Heikki Kyröläinen’s research while affiliated with National Defence University and other places

Background Resting heart rate (HR) and HR variability (HRV) are widely used parameters to assess cardiac autonomic nervous system function noninvasively. While resting assessments can be performed during sleep or after awakening, it would be relevant to know how interchangeable the results of these measurements are. This study aimed at examining the alignment between nocturnal and morning assessments during regular endurance training and in response to intensive training. A total of 24 recreational runners performed a 3-week baseline period (BL) and a 2-week overload (OL) period (Lucia’s training impulse + 80%). Their running performance was assessed with a 3000-m running test after the BL and OL. The participants recorded daily their nocturnal HR and HRV (the natural logarithm of the root mean square of successive differences; LnRMSSD) with a photoplethysmography-based wrist device and performed an orthostatic test (2-min supine, 2-min standing) every morning with a chest-strap HR sensor. The HR and LnRMSSD segments that were analyzed from the nocturnal recordings included start value (SleepStart), end value (SleepEnd), first 4-h segment 30 min after detected sleep onset (Sleep4h), and full sleep time (SleepFull). The morning segments consisted of the last-minute average in both body positions. All segments were compared at BL and in response to the 3000-m test and OL. Results All nocturnal HR and LnRMSSD segments correlated with supine and standing segments at BL (r = 0.42 to 0.91, p < 0.05). After the 3000-m test, the HR increased and LnRMSSD decreased only in the SleepStart, Sleep4h, and SleepFull segments (p < 0.05). In response to the OL, the standing HR decreased (p < 0.01), while the LnRMSSD increased (p < 0.05) in all segments except for SleepStart. The Pearson correlations between relative changes in nocturnal and morning segments were − 0.11 to 0.72 (3000-m) and − 0.25 to 0.79 (OL). The OL response in Sleep4h HR and LnRMSSD correlated with the relative change in 3000-m time (r = 0.63, p = 0.001 and r=-0.50, p = 0.013, respectively). Conclusions Nocturnal and morning HR and LnRMSSD correlated moderately or highly in the majority of cases during the BL, but their responses to intensive training were not similarly aligned, especially in LnRMSSD. The nocturnal segments seemed to be sensitive to physical loading, and their responses were associated with the performance-related training responses.

In this paper, the novel concept of using heat flux sensors (HFS) to measure arterial pulse on the skin surface is validated. The heat flux (HF) signal is compared with simultaneously measured electrocardiogram (ECG) and photoplethysmography (PPG) signals during both rest and initial recovery from exercise. It is found that the waveform measured with the HF sensor above the radial artery has similar shape to PPG and demonstrates a clear temporal alignment between the HF pulse waves and both the PPG and ECG signals. Further, it is shown that HF measurement can be used to consistently track the arterial pulse at varying skin-to-ambient temperature gradients.

The physical capacity of male and female warfighters is challenged on the modern battlefield by heavy loads and high-intensity work. When designing training programmes for warfighters, approaches for developing strength and power alongside endurance must be considered. Strength training often requires facilities that may not be available during deployments while multiple stressors may impair or decrease overall performance. Understanding the effect of military environments on warfighter performance and acknowledging the variation in demands for individuals during field training and deployments, including possible sex differences, is essential to promote the development of adequate physical reserves (strength, power and endurance), attenuate risk for injury and promote health during and after military careers. The purpose of this narrative review is to discuss considerations for programming physical training in a military environment where ‘one size does not fit all’. In addition, a brief description of physiological contributions (neural and muscular) to strength development is included.

INTRODUCTION: Strong neck muscles may decrease the risk of flight-induced neck pain and possible disability among fast jet pilots. The purpose of this study was to examine the intra- and interrater reliability of a commercial force gauge attached to a pilot’s helmet for measuring isometric force production of the neck muscles. METHODS: A total of 41 subjects performed maximal isometric cervical flexion, extension, and lateral flexion in two measurement sessions for intrarater reliability, and 31 of these subjects participated in a third session for measuring interrater reliability. Delayed muscle soreness and neck pain were assessed using the Visual Analog Scale before and after each measurement session. The intraclass correlation coefficient (ICC) was used to compare values between the test and retest assessments. RESULTS: The overall interrater reliability was good (ICC 0.79–0.90), whereas the intrarater reliability varied from moderate to good (ICC 0.58–0.84). In both intra- and interrater reliability, the flexion test had good (ICC 0.84–0.89) reliability, while the lateral flexion test results had moderate to good (ICC 0.73–0.90) reliability. The extension test had the lowest reliability in both intra- (ICC 0.58) and interrater (ICC 0.79) tests. The average visual analog scale score (from 1–100 scale) prior to the second measurement session was 16 ± 18 in delayed muscle soreness and 0 ± 0 in neck pain. DISCUSSION: The present study demonstrated that the helmet-attached force gauge is a reliable, safe, and clinically applicable method to evaluate isometric neck strength in the flexion and lateral flexion directions. Honkanen T, Mattila V, Kinnunen O, Janhunen M, Sovelius R, Vaara JP, Kyröläinen H. Reliability of a flight helmet-attached force gauge in measuring isometric neck muscle strength . Aerosp Med Hum Perform. 2024; 95(10):788–793.

