅min−1, an HR >95% of age-predicted maximum, or an RPE above 18. Respiratory analyses were determined using oxygen and carbon dioxide analyzers (Applied Electrochemistry, Models S-3A/I and CD-3A, respectively) while the participants breathed through a one-way valve (Hans Rudolph, Kansas City, MO). Ventilation was measured via an inspiratory pneumotachometer (Hans Rudolph). V˙O2, VCO2, and RER were continuously monitored throughout the exercise test. The highest 30-s average of V˙O2 was recorded as V˙O2peak.
Flow-mediated dilation
Endothelium-dependent vasodilation was evaluated via flow-mediated dilatation (FMD) with a semiautomated diagnostic ultrasound system (UNEX-EF38G; UNEX Corporation, Nagoya, Japan) while participants rested in a supine position as previously described (20). Briefly, a blood pressure cuff was placed on the forearm with the proximal edge of the cuff below the participant’s antecubital fossa. The cuff was inflated to 50 mm Hg above resting systolic blood pressure for 5 min to occlude blood flow. The position of the probe was noted for each participant to ensure that subsequent measurements were taken on the same region of the brachial artery. FMD was quantified as the peak diameter of the brachial artery observed postocclusion and reported as percent change from the baseline diameter.
Cardio-ankle vascular index
Arterial stiffness was measured using cardio-ankle vascular index (CAVI) with a computerized, semiautomatic, noninvasive screening device (Vasera, Fukuda Denshi, Tokyo, Japan) as previously described (21). Briefly, CAVI was calculated from pulse wave distance divided by transit time of pulse waves using blood pressure cuffs placed on the ankles and upper arms that were inflated to a pressure of 30–50 mm Hg. Pulse wave velocity was measured by summing closure of the aortic valve and brachial notch appearance upon the wave form, plus the time between the rise of the brachial and ankle pulse waves.
Exercise training
Each participant trained 3 times per week (M, W, F or T, Th, S) for 8 wk. Each session of training required a total of approximately 15 min with the total time actually exercising of only 60–120 s. The first week consisted of performing 15 sprints, each 4-s long on the ILE, beginning each sprint with 56 s of rest. During weeks 2–4, the rest was reduced to 41 s between sprints, completing 20 bouts. Finally, during weeks 5–8, participants took 26-s rest between sprints and completed 30 bouts.
Statistical analysis
Descriptive statistics are reported as mean ± SE. Student’s paired t-tests with Bonferonni correction were used to assess differences between pre- and posttraining. Correlations between variables of interest were performed using a Pearson product–moment correlation method. All data were analyzed using the Statistical Package for Social Science software version 25.0 for Macintosh (SPSS Inc., Chicago, IL). All data are presented as mean ± SE with P < 0.05 accepted as statistical significance. With 39 participants, the study had 53.8% power to detect a difference of 1.0 SD (i.e., Cohen’s d = 1.0).
RESULTS
Subject characteristics
Thirty-nine individuals (24 females and 15 males) completed this study. Subject descriptive statistics pretraining data are shown in Table 1.
TABLE 1 -
Subject descriptive statistics pretraining.
| Descriptors |
Mean ± SEM |
| Sex (n) |
F: 24; M: 15 |
| Age [range], yr |
58.5 ± 0.8 [50–68] |
| Height (cm) |
169 ± 1.7 |
| Body weight (kg) |
69.9 ± 2.5 |
| Body mass index (kg·m−2) |
24.3 ± 0.6 |
| V˙O2peak (L·min−1) |
2.30 ± 0.15 |
| Systolic blood pressure (mm Hg) |
117.7 ± 1.9 |
| Diastolic blood pressure (mm Hg) |
73.1 ± 1.5 |
Training power
The average weekly training power during the 8 wk was 629 ± 15 W. This was maintained despite increased training stress from increased number of sprints per session (from 15 to 30) and concurrent reduced rest between sprints (from 56 to 26 s).
