Water deprivation decreases strength in fast twitch muscle in contrast to slow twitch muscle in rat

AIM

The effects of dehydration on muscle performance in human are still contradictory, notably regarding muscle force. The effect of water deprivation (WD) on mechanical properties of skeletal muscle, and more precisely its impact on slow and fast muscles, remains largely unknown. The aim of this study was to determine for the first time whether WD leads to changes in contractile properties of skeletal muscle and whether these changes were muscle-type-specific.

METHODS

Sixteen-week-old male rats were assigned to either a control group (C) with water or a 96-hour WD group. At the end of the period, twitch and tetanus properties, as well as biochemical and structural analysis, were performed on soleus (SOL) and extensor digitorum longus (EDL) muscles.

RESULTS

Absolute twitch (Pt) and tetanic (P₀ ) tension were, respectively, 17% and 14% lower in EDL of WD rats as compared with C rats, whereas unexpected increases of 43% and 25% were observed in SOL. Tensions normalized with respect to muscle mass were not affected by WD in EDL, whereas they were increased by more than 40% in SOL. A 96-hour WD period leads to a decrease in fibre cross-sectional area and absolute myofibrillar content only in EDL.

CONCLUSION

It is hypothesized that differences in the results between slow and fast muscles may come from (i) a muscle-type-specific effect of WD on protein balance, EDL showing a greater myofibrillar protein breakdown and (ii) a greater sensitivity to osmolality changes induced by WD in EDL than in SOL.

Evaluation of HD-sEMG Probability Density Function deformations in ramp exercise

The aim of the present study is to propose a subject-specific screening approach of High Density surface EMG (HD-sEMG) Probability Density Function (PDF) shape evolution in experimental conditions following a ramp exercise from 0% to 50% of the Maximum Voluntary Contraction (MVC) during 25 seconds of isometric contractions of the Biceps Brachii from six healthy subjects. This method uses High Order Statistics (HOS), namely the kurtosis and the skewness for PDF shape screening examined on selectively positioned Laplacian sEMG channels obtained on an 8×8 HD-sEMG grid. For each subject, the position of the Laplacian channels was chosen based on the level of muscle activation obtained from the Signal to Noise Ratio (SNR) matrix computed for the 64 sEMG signals of the grid in order to obtain independent Laplacian configurations localized in areas with high SNRs indicating high muscle activation. Afterwards, we used the Principal Component Analysis (PCA) to obtain the principal trend of the kurtosis and the skewness computed from the selected Laplacian signals according to force level variation. The obtained results show a globally common increasing HOS trend according to force increase from 0% to 50% MVC for all the subjects regardless of the anatomical, instrumental and physiological variability that usually strongly influences these trends.

Training-induced changes in structural and mechanical properties of the patellar tendon are related to muscle hypertrophy but not to strength gains

To obtain a better understanding of the adaptations of human tendon to chronic overloading, we examined the relationships between these adaptations and the changes in muscle structure and function. Fifteen healthy male subjects (20+/-2 yr) underwent 9 wk of knee extension resistance training. Patellar tendon stiffness and modulus were assessed with ultrasonography, and cross-sectional area (CSA) was determined along the entire length of the tendon by using magnetic resonance imaging. In the quadriceps muscles, architecture and volume measurements were combined to obtain physiological CSA (PCSA), and maximal isometric force was recorded. Following training, muscle force and PCSA increased by 31% (P<0.0001) and 7% (P<0.01), respectively. Tendon CSA increased regionally at 20-30%, 60%, and 90-100% of tendon length (5-6%; P<0.05), and tendon stiffness and modulus increased by 24% (P<0.001) and 20% (P<0.01), respectively. Although none of the tendon adaptations were related to strength gains, we observed a positive correlation between the increase in quadriceps PCSA and the increases in tendon stiffness (r=0.68; P<0.01) and modulus (r=0.75; P<0.01). Unexpectedly, the increase in muscle PCSA was inversely related to the distal and the mean increases in tendon CSA (in both cases, r=-0.64; P<0.05). These data suggest that, following short-term resistance training, changes in tendon mechanical and material properties are more closely related to the overall loading history and that tendon hypertrophy is driven by other mechanisms than those eliciting tendon stiffening.