Nathalie Guissard

Effect of static stretch training on neural and mechanical properties of the human plantar flexor muscles

To determine the contributions of neural and mechanical mechanisms to the limits in the range of motion (ROM) about a joint, we studied the effects of 30 sessions of static stretch training on the characteristics of the plantar‐flexor muscles in 12 subjects. Changes in the maximal ankle dorsiflexion and the torque produced during passive stretching at various ankle angles, as well as maximal voluntary contraction (MVC) and electrically induced contractions, were recorded after 10, 20, and 30 sessions, and 1 month after the end of the training program. Reflex activities were tested by recording the Hoffmann reflex (H reflex) and tendon reflex (T reflex) in the soleus muscle. Training caused a 30.8% (P < 0.01) increase in the maximal ankle dorsiflexion. This improved flexibility was associated (r2 = 0.88; P < 0.001) with a decrease in muscle passive stiffness and, after the first 10 sessions only, with a small increase in passive torque at maximal dorsiflexion. Furthermore, both the H‐ and T‐reflex amplitudes were reduced after training, especially the latter (−36% vs. −14%; P < 0.05). The MVC torque and the maximal rate of torque development were not affected by training. Although the changes in flexibility and passive stiffness were partially maintained 1 month after the end of the training program, reflex activities had already returned to control levels. It is concluded that the increased flexibility results mainly from reduced passive stiffness of the muscle–tendon unit and tonic reflex activity. The underlying neural and mechanical adaptation mechanisms, however, showed different time courses. Muscle Nerve 29: 248–255, 2004

Effect of time of day on force variation in a human muscle

The effect of time of day on the neural activation and contractile properties of the human adductor pollicis muscle was investigated in 13 healthy subjects. Two different times of day were chosen, corresponding to the minimum (7 h) and maximum (18 h) levels of strength. The force produced was compared with the associated electromyographic (EMG) activity during voluntary and electrically induced contractions in order to determine whether peripheral or central mechanisms play a dominant role in diurnal force fluctuation. The results indicated that the force produced during a maximum voluntary contraction (MVC) was significantly higher (+8.9%) in the evening than the morning. Since the increase in force of the MVC and the tetanic contraction (100 Hz) were similar, it is suggested that peripheral mechanisms are responsible for diurnal fluctuations in force. This conclusion is supported by the observation that central activation, tested by the interpolated twitch method during an MVC, did not change, and that the EMG was less per unit force in the evening. In addition to the increase in maximum twitch and tetanus force, significant changes in muscle contractile kinetics were also observed. The maximum rate of tension development and the relaxation of the twitch and tetanus increased in the evening, and the twitch contraction time (CT) and the time to half‐relaxation (TR1/2) were reduced. Because the mean range of variation in skin temperature (2.6°C) observed over the course of the day was very low, this change cannot entirely explain those observed in muscle contractile properties.

Muscle stretching and motoneuron excitability

SummaryChange of motoneuron excitability has been studied during the three basic modalities of slow or static stretching of the human soleus muscle. Tendon (T) and Hoffmann (H) reflexes were analyzed during static stretching (SS). The H response was compared in SS, in SS preceded by a maximal isometric contraction of the muscle or contraction-relaxation (CR) and during stretching of the muscle by contracting the antagonistic muscles (AC). During progressive dorsiflexion of the foot there is a significant difference (p<0.05) between T and H reflexes during SS, although the amplitude of direct motor (M) response, evoked by a maximal stimulation of the motor nerve, is not changed. The maximal joint mobilization during SS, CR and AC modalities appears to be closely related to the decrease in the H response during stretching. This decrease is significantly (p<0.05) smaller in SS than in AC or CR. In this last method, the duration of the maximal isometric contraction does not affect the results. In these three basic stretching procedures, the H reflex quickly recovers as soon as the manoeuvre is interrupted. It is suggested that changes in muscle motoneuron pool excitability closely control joint mobilization during slow or static stretching. The inhibition of the motoneurons observed during SS, CR and AC modalities is limited to the duration of the stretching manoeuvre.

