Arno Schroll

Evidence of imbalanced adaptation between muscle and tendon in adolescent athletes

Adolescence may be regarded as a critical phase of tissue plasticity in young growing athletes, as the adaptation process of muscle‐tendon unit is affected by both environmental mechanical stimuli and maturation. The present study investigated potential imbalances of knee extensor muscle strength and patellar tendon properties in adolescent compared with middle‐aged athletes featuring long‐term musculotendinous adaptations. Nineteen adolescent elite volleyball athletes [(A), 15.9 ± 0.6 years] and 18 middle‐aged competitively active former elite volleyball athletes [(MA), 46.9 ± 0.6 years] participated in magnetic resonance imaging and ultrasound‐dynamometry sessions to determine quadriceps femoris muscle strength, vastus lateralis morphology and patellar tendon mechanical and morphological properties. There was no significant age effect on the physiological cross‐sectional area of the vastus lateralis and maximum knee extension moment (P > 0.05) during voluntary isometric contractions. However, the patellar tendon cross‐sectional area was significantly smaller (A: 107.4 ± 27.5 mm2; MA: 121.7 ± 39.8 mm2) and the tendon stress during the maximal contractions was significantly higher in adolescent compared with the middle‐aged athletes (A: 50.0 ± 10.1 MPa; MA: 40.0 ± 9.5 MPa). These findings provide evidence of an imbalanced development of muscle strength and tendon mechanical and morphological properties in adolescent athletes, which may have implications for the risk of tendon overuse injuries.

Asymmetry of Achilles tendon mechanical and morphological properties between both legs

Although symmetry of Achilles tendon (AT) properties between legs is commonly assumed in research and clinical settings, different loading profiles of both legs in daily life (i.e., foot dominance) may affect the tendon properties in a side‐depended manner. Therefore, AT properties were examined with regard to symmetry between legs. Thirty‐six male healthy adults (28 ± 4 years), who were physically active but not involved in sports featuring dissimilar leg load participated. Mechanical and morphological AT properties of the non‐dominant and dominant leg were measured by means of ultrasound, magnetic resonance imaging and dynamometry. The AT of the dominant leg featured a significant higher Youngs modulus and length (P < 0.05) but a tendency toward lower maximum strain (P = 0.068) compared with the non‐dominant leg. The tendon cross‐sectional area and stiffness were not significantly different between sides. The absolute asymmetry index of the investigated parameters ranged from 3% to 31% indicating poor AT side symmetry. These findings provide evidence of distinct differences of AT properties between both legs in a population without any sport‐specific side‐depended leg loading. The observed asymmetry may be a result of different loading profiles of both legs during daily activities (i.e., foot dominance) and challenges the general assumption of symmetrical AT properties between legs.

Insufficient accuracy of the ultrasound-based determination of Achilles tendon cross-sectional area.

The accurate assessment of the Achilles tendon cross-sectional area (CSA) is a crucial prerequisite to investigate tendon adaptation and to calculate the tendon Young׳s modulus. Besides magnetic resonance imaging (MRI), ultrasonography emerged as an alternative imaging technique. The purpose of the present study was to assess the objectivity and reliability of the ultrasound-based methodology and its validity with respect to the established MRI-based assessment. The Achilles tendon CSA from 17 healthy males was assessed by ultrasonography at the proximal, medial and distal position five times each on two separate days and three observers segmented the CSAs. For the validation, MRI-based CSA measurements were provided accordingly. The analysis of the ultrasound-based method revealed significant observer and day effects (p<0.05), despite high intra-class correlation coefficients for the three observers (>0.93) and both days (>0.89), respectively. The mean typical percentage error of both days was 7.1%. Comparing both methods, ultrasonography underestimated (19%) the CSA values obtained by MRI (p<0.05). Although the correlation coefficients of both methods were high in all three positions (>0.88), the respective absolute difference of in average 12mm2 and limits of agreement of up to +28mm2 and -2mm2 (~37% of the mean CSA) evidence considerable inconsistency. Besides the dependency of the CSA determination from the observer, the ultrasound method is not sensitive enough to detect physiological changes in tendon morphology of the magnitude that is reported by intervention studies. Furthermore, the validity analysis revealed a lack of agreement between both methods. Therefore, the ultrasound-based methodology cannot be recommended for an accurate Achilles tendon CSA determination in vivo.