The mechanical efficiency of human locomotion has been studied extensively. The mechanical efficiency of the whole body occasionally exceeds muscle efficiency during bouncing type gaits. It is thought to occur due to elasticity and stiffness of the tendinomuscular system and neuromuscular functions, especially stretch reflexes. In addition, the lower limb joint kinetics affect mechanical efficiency. We investigated the impact of varying external work on mechanical efficiency and lower limb kinetics during repeated sledge jumping. Fifteen male runners performed sledge jumping for 4 minutes at 3 different sledge inclinations. Lower limb kinematics, ground reaction forces, and expired gases were analyzed. Mechanical efficiency did not differ according to sledge inclination. Mechanical efficiency correlated positively with the positive mechanical work of the knee and hip joints and the negative contribution of the hip joints. Conversely, it correlated negatively with both the positive and negative contributions of the ankle joint. This may be attributable to the greater workload in this study versus previous studies. To achieve greater external work, producing more mechanical energy at the proximal joint and transferring it to the distal joint could be an effective strategy for improving mechanical efficiency because of the greater force-generating capability of distal joint muscles.

Purpose Physique athletes engage in rigorous competition preparation involving intense energy restriction and physical training to enhance muscle definition. This study investigates hormonal changes and their physiological and performance impacts during such preparation. Methods Participants included female (10 competing (COMP) and 10 non-dieting controls (CTRL)) and male (13 COMP and 10 CTRL) physique athletes. COMP participants were tested 23 weeks before (PRE), one week before (MID), and 23 weeks after the competition (POST). Non-dieting CTRL participants were tested at similar intervals. Measurements included body composition (DXA), muscle cross-sectional area (ultrasound), energy availability (EA) derived by subtracting exercise energy expenditure (EEE) from energy intake (EI) and dividing by fat-free mass (FFM), muscle strength, and various serum hormone concentrations (ACTH, cortisol, estradiol, FSH, IGF-1, IGFBP-3, insulin, and free and total testosterone and SHBG). Results During the diet, EA (p < 0.001), IGF-1 (p < 0.001), IGFBP-3 (p < 0.01), and absolute muscle strength (p < 0.01–0.001) decreased significantly in both sexes in COMP. Decreases in IGF-1 were also associated with higher loss in FFM. In males, testosterone (p < 0.01) and free testosterone (p < 0.05) decreased, while SHBG (p < 0.001) and cortisol (p < 0.05) increased. Insulin decreased significantly only in males (p < 0.001). Mood disturbances, particularly increased fatigue in males (p < 0.05), highlighted the psychological strain of competition preparation. All these changes were restored by increased EA during the post-competition recovery period. Conclusion Significant reductions in IGF-1 and IGFBP-3 during competition preparation may serve as biomarkers for monitoring physiological stress. This study offers valuable insights into hormonal changes, muscle strength, and mood state during energy-restricted intense training.

Introduction Casualty evacuation has been identified as a typical and essential single military task which every soldier should be able to perform rapidly during combat. Previous studies suggest that casualty evacuation is typically conducted by dragging and demands e.g., lean body mass and anaerobic performance. Association of physical fitness with casualty evacuation by dragging has been studied widely but previous studies lack comprehensive assessment of all physical fitness determinants. The purpose of the present study was to examine comprehensively how casualty emergency evacuation (CEE) performance associates with physical fitness and body composition. Materials and Methods A total of 25 conscripts (20 men, 5 women) volunteered for measurements of height, weight, waist circumference, body composition, 1-min sit-ups and push-ups, grip strength, isometric bench and leg press, standing long jump, 30-s cycle ergometer test, and 12-min run test. Subjects performed a CEE test in which evacuation time (ET), heart rate, blood lactate concentration, and rate of perceived exertion were measured. In the CEE test, subjects wore combat gear (11.7 ± 1.6 kg) and dragged a doll wearing combat gear (80.2 kg) 28 m while crawling (go round two cones, Z-pattern) and 20 m upright (straightforward). Correlations and backward regression analysis were used for statistical analyses. The level of significance was set to P ≤ .05. Results Evacuation time lasted on average 87 ± 32 s with a peak heart rate of 184 ± 6 bpm, lactate concentration of 9.4 ± 2.7 mmol/l, and RPE of 17 ± 1. Evacuation time correlated inversely and strongly with anaerobic capacity and power (r = −0.72-0.78, P ≤ .001), but not with aerobic fitness. Inverse and strong correlations were observed between ET and maximal strength variables (r = −0.58-0.69, P ≤ .01), whereas muscular endurance and ET revealed non-significant correlations. Evacuation time correlated moderate to strongly with body fat percentage (r = 0.48, P ≤ .05) and inversely with lean body mass (r = −0.74, P ≤ .001) and body height (r = −0.53, P ≤ .01). The backward regression analysis showed that anaerobic capacity (standardized β = −0.52, P ≤ .001), fat percentage (standardized β = 0.40, P ≤ .001), and isometric leg press (standardized β = −0.25, P ≤ .1) together explained the variance of ET significantly (adjusted R2 = 0.84, P ≤ .001). Conclusions This study examined thoroughly how different physical fitness dimensions and body composition relate to a CEE test performed by a combination of dragging while crawling and in upright position. Casualty emergency evacuation was discovered as a high-intensity military task, which demands most importantly high anaerobic performance, lean body mass, and maximal strength capabilities. Improving these dimensions of physical fitness should be considered highly important as CEE is essential and possibly one of the most demanding military tasks which every soldier should be able to conduct in combat. From operational perspective, it is relevant that soldiers are able to perform CEE during operations; therefore, further research is needed on how acute operational stress changes the nature of CEE and its physical determinants.

