Acute Effects of Heart Rate Variability Biofeedback on H-reflex and Maximal Voluntary Isometric Strength


 Background Heart rate variability biofeedback (HRV BFB) is a visually-guided paced breathing scheme that stimulates resonance in the cardiovascular system. Up to date, the influence of HRV BFB on neuromuscular function remains unknown. The purpose of this study was to investigate the effects of a single-session HRV BFB on Hoffman reflex (H-reflex) of the soleus muscle and maximal voluntary isometric contraction (MVC) of plantar flexors. Methods Eleven male healthy participants (height: 173.6 ± 7.5 cm; weight: 74.5 ± 17.3 kg; age: 24.6 ± 4.8 yrs) volunteered to undergo a randomized-crossover intervention involving a 10-minute HRV BFB and normal breathing (CON), separated by 48-hours. Pre and post indices for H-reflex, and post-only MVC in HRV BFB were evaluated using two-way repeated measures ANOVA and independent T-test respectively. Results No significant differences in H-reflex markers between HRV BFB and CON were found. Both groups posted non-significant difference in MVC. Conclusion An acute HRV BFB was not sufficient in eliciting significant changes in motoneuron excitability of the soleus muscle and maximal muscle strength of plantar flexors.


Background
Heart rate variability biofeedback (HRV BFB) is a non-invasive, visually-guided paced breathing intervention believed to improve psychophysiological indices [1][2][3]. HRV BFB utilizes a breathing pace (0.075-0.12 Hz or 4.5-7.2 breaths/min), also known as resonance frequency (RF), that generates rhythmical changes in the baroreflex and activates the resonant properties in the cardiovascular [4][5][6][7]. The baroreflex plays a crucial role in the regulation of blood pressure that operates in a negative feedback loop, such that increasing blood pressure reduces heat rate and vice versa. Key features in HRV BFB include occurrence of maximal heart rate oscillations, and simultaneous oscillations of heart rate and respiration. Regular HRV BFB practice enhances baroreceptor activity, vagal afferents and efferents that facilitate improvement in health outcomes [1,4,[8][9][10][11].
Recently, the application of HRV BFB has been extended to neuromuscular application and skill acquisition. For example, Gruzelier et al. [8] demonstrated that HRV BFB enhanced the technique and 3 artistry of dancers. Paul and Garg [10] posted improvement in shooting, dribbling, and passing among basketball athletes after HRV BFB. Similarly, Paul et al. [12] recorded enhancement in shooting ability of basketball athletes after HRV BFB. However, the existing mechanism in HRV BFB in relation to neuromuscular events is unknown.
To the researchers' knowledge, no previous study has investigated the influence of HRV BFB or any paced breathing technique on neuromuscular responses. An enhancement in baroreflex with HRV BFB might positively influence activity in the central autonomic network and facilitate improvement in neural excitability [1,[13][14][15][16][17][18][19][20]. Accordingly, neural excitability of the alpha motoneural pool could be revealed by the determination of the Hoffman reflex (H-reflex), a monosynaptic reflex in the spinal cord [21][22][23][24][25]. Thus, the primary purpose of this study was to determine motoneural excitability after HRV BFB utilizing the activity of H-reflex. In another light, if central autonomic network is associated with alpha motoneural pool, HRV BFB may improve the muscle-brain communication pathway and increase voluntary muscle contraction independent from H-reflex [26,27]. However, there is no study that examined influence of HRV BFB on voluntary muscle strength. Hence, the secondary purpose of this study was to identify the effect of HRV BFB on maximal isometric strength of the plantar flexors.

Participants
Eleven healthy males (height: 173.6 ± 7.53 cm; weight: 74.5 ± 17.3 kg; age: 24.6 ± 4.76 y) with no reported lower body injury 2 months prior to experimentation were recruited by YSC for this pilot study. Participants were asked to avoid any strenuous physical activity and caffeine containing food/beverage 24 hours prior to experimentation. Last meal intake was at least 2 hours before testing. Sample size in this pilot study was based on the study of Oza and colleagues [28]. A written informed consent was collected before any further experimentation. Ethical clearance for this study was obtained from the University of Tasmania Human Research Ethics Committee (H0016508), in agreement with the University of Taipei.