Mechanisms of decreased motoneurone excitation during passive muscle stretching

Abstract. The effect of pre- versus postsynaptic mechanisms in the decrease in spinal reflex response during passive muscle stretching was studied. The change in the electromyographic (EMG) responses of two reflex pathways sharing a common pool of motoneurones, with (Hoffmann or H reflex) or without (exteroceptive or E reflex) a presynaptic inhibitory mechanism, was compared. The EMG activities were recorded in the soleus muscle in response to the electrical stimulation of the tibial nerve at the popliteal fossa (H reflex), and at the ankle (E reflex) for different dorsiflexion angles of the ankle. The compound muscle action potential (M wave) in the soleus and the abductor hallucis was recorded in order to control the stability of the electrical stimulation during stretching. The results indicate that in the case of small-amplitude muscle stretching (10° of dorsiflexion), a significant reduction (–25%; P<0.05) in the Hmax/Mmax ratio was present without any significant change in the Emax/Mmax ratio. At a greater stretching amplitude (20° of dorsiflexion), the E reflex was found to be reduced (–54.6%; P<0.001) to a similar extent as the H reflex (–54.2%). As soon as the ankle joint returned to the neutral position (ankle at 90°), the two reflex responses recovered their initial values. In additional experiments, motor-evoked potential (MEP) induced by the magnetic stimulation of the motor cortex was recorded and showed a similar type of behaviour to that observed in the E reflex. These results indicate that reduced motoneurone excitation during stretching is caused by pre- and postsynaptic mechanisms. Whereas premotoneuronal mechanisms are mainly involved in the case of small stretching amplitude, postsynaptic ones play a dominant role in the reflex inhibition when larger stretching amplitude is performed.

Neural aspects of muscle stretching.

Neural mechanisms contribute significantly to the gains that occur in the range of motion about a joint with stretching exercises. In the acute condition, lengthening of a muscle-tendon unit decreases spinal reflex excitability, which reduces passive tension and increases joint range of motion. Similarly, participation in a stretch-training program decreases tonic reflex activity and increases flexibility.

The relative lengthening of the myotendinous structures in the medial gastrocnemius during passive stretching differs among individuals.

The increase in passive torque during muscle stretching may constrain the range of motion of a joint. As passive torque can vary substantially among individuals, the present study examined whether the relative lengthening of the myotendinous structures of the medial gastrocnemius (MG) during passive stretching differs among individuals. Sixteen subjects performed passive stretching of the plantar flexor muscles from ankle angles ranging from 10 degrees plantar flexion (-10 degrees ) to 30 degrees dorsiflexion (+30 degrees ). Changes in passive torque, muscle architecture (fascicle length and pennation angle) of the MG and electromyographic activity of MG and soleus were recorded. The results showed that passive torque produced by the plantar flexors increased exponentially (r(2) = 0.99; P < 0.001) with ankle dorsiflexion, whereas MG fascicle length increased linearly from 57.6 +/- 9.1 to 80.5 +/- 10.3 mm (P < 0.001), and pennation angle decreased linearly from 21.2 +/- 4.2 to 14.4 +/- 3.1 degrees (P < 0.001) when the ankle joint angle was moved from -10 degrees to +30 degrees . The relative contribution of muscle (fascicles and aponeuroses) and tendon elongation to the change in length of the muscle-tendon unit (MTU) at 30 degrees dorsiflexion was 71.8 and 28.2%, respectively. However, the adjustment differed across individuals during MTU lengthening; in subjects (62.5%) with small, passive stiffness, the elongation of the free tendon was less and that of the fascicles larger than for subjects (37.5%) with greater stiffness. In conclusion, the results indicate that the strain of muscle and tendon varies among individuals, and difference in the relative compliance of these structures influences MTU lengthening differently during passive stretching.

