Skeletal muscle remodeling in response to eccentric vs. concentric loading: morphological, molecular, and metabolic adaptations

Skeletal muscle contracts either by shortening or lengthening (concentrically or eccentrically, respectively); however, the two contractions substantially differ from one another in terms of mechanisms of force generation, maximum force production and energy cost. It is generally known that eccentric action s generate greater force than isometric and concentric contractions and at a lower metabolic cost. Hence, by virtue of the greater mechanical loading involved in active lengthening, eccentric resistance training (ECC RT) is assumed to produce greater hypertrophy than concentric resistance training (CON RT). Nonetheless, prevalence of either ECC RT or CON RT in inducing gains in muscle mass is still an open issue, with some studies reporting greater hypertrophy with eccentric, some with concentric and some with similar hypertrophy within both training modes. Recent observations suggest that such hypertrophic responses to lengthening vs. shortening contractions are achieved by different adaptations in muscle architecture. Whilst the changes in muscle protein synthesis in response to acute and chronic concentric and eccentric exercise bouts seem very similar, the molecular mechanisms regulating the myogenic adaptations to the two distinct loading stimuli are still incompletely understood. Thus, the present review aims to, (a) critically discuss the literature on the contribution of eccentric vs. concentric loading to muscular hypertrophy and structural remodeling, and, (b) clarify the molecular mechanisms that may regulate such adaptations. We conclude that, when matched for either maximum load or work, similar increase in muscle size is found between ECC and CON RT. However, such hypertrophic changes appear to be achieved through distinct structural adaptations, which may be regulated by different myogenic and molecular responses observed between lengthening and shortening contractions

Mechanisms of human skeletal muscle remodeling in response to concentric and eccentric loading paradigms

It is common knowledge that resistance exercise promotes muscle growth (hypertrophy) and increased strength and function: thus, regular exercise can help minimize the loss of muscle mass and function in healthy ageing. Skeletal muscle can contract by either shortening or lengthening (concentrically or eccentrically, respectively). A substantial number of studies focused on the effect of concentric versus eccentric training protocols on muscle morphological and functional changes: eccentric contractions are generally thought to result in more increased muscle hypertrophy and strength, because of the higher force produced by the muscle and the more severe exercise induced muscle damage, which may lead to a stronger adaptations in muscle remodeling and repair processes. Study 1 shows that ECC and CON exercise protocols lead instead to similar gains in muscle size, but through different architectural remodeling mechanisms: moreover, acute contraction-specific molecular responses have been characterised. Study 2 and Study 3 were then performed in order to gain novel insights into the relationship between these morphological adaptations and the metabolic responses (MPS, muscle protein synthesis) of human skeletal muscle in response to chronic ECC vs. CON loading paradigms. Study 2 was first carried out in order to validate the use of deuterium oxide isotope tracing technique for measuring changes in MPS in free-living subjects over longerterm periods (compared to normal AA infusion studies) of resistance exercise. After assessing the feasibility of deuterium oxide tracing technique in measuring MPS response during resitance-training protocols, study 3 investigated the chronic responses in MPS to ECC vs. CON loading in two 3 different sites of the human vastus lateralis, presenting novel insights into MPS and skeletal muscle homogeneity, attempting to link MPS changes to the different mechanisms of muscle morphological remodelling occurring after ECC vs. CON training

Reliability of panoramic ultrasound imaging and agreement with magnetic resonance imaging for the assessment of lumbar multifidus anatomical cross-sectional area

The aim of this study was to investigate the reliability of panoramic ultrasound (US) imaging and agreement with magnetic resonance imaging (MRI) for assessing the average lumbar multifidus anatomical cross-sectional area between the lumbar vertebral bodies L3-L5 (i.e., LMF ACSA$_{L3-L5}$). US and MRI scans of 20 male youth competitive alpine skiers were collected. To test the intra- and interrater reliability of US, transversal panoramic scans were analyzed on two different days by the same rater and the analysis of the first day was compared with the analysis of a second rater. To examine the agreement between US and MRI, Bland-Altman analysis was performed. Intrarater reliability was excellent, and interrater reliability was weak to good for both sides. The bias between MRI and US was - 0.19 ± 0.90 cm$^{2}$ (2.68 ± 12.30%) for the left side and - 0.04 ± 0.98 cm$^{2}$ (- 1.11 ± 12.93%) for the right side (i.e., for both sides US slightly overestimated LMF ACSA$_{L3-L5}$ on average). The limits of agreement were - 1.95 to 1.57 cm$^{2}$ (- 26.70 to 21.30%) for the left side and - 1.95 to 1.88 cm$^{2}$ (- 26.46 to 24.24%) for the right side. Panoramic US imaging may be considered a method with excellent intrarater and weak to good interrater reliability for assessing LMF ACSA$_{L3-L5}$. Comparison with MRI showed large individual differences in some cases, but an acceptable bias between the two imaging modalities

