Heart rate variability, task load and perceived exertion associated with a long-distance military ski exercise: A pilot study

INTRODUCTION

Skiing through mountainous terrains has been practiced by the armed forces, competitive and recreational skiers for years. Ski mountaineering is a demanding winter sport that requires climbing up several steep slopes as fast as possible then descend through rugged, off-piste snow completely unprepared and ungroomed.^[1,2] Early research evidences show that the skiing used to be practiced by military troops in Europe.^[1] In India, ski mountaineering is considered as a regular activity for mountain warfare. The troops need to overcome challenges such as high altitude, hypoxia, steep terrains, wind, snowfall and so on.^[1,3] The military activities at mountains are little different from the professional ski events. The operations include patrolling, covering of posts, skiing through snowy terrains to maintain law and order and taking up avalanche rescue missions to save lives and wealth. Sometimes, the authorities organise recreational and competitive skiing to utilise the environmental conditions and divert the monotony of the troops.^[3,4] Be it a mandatory or as recreational activity, skiing demands high level of stamina, energy, skill and motor control to generate propulsive force by the upper and lower body muscles to climb up and sliding down.^[1,3] The soldiers like the professionals and amateurs need to undertake extensive training programme lasting about 16 ± 0.9 h weekly with an average ascent of 50,814 ± 3654 m to build strength, aerobic and anaerobic endurance, coordination, motor skill, balance and mobility.^[3,5] They need to carry essential items such as food, water, ammunitions, communication devices and appropriate clothing during rescue operations or patrolling the area.^[4] Use of ski equipment with additional load at rucksack and bulky clothing make the movement more difficult.^[4] Scientific studies on training events performed at snowy and mountainous terrain are limited.^[5] Thus, further studies on snow and mountaineering training events may provide more scientific information regarding physiological and mental stress.

Monitoring of heart rate (HR) and its beat-to-beat modulation is applied for the prediction of effect of factors such as environment, terrain and operational strain on human physiology and behaviour. Heart rate variability (HRV) is the fluctuation in the time intervals between adjacent heartbeats.^[6] Investigating the changes in HR and different HRV spectral components are associated with different branches of autonomic nervous system (ANS). The ANS is divided into two branches, namely sympathetic nervous system and parasympathetic nervous system. There are two major components of HRV data such as time domain and frequency domain.^[7,8] Scientific studies on HRV have established its efficacy as tool to predict mental fatigue^[9] as well as a predictive marker of cognitive functioning.^[10] Physiological evaluation on professional and recreational skiers is rare apart from few case studies and studies considering group of participants.^[11] These studies generally focused on protein intake, bronchoconstriction and energy cost of skiers to ultimately establish skiing as a heavy physical activity same as other endurance sports such as cross-country cycling or marathon.^[11] However, few studies have mentioned that 70% of the entire workload remains above the first ventilatory threshold and more than 20% remains above second ventilatory threshold with HR exceeding 90% of maximum HR (HR_max) which is uncommon in many of the other physically demanding sports activities.^[11,12]

Subjective questionnaires are regularly used in experimental studies to predict overall workload and perceived exertion of a physical task.^[13] The subjective response of a person is influenced by current goal, motives and plans thus can be applied to scientific studies including physically intense tasks.^[14] Use of National Aeronautical and Space Administration Task Load Index (NASA-TLX) and Borg’s 14-point rating scale that predicts perfectly the perception of exertion are the most recurrently used subjective rating methods for experimental studies.^[15,16] These subjective scores may complement the findings from real-time monitoring physiological data and autonomic responses. With this background, the present study was undertaken with the aim to understand the physical and mental stress during a military skiing activity (training) through measurement of pre- and post-exercise HR and HRV, task load and perceived exertion. The second aim of the study was to find out the association of post-exercise physiological and mental workload variables to ascertain their interdependencies. The acquired data will be helpful to find out the autonomic response of the personnel to measure physiological and mental stress while carrying out intensive cold and snowy conditions.

MATERIALS AND METHODS

RESULTS

The ambient temperature, RH and barometric pressure during the snow and mountain climbing events were −6–−4°C, 50−65% and 570−590 mmHg. The baseline data of the participants are presented in [Table 1], which ensured appropriate acclimatisation to the environment through acceptable range of resting HR, blood pressure and oxygen saturation. Although medically cleared, the participants’ oxygen saturation (about 94%) was little lower than expected (above 95% is expected below 9000 ft); was not considered as a point of concern due to their tight schedule and lack of proper rest in between activities. All the participants needed about 3 h to complete the race which was within the time limit set by the institute.