Muscle and tendon adaptation in adolescent athletes: A longitudinal study

There is evidence that a non‐uniform adaptation of muscle and tendon in young athletes results in increased tendon stress during mid‐adolescence. The present longitudinal study investigated the development of the morphological and mechanical properties of muscle and tendon of volleyball athletes in a time period of 2 years from mid‐adolescence to late adolescence. Eighteen elite volleyball athletes participated in magnetic resonance imaging and ultrasound‐dynamometry sessions to determine quadriceps femoris muscle strength, vastus lateralis, medialis and intermedius morphology, and patellar tendon mechanical and morphological properties in mid‐adolescence (16 ± 1 years) and late adolescence (18 ± 1 years). Muscle strength, anatomical cross‐sectional area (CSA), and volume showed significant (P < 0.05) but moderate increases of 13%, 6%, and 6%, respectively. The increase of patellar tendon CSA (P < 0.05) was substantially greater (27%) and went in line with increased stiffness (P < 0.05; 25%) and reduced stress (P < 0.05; 9%). During late adolescence, a pronounced hypertrophy of the patellar tendon led to a mechanical strengthening of the tendon in relation to the functional and morphological development of the muscle. These adaptive processes may compensate the unfavorable relation of muscle strength and tendon loading capacity in mid‐adolescence and might have implications on athletic performance and tendon injury risk.

Muscle shape consistency and muscle volume prediction of thigh muscles

The present study investigated the applicability of a muscle volume prediction method using only the muscle length (LM), the maximum anatomical cross‐sectional area (ACSAmax), and a muscle‐specific shape factor (p) on the quadriceps vastii. LM, ACSAmax, muscle volume, and p were obtained from magnetic resonance images of the vastus intermedius (VI), lateralis (VL), and medialis (VM) of female (n = 20) and male (n = 17) volleyball athletes. The average p was used to predict muscle volumes (Vp) using the equation Vp = p × ACSAmax × LM. Although there were significant differences in the muscle dimensions between male and female athletes, p was similar and on average 0.582, 0.658, 0.543 for the VI, VL, and VM, respectively. The position of ACSAmax showed low variability and was at 57%, 60%, and 81% of the thigh length for VI, VL, and VM. Further, there were no significant differences between measured and predicted muscle volumes with root mean square differences of 5–8%. These results suggest that the muscle shape of the quadriceps vastii is independent of muscle dimensions or sex and that the prediction method could be sensitive enough to detect changes in muscle volume related to degeneration, atrophy, or hypertrophy.

Athletic training affects the uniformity of muscle and tendon adaptation during adolescence.

With the double stimulus of mechanical loading and maturation acting on the muscle-tendon unit, adolescent athletes might be at increased risk of developing imbalances of muscle strength and tendon mechanical properties. This longitudinal study aims to provide detailed information on how athletic training affects the time course of muscle-tendon adaptation during adolescence. In 12 adolescent elite athletes (A) and 8 similar-aged controls (C), knee extensor muscle strength and patellar tendon mechanical properties were measured over 1 yr in 3-mo intervals. A linear mixed-effects model was used to analyze time-dependent changes and the residuals of the model to quantify fluctuations over time. The cosine similarity (CS) served as a measure of uniformity of the relative changes of tendon force and stiffness. Muscle strength and tendon stiffness increased significantly in both groups (P < 0.01). However, the fluctuations of muscle strength were greater [A, 17 ± 7 (SD) N·m; C, 6 ± 2 N·m; P < 0.05] and the uniformity of changes of tendon force and stiffness was lower in athletes (CS A, -0.02 ± 0.5; C, 0.5 ± 0.4; P < 0.05). Further, athletes demonstrated greater maximum tendon strain (A, 7.6 ± 1.7%; C, 5.5 ± 0.9%; P < 0.05) and strain fluctuations (A, 0.9 ± 0.4; C, 0.3 ± 0.1; P < 0.05). We conclude that athletic training in adolescence affects the uniformity of muscle and tendon adaptation, which increases the demand on the tendon with potential implications for tendon injury.

Validation of a simplified method for muscle volume assessment

The present study investigated the validity of a simplified muscle volume assessment that uses only the maximum anatomical cross-sectional area (ACSAmax), the muscle length (LM) and a muscle-specific shape factor for muscle volume calculation (Albracht et al., 2008, J Biomech 41, 2211-2218). The validation on the example of the triceps surae (TS) muscles was conducted in two steps. First LM, ACSAmax, muscle volume and shape factor were calculated from magnet resonance image muscle reconstructions of the soleus (SO), gastrocnemius medialis (GM) and lateralis (GL) of a group of untrained individuals (n=13), endurance (n=9) and strength trained (n=10) athletes. Though there were significant differences in the muscle dimensions, the shape factors were similar across groups and were in average 0.497 ± 0.026, 0.596 ± 0.030, and 0.556 ± 0.041 for the SO, GM and GL respectively. In a second step, the shape factors were applied to an independent recreationally active group (n=21) to compare the muscle volume assessed by the simplified method to the results from whole muscle reconstructions. There were no significant differences between the volumes assessed by the two methods. In conclusion, assessing TS muscle volume on the basis of the reported shape factors is valid across populations and the root mean square differences to whole muscle reconstruction of 7.9%, 4.8% and 8.3% for SO, GM and GL show that the simplified method is sensitive enough to detect changes in muscle volume in the context of degeneration, atrophy or hypertrophy.

