Local muscle endurance (LME) is the ability of a given muscle/muscle group to resist fatigue when performing resistance exercise at a submaximal resistance/load. LME is vital for daily activities of life such as climbing stairs, lifting/moving objects, and in sport contexts such as, rock climbing, mixed martial arts, cross-fit, kayaking and canoeing. Therefore, understanding the mechanisms that underpin LME are of significant interest. Mitochondrial content, mitochondrial function and muscle capillarization have been purported as potential physiological factors that may influence LME. However, currently these mechanisms are speculative in nature and further research is required to draw more conclusive evidence. Furthermore, tolerance to exercise induced discomfort is another a potential mechanism of LME, whereby individuals who train under conditions that induce significant feelings of discomfort may possess a greater capacity to push through discomfort induced via LME tests. However, distinguishing between potential physiological and psychological/neural adaptations regarding LME improvements would require further investigations with nuanced methodology. Low load resistance exercise training (LLRET) has been definitively shown to improve local muscle endurance via numerous investigations. Resistance exercise training (RET), LLRET inclusive improves muscle strength which leads to greater repetition reserve capacity at lower loads. Although, Improvements in muscle strength are not specific to LLRET, yet, LLRET does yield greater gains in LME opposed to high load RET (HLRET). Therefore, LLRET likely induces vital physiological adaptations to greater extent than HLRET that drive improvements in LME such mitochondrial function, mitochondrial content and muscle capillarization. HIIT/Sprint interval training (SIT) induce significant discomfort and improve mitochondrial content/function and muscle capillarization, therefore, HIIT/SIT may be effective interventions to improve muscle endurance.
It is evident that RET of varying loads can improve strength, hypertrophy and LME and that endurance exercise training (EET) improves, VO2 Max, mitochondrial content, mitochondrial function and muscle capillarization. However, minimal research has investigated the impact of RET on single leg maximal aerobic capacity, mitochondrial content, mitochondrial function and muscle capillarization and of EET on muscle strength and muscle hypertrophy and muscle endurance. Furthermore, the findings that do exist from this body of literature are conflicted, with some suggesting RET can improve EET associated adaptions while others suggest no benefit or even decrements in aerobic condition are induced via RET. A similar pattern emerges surrounding the impact of HIIT and SIT on muscle hypertrophy, strength and local muscle endurance, whereby SIT and HIIT may induce gains in hypertrophy, strength and local muscle endurance or may yield no benefit at all. Interestingly, SIT and LLRET fall the closest to one another on the resistance exercise-endurance exercise (RE-EE) continuum suggesting that in theory there would be the largest "crossover" effect from these stimuli. Whereby SIT would elicit the greatest improvements in muscle strength and hypertrophy relative to other EET and LLRET would induce greater enhancement of EET associated adaptations relative to other RET. Although limited research has investigated this potential "crossover effect", evidence suggests that both stimuli may improve single leg maximal aerobic capacity ,mitochondrial content, mitochondrial function, muscle capillarization, muscle strength, muscle hypertrophy and local muscle endurance. However, results are in-consistent between investigations and findings are difficult to compare due to discrepancies in durations of studies, training architecture and intensity of sessions. Furthermore, to date no previous research has directly compared the effect of SIT/HIIT and LLRET on the aforementioned adaptations within the same study, leaving this topic up to speculation. The present study attempts to address this gap in the literature.
