Cardiac Autonomic Profile of Soccer, Field Hockey and Basketball Players: A Comparative Study

3.1. Participants

Fifty-five male collegiate athletes (20 soccer players, 18 field hockey players and 17 basketball players) were recruited from M. A. K. Pataudi Sports Complex, Jamia Millia Islamia, New Delhi, India. Subjects between the age of 18 - 25 years, with BMI (body mass index) ranging from 18.9 - 24.9 kg/m², indulging in sport-specific training for at least the last 3 months (4 - 5 days/week), and having participated in university level competition, were included in the study. Those with a history of cardiac or respiratory pathology, taking medications that alter cardiac response or recovery, diagnosed of a chronic health condition, smokers and alcoholics, were excluded. All participants gave their written consent, after verbal explanation of the procedure, risks involved and participation being entirely voluntarily. All uncertainties among the participants regarding the procedure were cleared. Prior to the experimental procedure, ethical approval for the study was granted from the institutional ethics committee, Jamia Millia Islamia, New Delhi, India.

3.2. Instrumentation and Data Acquisition

Measurements were undertaken in a quiet room, at a temperature between 24 to 30°C and at the same time (between 10:00 am and 1:00 pm) during the day to nullify the effect of circadian rhythm on HRV. Skin was prepared by wiping with alcohol swab for the electrodes placement to reduce impedance. Electrocardiograph (ECG) was recorded at rest and after sub-maximal exercise in supine lying for 10 minutes while breathing approximately at 15 breaths per minute. HRV analysis was done on final 5-minute epochs segment of ECG. ECG data was sampled at 1000 Hz, band pass filtered between 0.3 and 100 Hz, and stored on computer for analysis (Powerlab 8/30 Data Acquisition System with LabchartPro: AD Instrument, Australia). For HRV analysis, R-R intervals were selected by setting threshold at 0.5 mV and ectopics were excluded from analysis by “beat classifier” function of software. HR was recorded by a heart rate monitor (Polar Electro, RS 400, Kempele, Finland) with chest strap.

3.3. Procedure

Subjects were instructed to lie down supine for 10 minutes to attain complete relaxation during which, resting heart rate (RHR) and HRV were recorded. Following this, the participants performed an incremental treadmill test at sub-maximal exercise intensity. A 5 - 10 min familiarization session for treadmill running was given to each participant 24 hours before actual testing. The incremental running test began with an initial speed of 8 km/h with grade 0 and increased 1 km/h every minute until the subject reached sub-maximal target heart rate (THR), experienced adverse signs or symptoms, requested to stop, or experienced any emergency situation. Beat-by-beat HR responses were monitored during the running test. The THR was calculated using Karvonen formula, THR = [(HRmax - RHR) × 80% - 85% Intensity] + RHR, where maximal heart rate, HRmax = 206.9 - 0.69 × age (years).

After the exercise protocol, subjects returned to supine lying within 5 seconds. HRR was recorded at 1st, 2nd and 3rd minutes, post-exercise. The subject remained in this position for 15 minutes, during the final 5 minutes subjects were instructed to breathe at a frequency of around 15 breaths per minute. Talking and excessive movement during this time period was avoided. ECG was recorded for 10 minutes, but to ensure being stationary, the final 5 minutes were used to analyze HRV. Time domain HRV measures were obtained by mean RR intervals (meanNN) and the root mean square of successive differences (RMSSD) whereas frequency domain measures of HRV were derived by Fast Fourier Transformation: they were low-frequency (LF; 0.04 - 0.15 Hz) and high-frequency (HF; 0.15 - 0.4 Hz) spectral power. Temporal and spectral measures of HRV have been reported to be reliable and reproducible by previous literature (7, 39) (Figure 1).

Figure 1.

Evaluation of HRR and HRV

3.4. Statistical Analysis

Data were analyzed using SPSS version 21. The Shapiro-Wilk test was used to verify the normality of distribution for all variables. RHR and RMSSD scores that demonstrated non-normal distribution were log transformed for further analysis. The demographic characteristics and baseline measures were compared between the groups using one-way ANOVA. A repeated measure ANOVA was used for HRR. 3 × 2 mixed model ANOVA was employed for meanNN, HFnu to find out main effect of group (soccer, field hockey and basketball) and time (resting, post-exercise) as well as Group × Time interaction. 3 × 2 mixed model ANCOVA was used for RMSSD, LFnu and LF-HF ratio considering baseline measures as covariate. Post hoc Bonferroni test was used for multiple comparisons. Statistical significance level was kept at P ≤ 0.05.

5.1. Resting HR and HRV

The RHR of athletes is lesser than the sedentary individuals (29), due to increased vagal tone that causes resting bradycardia which is a cardiovascular adaptation to endurance training (40). This study revealed similar RHR between the groups, indicating comparable physical fitness.