Screening Tests for Assessing Athletes at Risk of ACL Injury or Reinjury-A Scoping Review

Various tests are available to assess athletes for factors associated with their susceptibility and risk of anterior cruciate ligament (ACL) injury or reinjury; however, it is unclear which tests are clinically meaningful and what should be considered when using them. Therefore, the aim of this scoping review was to screen and summarize testing and to derive evidence-based recommendations for clinicians, practitioners and future research. Five databases were searched to identify studies addressing musculoskeletal morphology or functional-performance-related screening tests with a clear conceptual link or an evidence-based relationship to ACL (re)injury. A quality rating was carried out using the National Institutes of Health (NIH) Study-Quality Assessment Tool. Six different categories of common screening tests were identified: balance and postural control, gait- and running-related tests, joint laxity, joint morphology and anthropometrics, jump tests and strength tests. Predicting future injury in a complex, dynamic system based on a single screening test is methodologically challenging, which is also reflected in the highly controversial findings in the literature regarding potential associations between specific screening tests and the occurrence of ACL injuries and reinjuries. Nonetheless, various screening tests can provide clinically relevant information on ACL-(re)injury-related factors and help to provide tailored preventive measures. A selection of corresponding evidence-based recommendations is derived and presented in this scoping review

Biceps femoris long head morphology in youth competitive alpine skiers is associated with age, biological maturation and traumatic lower extremity injuries

Lower extremity injuries are common in competitive alpine skiers, and the knee and lower leg are often affected. The hamstring muscles, especially the biceps femoris long head (BFlh), can stabilize the knee and the hip and may counteract various adverse loading patterns during typical mechanisms leading to severe lower extremity injuries. The aim of the present study was to describe BFlh morphology in youth competitive alpine skiers in relation to sex, age and biological maturation and to investigate its association with the occurrence of traumatic lower extremity injuries in the upcoming season. 95 youth skiers underwent anthropometric measurements, maturity offset estimations and ultrasound assessment, followed by 12-months prospective injury surveillance. Unpaired t tests showed that the two sexes did not differ in BFlh morphology, including fascicle length (Lf), pennation angle (PA), muscle thickness (MT) and average anatomical cross-sectional area (ACSAavg). In contrast, U16 skiers had longer fascicles than U15 skiers (9.5 ± 1.3 cm vs 8.9 ± 1.3 cm, p < 0.05). Linear regression analyses revealed that maturity offset was associated with Lf (R 2 = 0.129, p < 0.001), MT (R 2 = 0.244, p < 0.001) and ACSAavg (R 2 = 0.065, p = 0.007). No association was found between maturity offset and PA (p = 0.524). According to a binary logistic regression analysis, ACSAavg was significantly associated with the occurrence of traumatic lower extremity injuries (Chi-square = 4.627, p = 0.031, RNagelkerke 2 = 0.064, Cohen f = 0.07). The present study showed that BFlh morphology is age- and biological maturation-dependent and that BFlh ACSAavg can be considered a relevant modifiable variable associated with lower extremity injuries in youth competitive alpine skiers. Keywords: alpine ski racing; hamstrings; injury prevention; muscle morphology; ultrasound imaging; youth athlete

More than just a side effect: Dynamic knee valgus and deadbug bridging performance in youth soccer players and alpine skiers have similar absolute values and asymmetry magnitudes but differ in terms of the direction of laterality

From a preventative perspective, leg axis and core stabilization capacities are important for soccer players and alpine skiers; however, due to different sport-specific demands, the role of laterality clearly differs and may result in functional long-term adaptations. The aims of this study are 1) to determine whether there are differences in leg axis and core stability between youth soccer players and alpine skiers and 2) between dominant and non-dominant sides, and 3) to explore the outcomes of applying common sport-specific asymmetry thresholds to these two distinct cohorts. Twenty-one highly trained/national-level soccer players (16.1 years, 95% CI: 15.6, 16.5) and 61 alpine skiers (15.7 years, 95% CI: 15.6, 15.8) participated in this study. Using a marker-based 3D motion capture system, dynamic knee valgus was quantified as the medial knee displacement (MKD) during drop jump landings, and core stability was quantified as the vertical displacement during deadbug bridging exercise (DBB $_{displacement}$ ). For the analysis of sports and side differences, a repeated-measures multivariate analysis of variance was used. For the interpretation of laterality, coefficients of variation (CV) and common asymmetry thresholds were applied. There were no differences in MKD or DBB $_{displacement}$ between soccer players and skiers or between the dominant and non-dominant sides, but there was an interaction effect side*sports for both variables (MKD: p = 0.040, η$^{2}$ p = 0.052; DBB $_{displacement}$ : p = 0.025, η$^{2}$ p = 0.061). On average, MKD was larger on the non-dominant side and DBB $_{displacement}$ laterality on the dominant side in soccer players, whereas this pattern was reversed in alpine skiers. Despite similar absolute values and asymmetry magnitudes of dynamic knee valgus and deadbug bridging performance in youth soccer players and alpine skiers, the effect on the direction of laterality was opposite even though much less pronounced. This may imply that sport-specific demands and potential laterality advantages should be considered when dealing with asymmetries in athletes