Acute effect of muscle stretching on the steadiness of sustained submaximal contractions of the plantar flexor muscles

This paper examines the acute effect of a bout of static stretches on torque fluctuation during an isometric torque-matching task that required subjects to sustain isometric contractions as steady as possible with the plantar flexor muscles at four intensities (5, 10, 15, and 20% of maximum) for 20 s. The stretching bout comprised five 60-s passive stretches, separated by 10-s rest. During the torque-matching tasks and muscle stretching, the torque (active and passive) and surface electromyogram (EMG) of the medial gastrocnemius (MG), soleus (Sol), and tibialis anterior (TA) were continuously recorded. Concurrently, changes in muscle architecture (fascicle length and pennation angle) of the MG were monitored by ultrasonography. The results showed that during stretching, passive torque decreased and fascicle length increased gradually. Changes in these two parameters were significantly associated (r(2) = 0.46; P < 0.001). When data from the torque-matching tasks were collapsed across the four torque levels, stretches induced greater torque fluctuation (P < 0.001) and enhanced EMG activity (P < 0.05) in MG and TA muscles with no change in coactivation. Furthermore, stretching maneuvers produced a greater decrease (∼15%; P < 0.001) in fascicle length during the torque-matching tasks and change in torque fluctuation (CV) was positively associated with changes in fascicle length (r(2) = 0.56; P < 0.001), MG and TA EMG activities, and coactivation (r(2) = 0.35, 0.34, and 0.35, respectively; P < 0.001). In conclusion, these observations indicate that repeated stretches can decrease torque steadiness by increasing muscle compliance and EMG activity of muscles around the joint. The relative influence of such adaptations, however, may depend on the torque level during the torque-matching task.

Relation entre les modifications de l’architecture musculo-tendineuse et le développement de la tension pendant l’étirement passif du triceps sural

Resume L’etirement musculaire est aujourd’hui de pratique courante et de nombreuses etudes se sont interessees a ses effets. Au plan biomecanique, il est generalement propose que la modification de compliance, ou de raideur, induite par la manœuvre d’etirement soit principalement localisee au niveau du tissu tendineux. Moins d’attention a ete portee au comportement des tissus musculaires et plus particulierement des faisceaux cellulaires. Nous avons examine cette question par imagerie echographique et mesures de la tension mecanique, pendant un etirement passif du triceps sural induit par la dorsiflexion de la cheville. L’angle de pennation (forme par un faisceau de fibres et les aponevroses musculaires), ainsi que l’allongement fasciculaire du gastrocnemien medial, ont ete analyses chez 19 sujets humains. Les resultats experimentaux montrent que, lors d’un mouvement de dorsiflexion (de 30°), l’etirement musculaire induit une augmentation exponentielle de la tension passive developpee par l’entite musculo-tendineuse. A 30° de dorsiflexion, la longueur de l’unite musculo-tendineuse totale augmente de 24,7 ± 2,9 %, tandis que les angles de pennation profonds des fibres musculaires diminuent de 21,6 ± 1,6 % et que les fascicules s’allongent de 27,0 ± 2,4 %. Dans cette angulation articulaire (30 de dorsiflexion), les contributions tendineuse et fasciculaire a l’allongement musculaire total sont respectivement de 2,3 ± 0,5 % et de 22,4 ± 3,2 %. Ces resultats experimentaux suggerent que la limitation de la mobilisation articulaire, lors d’etirement passif important, serait davantage due a des modifications de compliance biomecanique des fascicules fibrillaires (fibres et proteines structurales) qu’a celles des tissus tendineux.

Effects of static stretching and contract relax methods on the force production and jump performance

For several years now, stretching has been integrated into the preparation of athletic performance. Today, more and more trainers are reluctant to advise stretching before a competition, except for...

Changes in muscle resting tension, architecture and spinal reflex after hook treatment in healthy subjects

The diacutaneous myo-aponeurosis technique is an instrument-based technique (hook treatment) used on patients suffering from pains of inflammatory or traumatic origin affecting the locomotor system (Ekman 1972). The purpose of this study was to investigate a possible effect of hook treatment of the triceps surae. Mechanical and neural parameters were recorded to try to identify the underlying mechanisms of the muscular relaxation perceived by therapists following the application of this technique (Veszely et al. 2000).