Morphological changes
Total body mass increased 1.4% ± 0.3% (P < 0.001) posttraining and TBLM increased 1.5% ± 0.4% (P < 0.01) (Table 2), suggesting the increased body mass was mostly skeletal muscle. As shown in Table 2, exercise training in the population increased TMV by 3.7% ± 0.9% (<0.001). There were no changes in intermuscular fat volume from pre- to posttraining (Table 2).
TABLE 2 -
Changes in outcome variables from pre- to posttraining in the entire population.
| Measures |
Pretraining |
Posttraining |
%Δ |
n
|
P
|
| Total body mass (kg) |
70.8 ± 2.7 |
71.6 ± 2.7 |
1.4 ± 0.3 |
39 |
<0.001 |
| TMV (mL) |
4034 ± 254 |
4183 ± 262 |
3.7 ± 0.9 |
27 |
<0.001 |
| TBLM (kg) |
46.0 ± 2.1 |
46.6 ± 2.1 |
1.5 ± 0.4 |
35 |
<0.01 |
| Intermuscular fat volume (mL) |
3726 ± 259 |
3767.1 ± 254 |
1.8 ± 1.4 |
27 |
NS |
| V˙O2peak (L·min−1) |
2.30 ± 0.2 |
2.52 ± 0.2 |
9.8 ± 1.8 |
35 |
<0.001 |
| Power at V˙O2peak (W) |
176 ± 12 |
187 ± 13 |
8.2 ± 1.5 |
36 |
<0.001 |
| Pmax (W) |
607 ± 40 |
684 ± 48 |
12.0 ± 1.5 |
37 |
<0.001 |
| FTLChair (s) |
17.1 ± 0.7 |
13.9 ± 0.5 |
−17.2 ± 2.0 |
35 |
<0.001 |
| FTLRamp (s) |
11.2 ± 0.4 |
10.2 ± 0.3 |
−8.5 ± 1.3 |
33 |
<0.001 |
| CAVI (AU) |
7.76 ± 0.17 |
7.58 ± 0.19 |
−2.35 ± 1.12 |
31 |
<0.05 |
| Flow-mediated dilation (%) |
4.90 ± 2.11 |
4.98 ± 1.79 |
25.3 ± 15.1 |
31 |
NS |
| Mean arterial pressure (mm Hg) |
88 ± 8 |
87 ± 8 |
−0.85 ± 0.01 |
31 |
NS |
Values are presented as mean ± SEM. P indicates the level of significance of change from pre- to posttraining.
Pmax
As shown in Table 2, training increased Pmax by 12.0% ± 1.5% (P < 0.01).
FTL
As shown in Table 2, training increased performance on FTLChair by 17.2% ± 2.0% (P < 0.01) and FTLRamp by 8.5% ± 1.3% (P < 0.01).
Correlations
Pretraining TMV was significantly correlated with pretraining Pmax (r = 0.94, P < 0.01) (Fig. 1A). Posttraining TMV was also significantly correlated with posttraining Pmax (r = 0.93, P < 0.01). Change in TMV (ΔTMV) was not correlated with change in Pmax (ΔPmax; r = −0.013, P = 0.95) (Fig. 1B), and ΔTBLM was not significantly correlated with ΔPmax (r = 0.25, P = 0.15) (Fig. 1C).
;)
FIGURE 1: A, Pre- and posttraining TMV vs pre- and posttraining Pmax in the entire population (r = 0.93, P < 0.01). B, Percent change (Δ) pre- to posttraining in TMV vs percent change (Δ) in Pmax (r = −0.013, P = 0.95). C, Percent change (Δ) in TBLM vs percent change in Pmax (r = 0.25, P = 0.15) all in the entire population.
V˙O2peak and power at V˙O2peak
As shown in Table 2, V˙O2peak improved 9.8% ± 1.8% (P < 0.01) and power at V˙O2peak improved by 8.2% ± 1.5% (P < 0.01).