Three-dimensional mapping of ultrasound-derived skeletal muscle shear wave velocity

Introduction: The mechanical properties of skeletal muscle are indicative of its capacity to perform physical work, state of disease, or risk of injury. Ultrasound shear wave elastography conducts a quantitative analysis of a tissue's shear stiffness, but current implementations only provide two-dimensional measurements with limited spatial extent. We propose and assess a framework to overcome this inherent limitation by acquiring numerous and contiguous measurements while tracking the probe position to create a volumetric scan of the muscle. This volume reconstruction is then mapped into a parameterized representation in reference to geometric and anatomical properties of the muscle. Such an approach allows to quantify regional differences in muscle stiffness to be identified across the entire muscle volume assessed, which could be linked to functional implications. Methods: We performed shear wave elastography measurements on the vastus lateralis (VL) and the biceps femoris long head (BFlh) muscle of 16 healthy volunteers. We assessed test-retest reliability, explored the potential of the proposed framework in aggregating measurements of multiple subjects, and studied the acute effects of muscular contraction on the regional shear wave velocity post-measured at rest. Results: The proposed approach yielded moderate to good reliability (ICC between 0.578 and 0.801). Aggregation of multiple subject measurements revealed considerable but consistent regional variations in shear wave velocity. As a result of muscle contraction, the shear wave velocity was elevated in various regions of the muscle; showing pre-to-post regional differences for the radial assessement of VL and longitudinally for BFlh. Post-contraction shear wave velocity was associated with maximum eccentric hamstring strength produced during six Nordic hamstring exercise repetitions. Discussion and Conclusion: The presented approach provides reliable, spatially resolved representations of skeletal muscle shear wave velocity and is capable of detecting changes in three-dimensional shear wave velocity patterns, such as those induced by muscle contraction. The observed systematic inter-subject variations in shear wave velocity throughout skeletal muscle additionally underline the necessity of accurate spatial referencing of measurements. Short high-effort exercise bouts increase muscle shear wave velocity. Further studies should investigate the potential of shear wave elastography in predicting the muscle's capacity to perform work

Maximal Eccentric Hamstrings Strength in Competitive Alpine Skiers: Cross-Sectional Observations From Youth to Elite Level

Competitive alpine skiers are subject to substantial risks of injury, especially concerning the anterior cruciate ligament (ACL). During “landing back weighted” episodes, hamstrings may partially counteract the anterior shear force acting on the tibia by eccentrically resisting the boot-induced drawer of the tibia relative to the femur. The aim of the present study was to provide novel descriptive data and sport-specific reference values on maximal eccentric hamstrings strength (MEHS) in competitive alpine skiers from youth to elite level, and to explore potential relationships with sex, age and biological maturation. 170 competitive alpine skiers were investigated: 139 youth athletes (51 females, 88 males; age: 13.8 ± 0.59 years) and 31 elite athletes (19 females, 12 males; age: 21.7 ± 2.8 years). MEHS was assessed by the (Vald Performance, Newstead, Australia). U15 female skiers presented lower MEHS compared to female elite skiers for both limbs (R = 210 ± 44 N vs. 340 ± 48 N, respectively, p &lt; 0.001, and L = 207 ± 46 N vs. 303 ± 35 N, respectively, p &lt; 0.001). Similarly, lower MEHS was observed in U15 male skiers compared to male elite skiers for both limbs (R = 259 ± 51 N vs. 486 ± 62 N, respectively, p &lt; 0.001, and L = 258 ± 57 N vs. 427 ± 54 N, respectively, p &lt; 0.001). Correlations between MEHS and chronological age were modestly significant only for the U15 group (r = 0.37 and p &lt; 0.001). When the correlations for the U15 group were performed between MHES and maturity offset (obtained from the calculation of biological age, i.e., age at peak height velocity), statistical significance was reached by all the correlations run for 3 variables (Males &lt; 0: r = 0.59, p &lt; 0.0001; Males &gt; 0: r = 0.70, p &lt; 0.0001; and Females &gt; 0: r = 0.46, p &lt; 0.0001, start of maturity offset = 0). This cross-sectional description of MEHS in alpine skiers from youth to elite level highlights the importance of biological maturation for MEHS values in youth athletes and presents novel data that may offer insights into new approaches for injury prevention

