Impact of unhealthy lifestyle on cardiorespiratory fitness and heart rate recovery of medical science students

This study was approved by the research ethics committee of the Faculty of Health and Medical Science at the University of Surrey, UK. Volunteers, both male and female, were recruited randomly and anonymously from the first-year medical science students who attended the human physiology practical classes during year 2011 to 2014 in the Faculty of Health and Medical Sciences, University of Surrey, UK. Prior to the study, volunteers were provided with the study information and consent documents. The inclusion criteria were first year undergraduate students between the ages of 18–35 without documented medical conditions. Exclusion criteria were above the age of 35 and evidence of medical conditions. Participants who failed to answer all questions in the survey form or failed to complete the fitness test or missing exercise data recording were excluded from the study. Data were collected anonymously and entered electronically for statistical analysis.

Information pertaining to individual demographics and lifestyle components were collected through pre-exercise questionnaires specifically designed in whole for this study (Supplemental Additional file 1). Demographics included gender, age, ethnic origin, medical conditions and home country of residence. Lifestyle components included alcohol consumption (units/week); current smoking status (cigarettes/week); levels and duration of physical activity (in terms of exercise and sport, h/week); sleep duration (h/day); weekend outdoor activity (h/day), and sedentary screen-time including computing, watching TV and gaming (h/day), and self-perceptions of physical fitness. Current smokers are defined as individual who has smoked 100 cigarettes previously and currently smoke according to Centre for Disease Control and Prevention (CDC, USA) [20]. Body height and weight were measured and recorded by the investigators before the exercise test, and used to calculate BMI. Resting blood pressure (BP) and heart rate (HR) were taken prior to the fitness test. Åstrand–Rhyming submaximal cycle ergometry test was performed using the protocol as described previously [21, 22]. It consisted of a 6 min exercise bout on a cycle ergometer (KORR Medical Technologies Int, USA). Power output of 75–125 W was used for female, and 100–150 W for male. Participants were instructed to maintain a pedal frequency of 50 rpm. Heart rate during exercise was recorded continuously using an infra-red ear lobe clip monitor. The criteria for exhaustion were heart rate at ~ 185 beats per minute and a subjective judgment by the investigators that the participant could no longer keep up. CRF was expressed as V̇O₂max (ml^.kg^-1.min^− 1) calculated using the Astrand and Ryhming nomogram [22]. The heart rate recovery and the BP were measured for 3 min after the exercise while the participant was still sitting on the bicycle.

Descriptive statistics included mean with standard deviation for continuous variables and frequency with percentage for categorical variables. If asymmetric then median with interquartile range were reported. A liner regression and hierarchical multiple regression analysis model was used to generate various regression equations for predicting the relative contribution of gender, BMI, PA levels and self-assessment of fitness with VO₂max values or HRR at 3 min as the dependent variables. Pearson correlation and values at 95% confidence intervals (CI) were used to measures the statistical relationship (or association) between two variables of VO₂max and HRR. Levene’s Test was used to determine the equality or homogeneity of variance of the groups. Statistical significance was set at alpha P < 0.05. Data analyses was performed using the Statistical Package for the Social Sciences (SPSS) version 25 software for windows.

The information of demographics and lifestyle components were given in Table 1. There were 58 (47.2%) male and 66 (52.8%) females with an average age of 20.6 ± 3.7 years. There were 56.5% EU Caucasians, 12.9% Asian and 9.7% Africans and the rests were other ethnical origins. The majority (80.5%) of the students had BMI within the normal range (18.5–24.9 kg/m²), 11.3% were overweight (BMI > 25 kg/m²) and 8.9% were underweight (BMI < 18 kg/m²). Among the participants, 8.9% described themselves as current smokers. Approximately 33.1% of the participants admitted to consume alcohol ≥12 units/week (112 g/week), which is close to the UK safe limit of alcohol consumption (≤14 units/week or 112 g/week) [23] with males occupying the greatest proportion of this category.

Self-reported physical activity (any form of exercise or sport) (h/week) were evaluated according to UK National Physical Activity Guidelines for adults [9]. Of the 124 participants (Table 1), 65.3% had about ≥3 h/week of moderate exercise, 34.7% had < 3 h/week exercise and 5.6% did not engage in any form of exercise or sport at all. When asked for reasons for not meeting National Physical Activity Guidelines, the majority responded “no time”. When asked about outdoor activity at weekend, 53.2% students claimed to have in average of ≥4 h outdoors/per weekend and the rest did not. In contrast, 83.1% of students admitted to spend > 3 h of screen time /per weekend-day to play games, watch film or TV. Regarding the sleeping time, 58.9% of students admitted to sleep < 7 h/day which is below the recommendation by UK National Health Serves [24]. For the self-assessment of physical fitness, 34.7% of participants perceived themselves as being physically unfit (feeling tired) with females being the majority in this category.

Significant correlations were found between the VO₂max distribution and the levels of physical activity, self-assessed fitness and BMI (Fig. 1). Those who participated ≥3 h/week of exercise had significant higher values of VO₂max distribution than others (F = 14.161; P < 0.001). The participants who believed themselves physically fit had significantly higher values of VO₂max distribution than those who felt not fit (F = 11.43; P = 0.001). Overweight and obese group had lower values of VO₂max distribution in comparison to the values of normal weight group (F = 4.555; P = 0.01).

The HRR was measured for 3 min after the cessation of physical exercise (at time 0) (Fig. 2). The HR decreased quickly in the first min (> 25 beats/min), and then slowed down (Fig. 2a). At 3 min post exercise, the mean HR (105 ± 20) was still above the resting HR (80.8 ± 16.6) measured before exercise. We found that HR at 3 min post-exercise had a good correlation with VO₂max distribution (Pearson correlation coefficient = 0.673; 95% CI: 0.646–1.175; p < 0.0001) (Fig. 2b). Participants who had < 3 h/week exercise had significantly slower HRR recovery at 3 min than others (F = 8.297; P = 0.005). Participants who believed themselves physically not fit had an attenuated HRR at 3 min in comparison to those who felt physically fit (F = 6.767; P = 0.01). Although approximately 33.1% of the new medical science students admitted to consume alcohol beyond the safe limit, we did not find significant association between the amount of alcohol consumption and VO₂max (or HRR) in this study. We did not find association between daily sleep time with CRF and HRR.

We also found gender-influence on SBP that females had slightly but significantly lower SBP than males at both rest and after exercise (F = 42.121; P < 0.0001) (Fig. 3a). There was no significant difference for diastolic BP. BMI also had an impact on resting SBP. The overweight and obese group had higher resting SBP than the normal weight group (F = 4.934; P = 0.009) and this difference disappeared after exercise (Fig. 3b), suggesting exercise is beneficial for the young overweigh adults to prevent pre-hypertension syndrome.