Robert Marzilger

Effects of load magnitude, muscle length and velocity during eccentric chronic loading on the longitudinal growth of the vastus lateralis muscle

The present study investigated the longitudinal growth of the vastus lateralis muscle using four eccentric exercise protocols with different mechanical stimuli by modifying the load magnitude, lengthening velocity and muscle length at which the load was applied. Thirty-one participants voluntarily participated in this study in two experimental and one control group. The first experimental group (N=10) exercised the knee extensors of one leg at 65% (low load magnitude) of the maximum isometric voluntary contraction (MVC) and the second leg at 100% MVC (high load magnitude) with 90 deg s−1 angular velocity, from 25 to 100 deg knee angle. The second experimental group (N=10) exercised one leg at 100% MVC, 90 deg s−1, from 25 to 65 deg knee angle (short muscle length). The other leg was exercised at 100% MVC, 240 deg s−1 angular velocity (high muscle lengthening velocity) from 25 to 100 deg. In the pre- and post-intervention measurements, we examined the fascicle length of the vastus lateralis at rest and the moment–angle relationship of the knee extensors. After 10 weeks of intervention, we found a significant increase (~14%) of vastus lateralis fascicle length compared with the control group, yet only in the leg that was exercised with high lengthening velocity. The findings provide evidence that not every eccentric loading causes an increase in fascicle length and that the lengthening velocity of the fascicles during the eccentric loading, particularly in the phase where the knee joint moment decreases (i.e. deactivation of the muscle), seems to be an important factor for longitudinal muscle growth.

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.

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.

Operating length and velocity of human M. vastus lateralis fascicles during vertical jumping

Humans achieve greater jump height during a counter-movement jump (CMJ) than in a squat jump (SJ). However, the crucial difference is the mean mechanical power output during the propulsion phase, which could be determined by intrinsic neuro-muscular mechanisms for power production. We measured M. vastus lateralis (VL) fascicle length changes and activation patterns and assessed the force–length, force–velocity and power–velocity potentials during the jumps. Compared with the SJ, the VL fascicles operated on a more favourable portion of the force–length curve (7% greater force potential, i.e. fraction of VL maximum force according to the force–length relationship) and more disadvantageous portion of the force–velocity curve (11% lower force potential, i.e. fraction of VL maximum force according to the force–velocity relationship) in the CMJ, indicating a reciprocal effect of force–length and force–velocity potentials for force generation. The higher muscle activation (15%) could therefore explain the moderately greater jump height (5%) in the CMJ. The mean fascicle-shortening velocity in the CMJ was closer to the plateau of the power–velocity curve, which resulted in a greater (15%) power–velocity potential (i.e. fraction of VL maximum power according to the power–velocity relationship). Our findings provide evidence for a cumulative effect of three different mechanisms—i.e. greater force–length potential, greater power–velocity potential and greater muscle activity—for an advantaged power production in the CMJ contributing to the marked difference in mean mechanical power (56%) compared with SJ.

Soleus H-reflex modulation during balance recovery after forward falling

Introduction: Our purpose was to examine the Hoffmann reflex (H‐reflex) during balance recovery after a simulated forward fall from 2 different inclination angles. Methods: The soleus H‐reflex of 15 healthy adults was measured in 2 different leaning positions (exerting a horizontal force at 15% and 30% of body weight, respectively), with no release (Int0) and at 2 different intervals (Int1, Int2) after the release (∼45 and ∼65 ms, respectively). Results: During Int2, the H‐reflex, which was evoked before the onset of the soleus electromyography, was significantly higher than the H‐reflex induced 20 ms earlier (Int1). No significant difference was observed between Int0 and Int1 and between the 2 leaning positions. Conclusions: These findings indicate that Ia afferent input is facilitated before muscle activation during forward falling. This could be important for the timely activation and increased rate of force development required during this task. Muscle Nerve 54: 952–958, 2016