Maximal eccentric hamstrings strength in competitive alpine skiers: cross-sectional observations from youth to elite level

Competitive alpine skiers are subject to substantial risks of injury, especially concerning the anterior cruciate ligament (ACL). During “landing back weighted” episodes, hamstrings may partially counteract the anterior shear force acting on the tibia by eccentrically resisting the boot-induced drawer of the tibia relative to the femur. The aim of the present study was to provide novel descriptive data and sport-specific reference values on maximal eccentric hamstrings strength (MEHS) in competitive alpine skiers from youth to elite level, and to explore potential relationships with sex, age and biological maturation. 170 competitive alpine skiers were investigated: 139 youth athletes (51 females, 88 males; age: 13.8 ± 0.59 years) and 31 elite athletes (19 females, 12 males; age: 21.7 ± 2.8 years). MEHS was assessed by the (Vald Performance, Newstead, Australia). U15 female skiers presented lower MEHS compared to female elite skiers for both limbs (R = 210 ± 44 N vs. 340 ± 48 N, respectively, p &lt; 0.001, and L = 207 ± 46 N vs. 303 ± 35 N, respectively, p &lt; 0.001). Similarly, lower MEHS was observed in U15 male skiers compared to male elite skiers for both limbs (R = 259 ± 51 N vs. 486 ± 62 N, respectively, p &lt; 0.001, and L = 258 ± 57 N vs. 427 ± 54 N, respectively, p &lt; 0.001). Correlations between MEHS and chronological age were modestly significant only for the U15 group (r = 0.37 and p &lt; 0.001). When the correlations for the U15 group were performed between MHES and maturity offset (obtained from the calculation of biological age, i.e., age at peak height velocity), statistical significance was reached by all the correlations run for 3 variables (Males &lt; 0: r = 0.59, p &lt; 0.0001; Males &gt; 0: r = 0.70, p &lt; 0.0001; and Females &gt; 0: r = 0.46, p &lt; 0.0001, start of maturity offset = 0). This cross-sectional description of MEHS in alpine skiers from youth to elite level highlights the importance of biological maturation for MEHS values in youth athletes and presents novel data that may offer insights into new approaches for injury prevention

Altered regional 3D shear wave velocity patterns in youth competitive alpine skiers suffering from patellar tendon complaints - a prospective case-control study

Patellar tendon (PT) complaints are frequent throughout the population, with increased occurrence in athletes and, particularly, in youth competitive alpine skiers. Timely detection and treatment might improve prospects of recovery. Diagnostic modalities in clinical use to date rely on pain symptoms, manual palpation, and potentially, magnetic resonance imaging (MRI); however, MRI-based imaging yields limited sensitivity. Quantitatively measuring the morphological and mechanical properties of PTs by means of B-mode ultrasound and shear wave elastography (SWE), instead, may allow improved diagnosis or even early detection. We performed B-mode scans and three-dimensional ultrasound shear wave velocity (SWV) mapping and MRI of the PT in 106 youth skiers. A prospective one-year survey on health problems combined with clinical assessments served to categorize symptomatic and asymptomatic youth skiers. Skiers suffering from distal or proximal tendon complaints showed lower SWV in the respective tendon region than asymptomatic skiers (p = 0.035 and p = 0.019, respectively). Youth skiers with distal tendon complaints additionally exhibited decreased SWV in the proximal region compared to asymptomatic counterparts (p = 0.020). Cross-validated analysis of retrospective prediction indicated sensitivity and specificity in detecting tendon complaints in the range of 0.606-0.621 and 0.536-0.650, respectively. MRI detected distal tendon complaints with a sensitivity of 0.410 (12/29) but failed to detect any proximal cases. This study agrees with the most recent literature in that SWE holds promise as a valuable adjunct modality for the diagnosis of PT complaints or even the detection of subclinical prestages. However, to evaluate its prospective predictive value, long-term studies are warranted. HighlightsPatellar tendon complaints are a frequent complaint in athletes, particularly in youth competitive alpine skiers, but timely quantitative detection of related tendon properties remains challenging.Quantitative B-mode US and three-dimensional ultrasound shear wave elastography assessments and magnetic resonance imaging were performed in youth competitive alpine skiers.Three-dimensional shear wave elastography was able to discern symptomatic from asymptomatic patellar tendons both in the distal and proximal tendon regions, whereas magnetic resonance imaging failed to detect any proximal cases. Keywords: Biomechanics; athletes; cumulative trauma disorders