Study participants
Competitive, but non-elite club-level collegiate triathletes, 7 males and 1 female, age 18–30 years were recruited to participate in this study. All subjects were required to have a minimum of one-year experience competing in triathlon distances ranging from Olympic/International to Ironman. Participants were required to report to the lab for three separate visits, each separated by at least 48 h. Participants were informed about the benefits and risks associated with the study and completed the informed consent before participating. The Appalachian State University Institutional Review Board approved this study before any procedures began and all study procedures were conducted in accordance with the Declaration of Helsinki.
Experimental design
The experimental design for this study was a counter-balanced crossover consisting of two randomized trials designed to simulate actual International/Olympic triathlon competition conditions. Each participant completed both trials at least 48 h apart. The Run–Run trial involved participants running a 5-km time trial (TT) at race pace, followed by a four-minute data collection run at race pace to measure running economy (RE), mechanical efficiency (MER), blood lactate, respiratory exchange ratio (RER), mechanical work, energy expenditure (EE), and muscle glycogen. The Cycle-Run trial involved participants cycling a 40-km TT at race pace, followed by another four-minute data collection run. Participants were allowed to complete a sufficient, non-fatiguing warm up of their choice before each TT, but were instructed to keep their warm up routine the same for each trial.
Procedures
During visit one, participants completed an informed consent, health screening questionnaire, anthropometric measures, and a VO2max test. The maximal graded exercise test was performed on the Bertec instrumented treadmill (Bertec; Columbus, OH) to assess individual maximal oxygen consumption. After obtaining baseline resting metabolic data (VO2, VCO2, RER, and VE; Parvo Medics 2400; Sandy, UT), participants were disconnected from the metabolic cart and asked to complete a 10-min warm up at a self-selected pace (no incline). Participants were then reconnected to the metabolic cart to obtain exercise metabolic data. Participants were instructed to begin the graded exercise test at their self-selected warm up pace and that the treadmill speed would increase 0.4 m•s−1 (no incline) for each successive stage until volitional exhaustion. Stages one through three were 4 min long, and every successive stage thereafter was 2 min long. Heart rate was recorded via a chest strap heart rate monitor connected to the metabolic cart (Polar Fit One; Kempele, Finland).
Upon arrival at the second visit, baseline resting blood lactate and muscle glycogen levels were recorded [23], and baseline metabolic data were obtained. Then participants randomly performed either the Run–Run trial or the Cycle-Run trial. All running was performed on the Bertec instrumented treadmill and cycling was performed on the triathlete’s own personal bicycle using a Computrainer® system (RacerMate; Seattle, WA). To ensure race pace consistency, heart rate was monitored throughout both the 5-km run TT and the 40-km cycle TT, but metabolic data were not collected. The 5-km TT was not intended to match the energy expenditure or work performed during the 40-km cycling TT, but rather to allow the triathletes a sufficiently high intensity, non-cycling workload prior to performing the four-minute data collection run. After completing the 5-km run TT (Run–Run trial) or the 40-km cycle TT (Cycle-Run trial), participants were allowed a 60- to 90-s transition (to simulate a triathlon transition period) to change shoes (for the Cycle-Run trial), record body mass, and don the face mask for metabolic measurements, before moving to the four-minute data collection run.
For the third visit, all procedures were repeated exactly the same as described above, however participants performed the trial not performed in the second visit. The second and third visits were completed at least 48 h apart.
Four-minute data collection run
The four-minute data collection run consisted of participants running on the Bertec instrumented treadmill at their competitive triathlon running race pace for four minutes. Data analysis for RE, MER, RER, mechanical work, and EE occurred during the final two minutes of the four-minute data collection run to ensure steady state exercise. Race pace running speeds remained constant for each individual between trials and ranged between 3.33 and 4.44 m•s−1 (~ 75% of VO2 max) for all participants. RE was determined by measuring submaximal relative VO2 at each individual's self-selected race pace (between 3.33 and 4.44 m•s−1), as described previously [24]. Immediately after the four-minute data collection run, blood lactate was measured via finger prick using a Lactate Plus portable lactate analyzer (Nova Biomedical; Waltham, MA) to determine anaerobic energy expenditure.