Previous studies have shown that training load significantly influences the cardiovascular autonomic activity control (21). Bricout et al. reported lesser LFnu and LF-HF ratio during rest in soccer players that increased with physical and psychological stress (27). Parrado et al. found higher value of LF-HF ratio in field hockey players showing changes in cardiac autonomic response during World Cup (31). HF indices of HRV more accurately indicate parasympathetic dominance, while RMSSD is shown to be less affected by the respiration at rest (41). RMSSD which is indicative of vagal activity was found to be significantly lower whereas, LFnu that reflects sympathetic activation was significantly higher in the basketball group. LF-HF ratio that represents sympatho-vagal balance related to arterial baroreflex activity and cardiovascular stresses was also shown to be significantly elevated. These suggest an enhanced sympathetic and depressed parasympathetic tone at rest, in basketball players, relative to soccer and field hockey athletes. While soccer and hockey athletes were comparable in terms of HRV indices, significant differences in the profiles of basketball players were recorded.

5.2. Heart Rate Recovery

Studies that have examined the relationships between HRR and physical activity, suggest that faster HRR is associated with good physical fitness (3, 42). The current study revealed faster HRR in the basketball players than soccer players and field hockey players. The difference found in the HRR in the first minute was about 62 beats in basketball players while for soccer and field hockey players it was about 49 beats and 40 beats respectively. This showed that the rapid change in autonomic control with sympathetic withdrawal or increased parasympathetic activity after cessation of exercise in basketball players was higher relative to soccer players and field hockey players. This could be attributed to the frequent, short-burst, sprinting activity in basketball. Faster HRR has been shown to be associated with sprinting (43). With the repeated bouts of high intensity activity and recovery bout over a period of time, adjustments in ANS could be expected, that might control the activity of heart via return of vagal tone. Studies have shown that athletes who engage in intermittent endurance sports have faster HRR than athletes involved in continuous endurance sports after maximal exercise (44). Also, previously found 1 min HRR to be about 40 beats in the first minute after exercise for soccer players as well as in sedentary during the competition (29). Although our study was performed during the competitive season, level of competition could be the reason for the difference because the participants were in the middle of intra-university competition.

5.3. Post-Exercise HRV

This study incorporated a submaximal exercise followed by 15 minutes of recovery in supine showing altered autonomic control post-exercise. RMSSD (69.79% in soccer players, 67.96% in field hockey players and 57.41% in basketball players) and HFnu indices (40.09% in soccer players, 38.58% in field hockey players and 55.48% in basketball players) of HRV showed significant reduction after 15 minutes of recovery when comparing with the baseline values; indicating lesser parasympathetic activity. We also found elevated LFnu (110.58% in soccer player and 112.73% in field hockey players and 62.98% in basketball players) and LF/HF ratio post recovery increased sympathetic outflow to the heart. The difference in resting and post-exercise values of HRV signifies that the shift in autonomic modulation during exercise did not return to resting levels. The findings are consistent with previous studies that emphasize parasympathetic withdrawal and increased sympathetic tone elicited during activity. Moreover, as demonstrated by Barak et al., Terziotti et al., Parekh and Lee, and Kamath et al. the recovery following activity was not completed in 15 minutes. Both the depressed vagal and elevated sympathetic tone failed to return to resting levels within the recovery period following exercise (14, 15, 20, 45).

The results of post-exercise HRV demonstrated an overall reduction in the indices of vagal activity and increase in the markers of sympathetic tone. This is expected, as the transition from rest to exercise entails parasympathetic withdrawal and sympathetic up regulation in order to raise the metabolic rate and meet the energy demands of physical activity. This elevated sympathetic outflow and reduced parasympathetic reactivation even after 15 minutes of exercise recovery could be due to the following i) increased level of metabolites, ii) elevation of catecholamines and other sympathetic transmitters and iii) arterial baroreflex stimulation during the exercise (46-48).

The magnitude of recovery (difference of resting and post-exercise HRV) was similar between the 3 groups for all variables except LFnu. LFnu which denotes sympathetic activity was found to be increased in all groups post-exercise but the increase was significantly greater in the basketball group. These sports have different aerobic and anaerobic contribution during the game, the player’s effort during the game, found to be 70% of the player’s effort in a field hockey game, 80% of player’s effort in a soccer and 50 % in the basketball in terms of aerobic demands (49-51). The aerobic fitness shifted the autonomic power distribution more towards parasympathetic activation (52). Moreover, increased sympathetic dominance in basketball players can be due to increased metabolites as a consequence of repeated anaerobic bouts of activity. Further changes in HRV might not be explained merely by the reflex mechanism, but weakening of baroreflex activity influence on post exercise autonomic control over cardiovascular activity.

Thus, it is reasonable that the current study explained the difference in the autonomic contribution in these different sports and gives cardiovascular fitness profile of athletes. This information is useful for incorporating different training methodologies or development of a specific training program for cardio-respiratory fitness in athletes. There was some limitation in the study. The type of training and amount of training done by the participating athletes was not fully explained in the study, although the training load is somewhat included into the inclusion criteria. Also, there was greater relation of HRV measures to the training load in the athletic population. Further research can be focused on considering the physiological (primarily aerobic and anaerobic) and psychological characteristics in these athletic groups.

In conclusion, the results of the present study suggest that basketball players exhibit greater sympathetic outflow, at rest and following activity. The autonomic profiles of soccer and field hockey were found to be comparable.