Procedures
The participants attended three experimentation sessions, separated by 48 h, at the Exercise 4 Performance Laboratory of University of Taipei. Measurement of anthropometrics (height and weight), identification of RF, and familiarization of MVC protocol were administered in the first session. In the second session, a randomized 10 min HRV BFB or CON was facilitated. Randomization was carried out by JCP using an online random allocation tool [29]. Upon arrival at the testing facility, an electrode attachment to the soleus muscle (SOL) for surface electromyography (EMG) and stimulation electrodes on popliteal fossa and over the patella of the right limb were applied. This was followed by pre-testing for H-reflex. Subsequently, the participants performed HRV BFB/CON with knee angle set at approximately 90-120 degrees. HRV BFB was carried out using a commercial HRV BFB equipment (HRV Starter System, Thought Technology, Canada). Participants under CON seated quietly and performed normal breathing for 10 min while asked to gaze at the computer screen. HRV BFB/CON was succeeded by post H-reflex test. Then, participants performed MVC testing. A crossover intervention was performed on Day 3. Figure 1 displays the experimental protocol for this study.

Resonance frequency
An elastic belt and finger sensor were placed on the participant to allow simultaneous recording of respiration and heart rate (HRV Starter System, Thought Technology, Canada). Then, a 5-minute familiarization of diaphragmatic breathing with visual feedback occurred. This was followed by RF testing that involves two-minute breathing at various frequencies (6.5, 6.0, 5.5, 5.0, 4.5 times per min). RF is established as the breathing pace that displayed the highest amplitude in the low frequency of HRV [5,30].

H -reflex
H-reflex was measured from the soleus (SOL) electromyography (EMG). A surface electrode (TSD150A, Biopac Systems, USA) was placed 2/3 from the medial condyles of the femur to the medial malleolus and central position in the medial-lateral direction of the border of SOL. In addition, a reference electrode was attached on the right hand of the participants. Further, an adhesive electrode (10 x 10 mm, FA 25, Gem-Stick, Australia) was placed on the right popliteal fossa as cathode and another (50 x 50 mm, Life Care, Taiwan) adhesive electrode was fixed over the right patella as the anode. After, participants were asked to seat with right knee extended at approximately 180 degrees, while the left knee flexed at 90-120 degrees. Hip flexion was 90-120 degrees. Recording of EMG signals were acquired using a commercial data acquisition system (MP150, Biopac Systems, USA) filtered with a band-pass range of 10-500 Hz and amplified with a gain of 1,000 times. The common mode rejection ratio for the EMG amplifier was 100dB. Sampling rate was set to 2.5 kHz. A single electrical impulse was applied to the right posterior tibial nerve via an electrical stimulator (DSH7, Digitimer, Herfordshire, UK) with 1000µs pulse duration to identify SOL H-reflex response. To determine maximal levels of H-wave and M-wave, stimulation intensity was increased with 10 mA increments until maximum M-wave (Mmax) was identified. The stimulation intensity for exhibiting maximum H-wave (Hmax) was determined by using 2 mA increment when the H-reflex threshold was identified (see Figure 2). To record the H-reflex responses for comparison, stimulation intensity was controlled to elicit an H-reflex response with a small incidence of M wave at 10% to 20% Mmax [31].  [22,23]. Mmax represents the entire activation of descending inputs to motor axons [24,30,31].
Hmax/Mmax ratio is the proportion of the entire motoneuron pool capable of being recruited by Іa afferents [24,32]. M wave at Hmax refers to the proportion of motor fibers activated by electrical stimulations [23,31]. During H-reflex testing, participants were encouraged to breathe normally. A sample of H wave and M wave recruitment curves from one representative participant is displayed on Figure 2.

Maximal voluntary isometric contraction
An isokinetic dynamometer (Biodex System 4, Shirley, New York) was utilized to measure MVC of plantar flexors. In this test, participants performed three x 3-second plantar flexor MVC. Prior to MVC 6 testing, the participants performed plantarflexion-dorsiflexion at force intensity of submaximal level as warm-up for 30 times, followed by a 4-minute rest. A visual feedback of torque production via a computer monitor located in front of the participant, and standardized verbal encouragement were provided to facilitate maximal effort during each trial. Rest interval between trials was one-minute.
The highest MVC torque value was recorded for analysis.