Mechanical efficiency of running
Baseline/resting metabolic data were obtained before any activities were performed. During the resting data collection period, total O2 consumed in liters was recorded for two minutes to measure aerobic energy expenditure in kJ•L of O2−1 [17, 25, 26]. To assess MER, forces from the footstrikes were utilized to calculate external mechanical work, and O2 consumption and RER were utilized to determine aerobic EE (EEAer). EE was also calculated from changes in RER through a linear equation (kJ•L of O2−1 = 5.254•RER + 15.986) created by Zuntz and Schumburg [27]. Total O2 consumed for the data collection time period [∆time (min)•VO2 (in L•min−1)] was then multiplied by the kJ•L of O2−1 calculated from RER to provide energy produced. The sum of kJ of energy produced from the RER and total O2 consumed was considered baseline EEAer. The baseline EEAer was subtracted from the kJ of energy produced from the RER and total O2 consumed during the exercise data collection period to measure changes in EEAer. Anaerobic energy expenditure (EEAn) was measured through changes in blood lactate. A resting lactate value was obtained during baseline metabolic data collection and subtracted from the lactate measured immediately after the four-minute data collection run. The change in lactate was then converted to O2 equivalents as 3 mL of O2•kg−1•mM−1 and multiplied by 21.1 kJ•L of O2−1 [28, 29].
To calculate external mechanical work (We), vertical and horizontal center of mass (COMb) velocities (v) and displacements (h) were calculated from integration of acceleration values obtained via the force plates mounted within the Bertec treadmill [15]. The energy-time curve of the COMb was provided by the summation of the potential (Ep = mgh) and kinetic energies (Ek = ½mv2), where m is the mass of the subject and g is acceleration due to gravitational force (9.81 m•s−2). Thus, We (We = mgh + ½mv2) is represented by the incremental summation of this curve [30]. We was calculated as the positive work completed from each footstrike during the final two minutes of the four-minute data collection run to obtain steady state values [16, 17, 25, 26]. Every 15th footstrike was analyzed during the final two minutes, resulting in an average of 22 analyzed footstrikes at each participant’s race pace running velocity. Then, the total number of footstrikes analyzed was multiplied by the average positive work to obtain work values. MER was then calculated as the ratio between We and net (or total) energy expenditure (EEn = EEAer + EEAn), thus MER = We/EEn, in accordance with previously published methods [16,17,18, 25].
Muscle glycogen assessment
Non-invasive muscle glycogen levels were obtained using the MuscleSound® ultrasound system, according to previous validation studies [23]. Participants were asked to lay supine while glycogen levels were measured in the rectus femoris muscle of the left leg in each subject before and after activity on visits two and three. A mark was made at half the distance from the patella to the inguinal crease to enable pre to post glycogen measurements at the same location for each trial and four images were obtained.
Statistical analyses
All data are expressed as mean ± SEM. A Repeated Measures ANOVA was used to compare changes in blood lactate and muscle glycogen before and after running for the Run–Run and Cycle-Run trials. If significant F-ratios were found, within condition changes were compared post hoc using two-tailed t-tests with significance set after Bonferroni adjustment at p ≤ 0.0125. Paired sample t-tests were used to compare RE, MER, absolute VO2, RER, EE, and mechanical work during running after the Run–Run and Cycle-Run trials. Effect sizes were computed for time × condition interactions using Cohen’s d and were interpreted such that 0.2, 0.5, and 0.8 were considered small, medium, and large effect sizes, respectively. Significance was set at p ≤ 0.05. All statistical analyses were performed using SPSS (IBM: Version 21.0. Armonk, NY). The data associated with this study are not publicly available, but are available from the corresponding author upon reasonable request.