The effect of age and sex on the cervical range of motion – A systematic review and meta-analysis

Cervical-flexibility examination is routinely performed in neck-pain patients. However, diagnosis of cervical-flexibility impairment requires physiological reference values, which vary widely among the population. Although there is a general understanding that the cervical range of motion (RoM) alters with age and sex, the consolidated details of these variations remain lacking. A systematic review and meta-analysis was performed to evaluate the difference of cervical RoM in different age and sex populations. The quality-assessment tool for quantitative studies was applied to assess methodological quality. We identified 4,034 abstracts through a database search and 3 publications through a manual search. Thirty-four cross-sectional studies were selected for the systematic review and measuring technologies were identified. The difference in age descriptions was substantial and a strong discrepancy existed between the mobility measured by radiological and non-radiological devices. Therefore, only 11 non-radiological studies with similar age descriptions were selected for meta-analysis. Cervical RoMs varied considerably among the populations and generally decreased with age. However, this diminishment started earlier and ended later in males, and was not continuous across age in both sexes. Females normally displayed a greater RoM than males, except in lateral bending. In young subjects, the difference between males and females was not significant. For subjects in their 50s, males displayed a non-significantly greater RoM than females. The variability of cervical RoMs can be explained by different devices as well as age and sex. However, the age-dependent reduction is not continuous and differs between males and females. These findings lay the foundation for a better understanding of the incidence of age- and sex-dependent cervical disorders, and may have important implications for the long-term success of different clinical interventions.

The Maximum Lyapunov Exponent During Walking and Running: Reliability Assessment of Different Marker-Sets

The maximum Lyapunov exponent (MLE) has often been suggested as the prominent measure for evaluation of dynamic stability of locomotion in pathological and healthy population. Although the popularity of the MLE has increased in the last years, there is scarce information on the reliability of the method, especially during running. The purpose of the current study was, thus, to examine the reliability of the MLE during both walking and running. Sixteen participants walked and ran on a treadmill completing two measurement blocks (i.e., two trials per day for three consecutive days per block) separated by 2 months on average. Six different marker-sets on the trunk were analyzed. Intraday, interday and between blocks reliability was assessed using the intraclass correlation coefficient (ICC) and the root mean square difference (RMSD). The MLE was on average significantly higher (p < 0.001) in running (1.836 ± 0.080) compared to walking (1.386 ± 0.207). All marker-sets showed excellent ICCs (>0.90) during walking and mostly good ICCs (>0.75) during running. The RMSD ranged from 0.023 to 0.047 for walking and from 0.018 to 0.050 for running. The reliability was better when comparing MLE values between blocks (ICCs: 0.965–0.991 and 0.768–0.961; RMSD: 0.023–0.034 and 0.018–0.027 for walking and running respectively), and worse when considering trials of the same day (ICCs: 0.946–0.980 and 0.739–0.844; RMSD: 0.042–0.047 and 0.045–0.050 for walking and running respectively). Further, different marker-sets affect the reliability of the MLE in both walking and running. Our findings provide evidence that the assessment of dynamic stability using the MLE is reliable in both walking and running. More trials spread over more than 1 day should be considered in study designs with increased demands of accuracy independent of the locomotion condition.

Muscle Strength and Neuromuscular Control in Low-Back Pain: Elite Athletes Versus General Population

The purpose of the study was to investigate the athletic-based specificity of muscle strength and neuromuscular control of spine stability in chronic non-specific low-back pain (LBP). Thirty elite athletes and 29 age-matched non-athletes with (15 athletes and 15 non-athletes) and without LBP (15 athletes and 14 non-athletes) participated in the study. Muscle strength was measured during maximal isometric trunk flexion and trunk extension contractions. The neuromuscular control of spine stability was analyzed by determining trunk stiffness, trunk damping, and onset times of the lumbar and thoracic erector spinae muscles after sudden perturbations (quick release experiments) as well as maximum Lyapunov exponents (local dynamic stability) using non-linear time series analysis of repetitive lifting movements. LBP was assessed using the visual analog scale. We found lower maximal trunk extension moments (p = 0.03), higher trunk damping (p = 0.018) and shorter onset times (p = 0.03) of the investigated trunk muscles in LBP patients in both athletes and non-athletes. Trunk stiffness and the local dynamic stability did not show any differences (p = 0.136 and p = 0.375, respectively) between LBP patients and healthy controls in both groups. It can be concluded that, despite the high-level of training in athletes, both athletes and non-athletes with LBP showed the same deconditioning of the lumbar extensor muscles and developed similar strategies to ensure spine stability after sudden perturbations to protect the spine from pain and damage. The findings highlight that specific training interventions for the trunk muscles are not only crucial for individuals of the general population, but also for well-trained athletes.