Purpose : To examine the reproducibility and sensitivity of a self-paced field running test (SFT) in monitoring positive and negative changes in endurance performance. Methods : A total of 27 (11 women) recreational runners participated in a 6-week training intervention. The intervention was divided into a 3-week baseline period, a 2-week overload period, and a 1-week recovery period. An incremental treadmill test was performed before the baseline period and a 3000-m running test before and after all periods (T1–T4). In addition, the participants performed once a week an SFT (SFT1–6), which consisted of submaximal (6 + 6 + 3-min test at perceived exertion of 9/20, 13/20, and 17/20) and maximal sections (6 × 3-min intervals at maximum sustainable effort). The associations between the incremental treadmill test and the SFT1 performance were examined with the Pearson correlation, and the intraclass correlation was analyzed for the parameters of SFT1 to SFT3 sessions during the baseline period. The repeated-measures correlation was calculated for the 3000-m speed at T1 to T4 and the corresponding speeds at SFT. Results : Significant associations ( r = .68–.93; P < .001) were found between the speeds of the SFT and the peak and lactate threshold speeds of the incremental treadmill test. Intraclass correlations varied between .77 and .96, being the highest for the average speed of 6 × 3-minute intervals. Repeated-measures correlation was significant ( P < .05) for the 9/20 ( r = .24), 13/20 ( r = .24), and 6 × 3-minute intervals ( r = .29). Conclusions : The SFT seemed a reproducible method to estimate endurance performance in recreational runners. The sensitivity to track short-term and small-magnitude changes in performance seems more limited and might require more standardized conditions.

Background Vitamin D supplementation may lower the risk of acute respiratory infection (ARI), and the effects may be mediated through the induction of cathelicidin production. Objective To study the effect of vitamin D supplementation on ARI and cathelicidin concentration in a randomized controlled trial (RCT) and to study the associations between baseline serum 25(OH)D and ARIs and cathelicidin concentrations in a 14-week follow-up study. Methods In the RCT study, the participants were randomized into two groups to receive either 20 µg of vitamin D3 or an identical placebo daily. Blood samples were obtained three times, at the beginning (study week 0), mid-term (study week 6), and at the end of the study period (study week 14). The follow-up study had 412 voluntary young men from two different locations and seasons (January and July). The primary outcomes were the number of ARIs diagnosed and the number of days off due to ARI. Results In the RCT, vitamin D supplementation had no effect on ARI or days off due to ARI. However, regardless of the group, vitamin D insufficiency (<50 nmol/l) was associated with increased ARI. In the 14-week follow-up study, insufficient serum 25(OH)D at baseline was also associated with increased risk of ARI (OR 2.1 95% CI 1.2-3.7) and also days-off duty (OR 2.3 95% CI 1.3-4.0) and was inversely associated with cathelicidin concentration (OR 0.49 95% CI 0.24-0.99). Conclusions Sufficient serum 25(OH)D may be preventative against acute respiratory infection and the preventive effect could be mediated through the induction of cathelicidin production.

Purpose Very low intensity endurance training (LIT) does not seem to improve maximal oxygen uptake. The purpose of the present study was to investigate if very high volume of LIT could compensate the lack of intensity and is LIT affecting differently low and high intensity performances. Methods Recreationally active untrained participants (n = 35; 21 females) cycled either LIT (mean training time 6.7 ± 0.7 h / week at 63% of maximal heart rate, n = 16) or high intensity training (HIT) (1.6 ± 0.2 h /week, n = 19) for 10 weeks. Two categories of variables were measured: Low (first lactate threshold, fat oxidation at low intensity exercise, post-exercise recovery) and high (aerobic capacity, second lactate threshold, sprinting power, maximal stroke volume) intensity performance. Results Only LIT enhanced pooled low intensity performance (LIT: p = 0.01, ES = 0.49, HIT: p = 0.20, ES = 0.20) and HIT pooled high intensity performance (LIT: p = 0.34, ES = 0.05, HIT: p = 0.007, ES = 0.48). Conclusions Overall, very low endurance training intensity cannot fully be compensated by high training volume in adaptations to high intensity performance, but it nevertheless improved low intensity performance. Therefore, the intensity threshold for improving low intensity performance is lower than that for improving high intensity performance. Consequently, evaluating the effectiveness of LIT on endurance performance cannot be solely determined by high intensity performance tests.