Compatibility of strength and endurance training

Combined (or concurrent) training is defined as simultaneously incorporating both strength and endurance exercise within a periodized training regime (Fyfe et al. 2014; Hickson 1980; Häkkinen et al. 2003; Paavolainen et al. 1999a). In this thesis, combined training refers to training both strength and endurance during the same microcycle (e.g. the same week) but not in the same session, whereas in concurrent training the same training session includes both training regimes.

Many typical military tasks such as load carriage, moving under fire, casualty evacuation and manual materials handling require both qualities of physical fit-ness, so the development of optimal occupational performance capacity of a sol-dier most likely requires a combination of strength and endurance training.

The effects of combined and concurrent training have been studied exten-sively since the early 1980´s, when Hickson (1980) presented the theory of inter-ference effect. The original interinter-ference effect refers to attenuated adaptation of muscular (i.e. maximal) strength following 6-10 weeks of high frequency (5 times strength + 6 times endurance training per week) and high volume (each mode for 30-40 min per session) concurrent training. Later, it was shown that intensive concurrent or combined strength and endurance training interferes especially with the development of muscular power (i.e. explosive strength) during a pro-longed (>12 weeks) training period (Häkkinen et al. 2003). In fact, Häkkinen et al.

(2003) reported no interference of muscular strength or hypertrophy during a 21-week (gradually increasing intensity) strength training intervention performed only twice a week versus the same training program combined with additional endurance training, also performed twice a week. However, no development in explosive force production (i.e. muscular power) was observed in the combined training group, with a significant difference compared to strength training only.

Several studies supporting the findings of Häkkinen et al. (2003) have been pre-sented thereafter (Eklund et al. 2015; Schumann et al. 2015), also within the mili-tary context (Santtila et al. 2009a).

In a 6-week follow-up (Jones et al. 2013), training adaptations were exam-ined in twenty-four strength traexam-ined men in response to concurrent strength and endurance training three times per week, but with varying training ratios (e.g.

1:1 = endurance training session followed each strength training session; 3:1 = endurance training session was performed after every third strength training ses-sion). The training groups were also compared to a control group that performed no training and a group that only performed strength training. Both strength (5

x 6 repetitions at 80% 1RM) and endurance (30 min of continuous repetitions at 30% 1RM) training included the performance of unilateral leg extensions. The main findings in the concurrent training groups were that one weekly endurance training session (group 3:1) did not interfere with muscular strength develop-ment, whereas three weekly endurance training sessions led to an interference effect. In addition, compared to the control group, muscle mass (thigh girth) only increased in the group that performed one weekly endurance training session.

Thus, Jones et al. (2013) concluded that the magnitude of the interference effect was related to endurance training frequency (and/or overall volume of endur-ance training), with higher endurendur-ance training frequency resulting in larger in-terference.

Fyfe et al. (2014) suggested that a combination of a high endurance training load and inadequate recovery induces residual fatigue, which may attenuate strength development due to poorer training quality and thus a compromised adaptation stimulus. The suggested major modulators of interference include ex-ercise order (endurance first), proximity (inadequate recovery time between the training modes), and high endurance training load (Fyfe et al. 2014). Häkkinen et al. (2003) demonstrated that attenuated neural drive, caused by fatigue from pre-ceding endurance exercise, may ultimately lead to interference in explosive strength development. Jones et al. (2017) reported that endurance exercise per-formed prior to strength training induced greater blood cortisol and lactate con-centrations compared to the opposite exercise order and thus, impaired the sub-sequent strength training performance. Other suggested mechanisms that impair the development of muscular strength and power include endurance training-induced muscle damage, depletion of muscle glycogen stores, skeletal muscle fi-ber-type transformations towards slower types, and decreased muscle mass (García-Pallarés & Izquierdo 2011; Leveritt et al. 1999). Various differences in the setting of combined or concurrent training studies have made the investigation of interference challenging. These differences include the training history of the study population, training mode, volume and frequency, as well as sequencing and length of recovery between the training modes. Moreover, different studies have employed different methods of measuring strength and endurance perfor-mance.

