Search

We investigated the effects of intermittent long-term stretch training (5 weeks) on the architectural and mechanical properties of the muscle-tendon unit (MTU) in healthy humans. MTU's viscoelastic and architectural properties in the human medial gastrocnemius (MG) muscle and the contribution of muscle and tendon structures to the MTU lengthening were analyzed. Ten healthy volunteers participated in the study (four females and six males). The passive stretch of the plantar flexor muscles was achieved from 0° (neutral ankle position) to 25° of dorsiflexion. Measurements were obtained during a single passive stretch before and after the completion of the stretching protocol. During the stretch, the architectural parameters of the MG muscle were measured via ultrasonography, and the passive torque was recorded by means of a strain-gauge transducer. Repeated-measure ANOVA was applied for all parameters. When expressed as a percentage for all dorsiflexion angles, the relative torque values decreased ( p < 0.001). In the same way, architectural parameters (pennation angle and fascicle length) were compared for covariance and showed a significant difference between the slopes (ANCOVA p < 0.0001 and p < 0.001, respectively) suggesting a modification in the mechanical behavior after stretch training. Furthermore, the values for passive stiffness decreased ( p < 0.05). The maximum ankle range of motion (ROM) ( p < 0.01) and the maximum passive torque ( p < 0.05) increased. Lastly, the contribution of the free tendon increased more than fascicle elongation to the total lengthening of the MTU (ANCOVA p < 0.001). Our results suggest that five weeks of intermittent static stretch training significantly change the behavior of the MTU. Specifically, it can increase flexibility and increase tendon contribution during MTU lengthening.

This study investigated the adjustments in muscle activation and oxygenation in biceps and triceps brachii during two tasks sustained to failure at 20 and 60% of the maximal voluntary contraction (MVC) force. The tasks required participants either to push against a rigid restraint (force task) or to support an inertial load (position task) with the elbow flexor muscles. The surface EMG was recorded for biceps brachii, brachioradialis, triceps brachii and trapezius superior muscles. Muscle oxygenation of biceps and triceps brachii was measured by near-infrared spectroscopy. The position task was briefer (404 ± 159 s) than the force task (533 ± 194 s) when performed at 20% MVC (P = 0.011), but endurance time did not differ at 60% MVC (54 ± 19 versus 64 ± 16 s, respectively; P = 0.13). Biceps brachii oxygenation decreased slightly (by ~7%) during tasks performed at 20% MVC, whereas it dropped (-40%) for tasks sustained at 60% MVC. However, the decrease in muscle oxygenation was not a significant predictor of time to failure at the two target forces, although its contribution to muscle fatigue cannot be completely ruled out at 60% MVC. In contrast, time to failure was predicted by the increase in EMG of biceps brachii for both tasks at 20% MVC, and EMG of brachioradialis and trapezius for both tasks at 60% MVC. These results suggest that neural adjustments rather than muscle oxygenation limited the time to failure for the force and position tasks at low and high target forces.