Vascular functions
CAVI decreased significantly after 8 wk of inertial load exercise training (n = 31, pretraining = 7.76 ± 0.92 vs posttraining = 7.58 ± 1.04, a decrease of 2.4% ± 1.1%; P = 0.048). In addition, grouping the participants by age or sex did not influence the reduction observed with CAVI. FMD (pretraining = 4.90% ± 2.11% vs posttraining = 4.98% ± 1.79%) and mean arterial pressure (pretraining = 88 ± 8 mm Hg vs posttraining = 87 ± 8 mm Hg) did not change significantly pre- to posttraining (P = 0.819 and P = 0.378, respectively).
DISCUSSION
This investigation found that PC training composed of only 60–120 s of exercise per training session resulted in statistically significant improvements in TMV (3.7% ± 0.9%, P < 0.01), TBLM (1.5% ± 0.4%, P < 0.01), Pmax (12% ± 1.5%, P < 0.01), FTLChair (17.2% ± 2%, P < 0.01) and FTLRamp (8.5% ± 1.3%, P < 0.01), V˙O2peak (9.8% ± 1.8%), power at V˙O2peak (8.2% ± 1.5%, P < 0.05), and CAVI (2.4% ± 1.1%, P < 0.05) (Table 2). Given that the most common reason people give for not exercising is lack of time (22) and because exercise training should ideally increase both Pmax (i.e., anaerobic) and maximal oxygen consumption (i.e., aerobic), we devised a 15-min program of maximal anaerobic energy breakdown during 4 s of sprint cycling and high rates of aerobic recovery between sprints. The aerobic component progressively became more stressful over the weeks by reducing the recovery time from 56 to 26 s. Of course, this is a form of interval training that might best be described as sprint interval training, although it is unique because of the short 4-s sprint duration and short rest duration, which reduce fatigability compared with all-out 20- to 30-s sprints (23).
Resistance training to stimulate hypertrophy for the elderly was emphasized 30 yr ago and involved heavy weights moved at relatively slow velocities (9,24). Thereafter, the concept of power training for the elderly was introduced by Fielding and colleagues (25–29), with power defined as moving a given force (weight) at the fastest velocity. Power training has been demonstrated to be more effective at improving FTL than resistance training (30–32). Several isokinetic and pneumatic devices have been used to demonstrate the performance benefits of training for higher power (12,33). These studies generally supported the concept of the specificity of training whereby the greatest improvement in maximal power is seen at the velocity of training and at slower velocities (12). Furthermore, it was observed that high-velocity and high-power knee extension training stimulated hypertrophy in fast-twitch (Type 2) muscle fibers (12). There is a need for power training devices for older adults that are safe and do not generate high impact forces and do not involve eccentric contractions, such as cycling. Furthermore, the IL ergometers contained a freewheeling mechanism that disengages the flywheel from the cranks and pedals to prevent eccentric contractions. Among the 39 subjects training for 8 wk in the present study, there were no injuries or even cases of delayed onset muscle soreness that were worth noting. Furthermore, there is a need for exercise that both trains the individual at the velocity of maximal power and additionally at their maximal velocity, and the ILE accomplishes that goal.
The age-related decline in neuromuscular power is preceded by a decline in skeletal muscle mass (sarcopenia) (5). Heavy strength training results in improvements of 4.8%–9% in thigh muscle area (9,24) in older men and women, to a 12% increase (34) in MRI measured knee extensor volume. However, other studies have failed to show improvements in leg lean mass in older adults performing heavy resistance training (27,33). In comparison, the muscle mass improvements we observed were smaller than those from early heavy strength training studies of comparable length (9,24,34). However, PC training with an ILE was able to elicit moderate hypertrophy and increased neuromuscular power while also stimulating increases in maximal aerobic power. More importantly, this was achieved with much less time spent in each exercise training session.