Regarding the military context, Burley et al. (2020) hypothesized that by re-ducing the overall training load of endurance-type military training, lower vol-ume and higher intensity combined strength and endurance training may induce more positive adaptations in physical performance of recruits during basic mili-tary training. During the 12-week study, milimili-tary recruits in the experimental group performed 40 individualized high-intensity strength (6-8RM free weight deadlifts, squats etc.) and endurance (3-min interval runs above 80% of heart rate reserve) training sessions, whereas the control group performed a similar volume of standard military physical training (i.e. moderate-to-high intensity running, circuit training, load carriage). The main outcomes of this study included im-provements in muscular strength, power and endurance in response to individ-ualized, high-intensity, low-volume training, which also required a smaller time

commitment during military basic training. In addition, the experimental group improved aerobic fitness and load carriage performance more than the control group, despite performing 50% fewer endurance training sessions, with a corre-sponding reduction in objectively measured physical activity. Together, these findings suggest that more individualized progressive training periodization with lower total training load may improve overall training quality, and result in more effective training adaptations during military training.

Taking into consideration the methodological challenges, combined strength and endurance training has proven to be an effective and time-efficient method of improving physical fitness and occupational performance within the military context (Kyröläinen et al. 2018). It has been suggested that at least eight hours should separate the two training modes to ensure proper recovery, and if possible, strength and endurance training should target different muscle groups to maximize recovery and adaptations (Jones & Howatson 2019, 150-151). More recently, Doma et al. (2019) suggested that due to similar mechanisms including residual neuromuscular fatigue, also endurance training outcomes may be im-paired following a strength training session. However, longitudinal negative ef-fects have not been consistently observed, especially among recreationally active individuals (Rønnestad & Mujika 2014; Schumann et al. 2014; Taipale et al. 2010).

In order to optimize endurance training adaptations, acute fatigue resulting from high volume, high intensity strength training (up to 24 hours) should be avoided (Eklund et al. 2016), especially when preparing for a high intensity endurance training session (Doma et al. 2019).

The present thesis investigated physical workload and associations between occupational performance and body composition, as well as physical fitness variables during a 6-month crisis management operation in the Middle East. An additional purpose of the thesis was to determine the optimal distribution of strength and endurance training to maintain or improve the physical fitness of crisis management soldiers during deployment. The specific aims and hypothe-ses of each study in this thesis were:

1) To investigate changes in body composition, serum and saliva stress bi-omarkers, objectively measured volume and intensity of physical activity, and heart rate responses in soldiers during a 6-month international crisis management operation. Based on previous studies (Henning et al. 2011; Nindl et al. 2013), it was hypothesized that the occupational workload would result in symptoms of accumulative stress but to a lesser extent than observed pre-viously during combat operations (Original paper I).

2) To evaluate cross-sectional associations between physical fitness and body composition characteristics and simulated high-intensity military task perfor-mance whilst wearing combat load. The primary hypothesis was that muscu-lar power of the lower extremities, together with greater muscle mass and less body fat (Lyons et al. 2005; Mala et al. 2015), would be associated with better performance whilst wearing combat load (Original paper II).

3) To examine the effects of different combinations of strength and endurance training on body composition, physical performance and serum anabolic and catabolic biomarkers during a six-month crisis management operation in the Middle East. Based on previous literature (Dyrstad et al. 2007; Haff 2017, 181-205; Warr et al. 2013), it was hypothesized that aerobic fitness in particular would decrease during the operation, but with a periodized strength and en-durance training program performed 2-3 times per week, enen-durance perfor-mance would be preserved during deployment (Original paper III).

3 PURPOSE OF THE THESIS

4) To investigate individual training responses and adaptations of endurance performance to combined strength and endurance training during a 6-month crisis management operation in the Middle East. It was hypothesized that in-dividual-specific training factors such as fitness status, training history and body composition (Burley et al. 2018; Impellizzeri et al. 2019; Kyröläinen et al.

2018; Pihlainen et al. 2020) would influence training adaptations during the operation (Original paper IV).