Statistical Analyses
Data are expressed as mean ± standard deviation. Normality of data was analysed using Kolmogorov-Smirnov method. A 2 x 2 repeated measures ANOVA was employed to analyse any significant main effects (condition and time) and interaction (condition x time). Effect size was estimated using eta squared. Paired T-test was carried out for any significant parameter main effect. The same method was also used to establish any difference in MVC between HRV BFB and CON. Statistical procedures were carried out using a commercial statistical package (SPSS ver 22, IBM Corp, USA). Significance was set at alpha of 0.05.

Results
Kolmogorov-Smirnov test confirmed data normality. RF of participants ranged from 4.4 to 7.4 breaths per minute (0.09 ± 0.02 Hz).

MVC
The difference in MVC between HRV BFB and CON was not significant at t(10) = -1.045, p = 0.321. Table 1 displays the H-reflex parameters and MVC with HRV BFB and CON.

Discussion
This is a novel study examining the effect of an acute HRV BFB on the neural pathway using the Hreflex of the soleus. Change of H-reflex amplitude reflects the inhibitory and excitatory neural 8 activation of alpha motoneuronal pool [27]. Results revealed no significant difference in M wave at Hmax between HRV BFB and CON. Non-significant difference in Hmax was also found in both schemes. Similarly, Mmax was not significantly different between HRV BFB and CON. The difference in Hmax/Mmax ratio from HRV BFB and CON was non-significant. M wave at Hmax/Mmax ratio were not significant different among conditions.
The non-significant findings in H-reflex properties between the HRV BFB and CON can be attributed to An interesting finding in this pilot study showed decreased Hmax/Mmax ratio after HRV BFB. Although not significantly different with CON, this finding may reflect increased presynaptic inhibition from primary afferent depolarisation [25,27,32]. Presynaptic inhibition is a primary neural mechanism to regulate the H-reflex modulation. It seems that a single HRV BFB intervention session may have reduced the motoneural pool excitability between Ia afferents and alpha motor neurons. The neural inputs at the presynaptic level inhibits neurotransmitters releasing to Ia afferent terminals and is related to HRV BFB intervention [29]. However, the exact mechanisms to influence of neural transmission via HRV BFB was unclear in our study. The reduced Hmax/Max ratio after HRV BFB in this study may be connected to stress-reduction capacity of HRV BFB. This is partially supported by the study of Bagheri et al. [33] which exhibited lower Hmax/Mmax ratio in low-stressed athletes compared to high-stressed athletes. Thus, the decreased Hmax/Mmax ratio after HRV BFB in this study indicates a promising ergogenic effect of HRV BFB.
The secondary purpose of this study was to determine the maximal isometric strength of plantar flexors after HRV BFB and CON. It was discovered that no difference in MVC existed after HRV BFB and CON. This means that both conditions exhibited similar muscle force production [34]. Possible factors that led to the outcomes can be linked to variability in interindividual responses to HRV BFB 9 and CON. It may also be possible that a single-session HRV BFB was not sufficient in improving muscle force mechanics. Thus, long-term studies in HRV BFB and MVC should be administered to elucidate information on possible muscle force capability adaptations with HRV BFB.

Conclusion
In conclusion, a single session HRV BFB does not influence the enhancement of motoneural excitability of the soleus muscle and muscle force output of plantar flexors in healthy males.

Limitations
This novel study serves as an impetus for future research within the context of HRV BFB, or any paced breathing method and neuromuscular function. However, there are limitations that should be noted.
First, generalizability of results in this study should be avoided as the outcomes are only applicable to the sample population involved. Also, H-reflex testing entails vigorous procedure, thus it was prudent to use a small sample size for this demonstration. Lastly, this study was administered in an acute session. Long-term HRV BFB may facilitate plausible results in musculoskeletal performance that CONSORT Checklist.doc