SR conditions were determined following observations of 290 individuals (151 men, 139 women) running with strollers in a public recreational area. Of those observed, approximately 78% chose a single method for the 400 meter duration of observation; the other 22% alternated between pushing methods. The three pushing methods observed in either case were classified as 1-Hand (42%), 2-Hands (51%), and Push/Chase (7%) (Push/Chase was defined as alternating between pushing the stroller ahead of the runner with both hands and running independently behind the stroller). As all runners observed used these three methods, each was compared against running independently in order to determine how pushing method might influence SR energetic cost and kinematics.

Running economy and kinematics were measured in 16 participants (10 men, 6 women), who had no previous SR experience. Participants with no SR experience were selected to prevent bias towards a single pushing method. Mean subject age was 22.8 ± 3.3 (SD); mean body mass was 68.2 kg ± 11.9 (SD); and all participants ran at least 3 times a week for a minimum duration of 30 min. This study adhered to ethical research standards and was approved by the Seattle Pacific University Institutional Review Board. All participants signed consent forms prior to participation. This study excluded participants who had experienced an orthopedic injury within six months prior to study participation.

Each participant ran 800 meters on an outdoor 400 meter track for three stroller conditions (1-Hand, 2-Hands, and Push/Chase) and one control which required running independently. No other people were on the track for the duration of the trial. Previous research has shown that 800m of comfortable jogging (see below for ‘comfortable’ definition) is the appropriate amount of time to have runners reach steady state values for each condition, and also not have a fatigue effect influence the later conditions [14–15]. All stroller conditions were performed with a Single Sport Stroller (phill&teds, Wellington, NZ) loaded with a 16kg weighted infant model in order to simulate the presence of a 3 year-old child [16]. There was an opportunity for participants to familiarize themselves with the stroller and adjust handle height prior to being fitted with the respirometer and HR monitor. All conditions were randomized for each subject and completed consecutively. After the completion of all the conditions, participants were asked to select which SR condition was the ‘most comfortable’ for them.

To measure running economy, subjects ran at self-selected speeds (mean speed = 3.1 ms^-1 ± 0.5) while wearing a mobile respirometer (Oxycon Mobile, CA, USA) to collect breath-by-breath metabolic data [17]. The mobile respirometer was calibrated on-site immediately prior to each data collection and there were no significant differences in environmental conditions for each trial. Heart rate (HR) was also collected using a chest-strap HR monitor (Polar Electro, Finland). Participants consumed no caffeine or alcohol within 24 hours or any food within 4 hours of the trial to ensure accurate metabolic readings. Participants initially sat quietly for 5 minutes in order to obtain baseline metabolic rate and were then instructed to run at a speed they were comfortable maintaining for 60 minutes across all four conditions. Participants received the same verbal description of the requested pace at the beginning of their trial and this was repeated at the beginning of each condition. Participants were given a minimum of 4 minutes of rest between conditions to ensure metabolic rate returned to pre-exercise values; metabolic rate was monitored during the rest so that values did reach the initial resting values between each condition. Mean condition duration for each 800m run was 4.5 min ± 0.8 (SD); the metabolic data used for analysis was collected from the final 400 meters of the condition to ensure participants had reached a steady metabolic rate. Metabolic rate was converted from VO₂ and VCO₂ into Joules following Weir’s standard equation [18].

Stride frequency and running speed were measured with a stopwatch. Speed (ms^-1) was measured every 100 meters and stride frequency (strides*s^-1) was measured every 200 meters (during straightaways). Speed and stride frequency were averaged for the final 400 meters of each condition, and stride length (m/stride) was calculated from speed and stride frequency.

A repeated-measures ANOVA was performed to investigate the effects of pushing method on energetic cost, HR, running speed, and stride length. Sex was included as a fixed factor. A post-hoc Tukey HSD test was performed to compare whether pushing methods differed from each other in their influence of the main variables. All statistical analyses were conducted using SPSS 23.0 (Armonk, NY, USA).

Of the 16 participants, 11 preferred the 2-Hands, 4 preferred Push/Chase, and 1 preferred the 1-Hand SR condition to the other SR conditions.

The results of the present study indicate that SR speed and stride length differ from independent running and that there are significant differences between pushing methods. The post-hoc Tukey HSD test revealed gradients in speed and stride length changes when comparing Non-SR to SR conditions. Non-SR stride length was significantly longer than stride length during 1-Hand (p = 0.020) and Push/Chase (p = 0.022) SR conditions, with similar patterns observed for speed. However, 2-Hand SR showed no significant differences in speed and stride length from the other SR conditions or Non-SR (Table 2). These results show then that the Push/Chase and 1-Handed SR methods are the most disruptive to running kinematics, while the 2-Hands method results in a speed and stride length most similar to Non-SR. These findings corroborate those of O’Sullivan et al (2015) who observed no significant change in stride length or speed when pushing a stroller with both hands. Unfortunately other studies did not explicitly control for pushing method so it is not clear how their results are influenced by stroller posture. The differences between SR conditions themselves may explain conflicting results between prior studies regarding the effect of SR on lower limb kinematics (Table 1).

Given that 69% of our participants and 51% of the observed stroller runners on public paths all preferred the 2-Handed SR, it seems this is an SR condition that results in the least perturbation of typical running behavior. The results here show that 2-handed SR is in the middle of a continuum of changes in metabolic cost, speed and stride length. Future work should investigate how pushing method preferences might change as an individual becomes more attuned to running with a stroller, how methods might change over very long distances, and how energetic cost patterns might differ between individuals familiar or unfamiliar with SR.

Postpartum men and women experience declines in physical and mental health and maintaining physical activity during this period of parenthood is often difficult with the added responsibility of caring for a child [2–3]. The present findings suggest that individuals looking to run with a stroller as a form of physical activity should consider implementing different pushing methods depending on their specific fitness objectives. For example, the 2-Hands method may be optimal for individuals looking to reduce the physiological burden of SR as it showed the smallest kinematic change of all SR conditions. However, the Push/Chase or 1-Hand method may be optimal for those looking to further improve cardiovascular health or physical fitness through SR, given the increased energetic burden.

In order to predict the impact of the different SR conditions on metabolic cost, a linear regression was created for each condition which included mass, speed and energetic cost. The models suggest that when speed is maintained, running with a stroller increases cost between 5–8% depending on the pushing method. As expected, the 2-Hands method is the most economical and the Push/Chase method is the most energetically costly. To predict the cost of a runner of a different mass, please go to https://tinyurl.com/stroller-running-calculator.