DISCUSSION

Physiological studies have classified different types of skiing events, namely Alpine skiing, Nordic skiing, cross-country skiing and ski mountaineering. Alpine skiing is practiced in an altitude below 1800 m consisting of several short and long duration events which require climbing up and down through unbitten snow.^[19] The cross-country skiing is carried out for long duration like more than 5 h to cover long distances like 50 km in well-prepared snow at less hilly terrain. The ski mountaineering is generally practiced at an altitude more than 1800 m including multiple activities like climbing up on steeper gradients (2–3 slopes). Other activities include walking without ski and sliding down through off-piste snow.^[11] Research work covering ski mountaineering has categorised the physiological demand as ‘heavy’ while the HR reaching above 90% of the HR_max for major parts of the exercises same as 50–60 km cross-country cycling or long-distance running as marathons.^[11] The ski activity reported in the present study somehow resembled ski mountaineering due to its variations in activities carried out at moderate high altitude. The activities include: Climbing up multiple slopes with/without ski, reaching certain point, gathering valuable information like weather report and avalanche prediction. Besides this, they need to carry out patrolling certain areas to inspect presence of threat and carry out dummy rescue operations as well which play crucial role in soldiers’ preparation for winter and mountain duties. Here, precision, skill and stamina play an important role to provide additional edge to the troops.^[20] Thus, sometime, the physiological workload may not appear to be as hard as expected. However, the results indicated 34.8% increase in HR from pre- to post-exercise recordings. Continuous monitoring of physiological variables was beyond the scope of the present study. Hence, the physiological stress was predicted through such a significant difference in pre- and post-exercise HR.^[21] The participants rated their perceived exertion ‘somewhat hard’ (about 14) and scored as high as ‘79’ as ‘PD’ (NASA-TLX).^[15,16] Such findings predicted presence of physical stress in the ski mountaineering event.

The results of HRV analyses showed post-exercise decrease in the time domain parameters such as mean RR, RMSSD and pNN50 compared to the pre-exercise values. Whereas, the frequency domain parameters showed increase in LF% and LF/HF ratio and decrease in HF% after the exercise. Reduction of the time domain parameters generally occurs after any heavy physical activity mainly due to withdrawal or diminished functions of the parasympathetic domain followed by delayed onset of recovery due to the exertion.^[21] Increase in LF% and LF/HF ratio indicates central modulation of sympathetic tone through cardiovascular active neurons in the lower brainstem by adrenergic dominance during physical exercise.^[22] Such responses can also be linked to additional mental efforts needed to execute various physical tasks in the ski mountaineering event.^[23,24] The sympathetic hyperactivity during the beginning of the recovery could be attributed to exercising in freezing cold and hypoxic conditions.^[25,26] Cold stimulus affecting physical performance in skiers has been published in past and proved that the autonomic responses of the participants of the current work have occurred as normal consequences of skiing at sub-zero temperatures.^[19] In a study on cross-country skiers, the researchers have established that recording of HR recovery play a crucial role in predicting their physical performance and also their ability to tolerate stress.^[27] These evidences point out the utility of recording of post-exercise recovery to understand the overall requirement of physical and mental efforts to carry out ski mountaineering with equipment at moderately high-altitude areas for 3 h.

The total average score (NASA-TLX) of the military ski mountaineering event was ‘77’ which depicted expression of a heavy overall task load. Studies in the area of exercise physiology in the past have proved that exercising outdoor puts extra burden on human system with additional requirements of planning, coordinating sensory and motor activities, avoiding injuries or accidents, etc.^[28,29] The subjective responses of the participants: MD – 69.7, PD – 78.8, TD – 69.1, PF – 70.9, EF – 68.2, FR – 43.2, TS – 77.0 and RPE – 13.8 provided logical and acceptable insight into the perception of physical and mental effort requirements. They had to utilize their technical and tactical adaptability to overcome challenges like the variations in quality and the texture of the snow and encounter sudden slopes and turns that could affect the interaction of the equipment.^[2,30] Positive significant correlations between EF and pNN50 and RMSSD and negative significant correlations between LF/HF ratio and PD partially supported the notion that the 15 km skiing-induced physical and mental burden on the participants. However, more accurate prediction of the physical stress, autonomic response to it and associated mental stress could be established with a larger sample size.

CONCLUSION

The significant increase in post-exercise ‘mean HR’ than pre-exercise values could identify presence of physical stress due to 15 km ski mountaineering event. This finding was supported by ‘somewhat hard’ perceived exertion (about 14 out of 20) and ‘high’ PD (79 out of 100) expressed through NASA-TLX score. Recordings of pre- and post-exercise HRV indicated withdrawal of parasympathetic control and subsequent delayed onset of recovery after exercise as well as dominance of sympathetic system. The sympathetic hyperactivity could also be linked to extra mental effort requirements of the ski mountaineering with overall high task load expressed through NASA-TLX scores (especially total score – 77). Overcoming the natural adversities such as snow, freezing temperatures, hypoxia, upward and downward slopes, turns and control of speed of movement and other task demands can be attributed to the above-mentioned findings. Such results are important to understand exact requirements of the mountain warfare events carried out by the forces. The results could be utilised by the instructors, commanders for training and real-time operation purposes.