Pmax improved ~12% over the span of 8 wk of training in the present study. These results are lower than the average changes in power (22%–34%) reported by Katsoulis et al. (35) in a meta-analysis of power training studies in older adults. However, average study length was ~13 wk, thus providing more time for adaptations than the 8 wk of training in the present study. Fielding and colleagues have reported improvements in power ranging from 12% (36) to 34% (29) with results differing by study duration, population studied, and modality of training. These studies most likely did not train participants at velocities high enough to elicit maximal power as they all used heavy resistances (>70% 1 repetition maximum), and thus the velocity reached with each repetition was not optimal for maximal power generation. Coyle et al. (12) demonstrated that only those participants training at fast velocities of knee extension (300°·s−1) showed hypertrophy of fast-twitch muscle fibers. Thus, we used PC training because participants reach true maximal power at approximately 110 ± 20 rpm, and their maximal velocity of contractions reached approximately 155 ± 18 rpm with each repetition. It is likely that this was optimal for stimulating possible hypertrophy in fast-twitch fibers (2,5). The fact that the change in Pmax was proportionally greater than the increase in TMV or TBLM suggests that adaptations other than muscle hypertrophy enabled the 12% increase in maximal power, and this other adaptation is most likely enhanced motor unit recruitment (37).
In addition, we sought to investigate the ability of PC training with ILE to improve aerobic power in healthy untrained 50- to 68-yr-old participants. V˙O2peak improved by 9.8% ± 1.8% and power at V˙O2peak by 8.2% ± 1.5%. High-intensity intervals are widely established as effective for improving V˙O2peak (38,39) with minimal time commitment (22). Conversely, regularly performed moderate-intensity continuous exercise performed over 13–48 wk resulted in an improvement in V˙O2max in elderly adults of 11%–24% (40,41). Thus, the 9.8% improvement in V˙O2peak over the span of 8 wk of PC training in this study might be viewed as somewhat rapid. This suggests that PC is a time-efficient training modality for improving aerobic power. We have no information as to the mechanisms for the improved V˙O2peak but theorize it might involve increases in stroke volume, muscle oxidative capacity, or blood volume. The 8 wk of exercise training using the ILE significantly decreased arterial stiffness as assessed by CAVI by 2.4% (P < 0.05) in these men and women 50–68 yr old. In other studies, improvements in pulse wave velocity were ~5% after 16 wk of high-intensity interval training in older adults, and thus the 2.4% improvement in arterial stiffness over 8 wk of the present PC training appears proportional to the fewer weeks of training. Although our study did not find an improvement in endothelial function as evaluated by brachial FMD after exercise training, sprint interval exercise has been demonstrated to increase popliteal FMD (42). The difference in findings could be a result of the modality of exercise as our study included bouts of cycling of 4 s, whereas Rakobowchuk et al. (42) used 30-s Wingate tests. In addition, our measurement of endothelial function was taken at the brachial artery, and the previous study measured endothelial function in the leg at the popliteal artery. It should be noted that the design of the present study suffered by not having a control group and by using a relatively short duration of the exercise program. Furthermore, the 50- to 68-yr-old subjects were healthy, which limits the generalizability of these results to diseased populations.
In summary, this is the first study to demonstrate that PC training with inertial loading can simultaneously improve Pmax as well as V˙O2peak in men and women 50–68 yr using training sessions with only 60–120 s of actual exercise. These results should help inform the growing understanding of the kinds of time-efficient exercise that older individuals might use to attenuate age-related reductions in maximal neuromuscular and aerobic power.
This study was funded by POM Wonderful LLC.
The results of the study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.
Dr. Coyle holds equity in Sports Texas: Fitness, Training and Nutrition Inc., a company that sell the ILE used in the study. The business interests of Sports Texas: Fitness, Training and Nutrition Inc. overlap with this area of research.
The results of the study do not constitute endorsement by the American College of Sports Medicine.
J. R. A. and E. F. C. conceived the research and designed the experiment; J. R. A., R. S., E. V., and B. F. recruited participants and performed the experiments; J. R. A. and E. F. C. interpreted results of the experiments; J. R. A. prepared figures, performed statistical analyses, and drafted manuscript; J. L. created the analysis software used in the study for MRI analyses; J. R. A., R. S., E. V., B. F., H. T., and E. F. C. edited and revised manuscript; J. R. A., R. S., E. V., B. F., J. L., H. T., and E. F. C. approved final version of the manuscript.
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