Resistance training is known to result in an array of neural, musculo-tendinous and skeletal adaptations that enhance function/performance, reduce injury risk and improve physical and mental health. Consequently, physical activity guidelines typically recommend regular muscle strengthening exercise in perpetuity, and sporting populations frequently engage in continuous resistance exercise for months and years. However most resistance training studies, particularly those focused on neuromuscular adaptations, typically examine short- (≤4 weeks) or medium-term (≤4 months) adaptations, which limits our understanding of more prolonged adaptations. For example, during short and medium duration studies the primary adaptations may be neural, whereas the adaptations to prolonged RT may be primarily structural (in muscle, tendon and bone). In fact the importance of neural changes for prolonged RT (>4 months) remains somewhat unclear. Insight has been gained by some recent studies examining the differences between long-term resistance trained and untrained groups as well as the relatively few prolonged longitudinal investigations that have been carried out.
ECSS Paris 2023: IS-PN05
It has long been believed that neural changes are the main contributors to strength gains after short- and medium-term resistance training (RT). Indeed, there is a compelling body of evidence demonstrating that neural changes, such as increased agonist neural drive (assessed with surface electromyography: sEMG) during maximal voluntary contraction, can explain a majority of the strength gains after ≤12 weeks of RT, with muscular changes (hypertrophy) accounting for only a minor portion of the strength gains (Balshaw et al. 2017; Maeo et al. 2018). However, relatively few early studies with prolonged (6 months) longitudinal RT interventions (Narici et al. 1996; Hakkinen et al. 1998), as well as a recent cross-sectional study (Balshaw et al. 2018), suggest that increases in the agonist neural drive tend to plateau after 2-4 months of RT, whereafter muscle hypertrophy appears to play a greater role in contributing to further increases in strength. Although these findings might suggest neural adaptations to RT are completed within 2-4 months, lack of further change (i.e., plateau) in the agonist neural drive does not exclude continued adaptation of other neural mechanisms. Indeed, there is some evidence that intermuscular coordination continues to adapt with prolonged RT, and likely to contributes to continued strength gains. For example, antagonist coactivation was found to be progressively lower, between untrained controls, medium-term (12 weeks) and long-term (>3 years) resistance trained individuals (Balshaw et al. 2018). Apart from the traditional sEMG measurements, recent studies have utilized more advanced techniques such as high-density surface electromyography (HDsEMG, Casolo et al. 2021) and transcranial magnetic stimulation (TMS, Maeo et al. 2021) when comparing neural characteristics in long-term resistance trained versus untrained controls. To date, there is no clear evidence for differences in agonist neural drive between long-term resistance trained individuals and untrained controls that explain the distinct maximal strength differences between these groups. Nevertheless, recent findings and suggestions from these studies will facilitate future studies using both longitudinal and cross-sectional approaches. Based on the above, this lecture will mainly focus on the potential neural adaptations that may (or may not) contribute to further increases in maximal strength during prolonged RT. In addition, neural strategies during submaximal or rapid contractions that may be associated with improved neuromuscular performance (e.g., improved force steadiness and rapid force production) after long-term RT will be discussed. Finally, methodological considerations for other commonly used techniques (e.g., sEMG normalization to M-max, Skarabot et al. 2021; interpolated twitch technique, Follland & Willliams 2007) and one less explored but likely promising technique (e.g., fMRI) will also be introduced.
Read CV Sumiaki MaeoECSS Paris 2023: IS-PN05
Resistance training (RT) has been suggested to lead to a range of muscular and tendinous tissue adaptations that contribute to changes in function (e.g. strength, power), improved metabolic health and reduced injury risk. Yet the majority of the evidence is from short-term studies that document relatively modest musculotendinous changes, that are likely secondary to neural adaptations in this time frame, and therefore this approach may limit our understanding of the magnitude and importance of musculotendinous adaptations. In contrast prolonged exposure to resistance training may stimulate more pronounced structural changes and provide an opportunity to better understand musculo-tendinous adaptations. This lecture will describe the insights we have gained from the study of long-term RT. Much of what is known about long-term RT has been drawn from cross-sectional studies comparing long-term trained with untrained individuals, due to the limited data from more rigorous longitudinal studies of human participants training for several months or years, and the reasonable insights and weaknesses of this approach will be discussed. Increased whole muscle size, skeletal muscle hypertrophy, is often regarded as the primary adaptation to prolonged resistance training and appears to proceed at an approximately linear rate during the first 6 months of systematic progressive RT (Narici et al., 1996). Thereafter the rate of hypertrophy is assumed to slow even in response to continued training, but with very little quantitative evidence. Evidence suggests that increases in the number of sarcomeres in parallel (due to fibre hypertrophy and/or fibre hyperplasia), rather than sarcomeres in series (due to fibre/fascicle lengthening), may be more important factor for muscle growth (Maden-Wilkinson et al., 2020). Although geometric modelling suggests these adaptations may be related with an increase in the number of sarcomeres in series, having a disproportional effect on the number of sarcomeres in parallel (Jorgenson and Hornberger, 2019). Regional hypertrophy, muscle architecture and the structural changes within fibres will also be discussed. From short-term RT studies the evidence for tendon hypertrophy appears to be equivocal with some observations of region-specific increases in tendon and other reports of no change. Cross-sectional studies of long-term trained vs untrained individuals (Massey et al., 2018) may provide further insight into this debate and inform the capacity of both free tendon and aponeuroses to adapt to RT. Enhanced tendon mechanical properties (e.g. stiffness) may be a relatively early adaptation as there is evidence for no further changes after the first few months of RT (Massey et al, 2018).
Read CV Jonathan FollandECSS Paris 2023: IS-PN05
The effect of RT on bone health has received much attention as muscle contractions generated by this type of training activate many molecular mechanisms of mechanosensing and mechanotransduction that are common pathways for muscle and bone. RT has been primarily studied in adults and older adults (with or without comorbidities), but results from prolonged RT are conflicting. Several systematic reviews (with and without meta-analysis) exploring the effect of exercise on bone have been conducted. However, when we select only prolonged RT, only a smaller number of studies are available, and we observe large variations between the individual study findings. The main reason for this outcome can be attributed to the complexity of exercise interventions concerning exercise variables (e.g., exercise intensity, contraction velocity, frequency), training principles (e.g., progression, periodisation), and training conditions (e.g., supervision, devices). Also, the techniques used to quantify bone changes (Dual x-ray absorptiometry (DXA), computed tomography (CT), peripheral quantitative CT, and magnetic resonance imaging (MRI) or bone biomarkers) and the selected locations make the findings even harder to combine and draw firm conclusions. This symposium will present the results of our previous work and others, highlighting what is currently known and where more research is needed.
Read CV Elisa MarquesECSS Paris 2023: IS-PN05