Effect of Whole-body Vibration frequency on muscle tensile state during graded plantar flexor isometric contractions.

Background: Acute physiological and biomechanical alterations have been reported following whole-body vibration (WBV). Stiffening of muscles has only been anecdotally reported in response to WBV. Accordingly, this study investigated active plantar flexor muscle stiffness in response to a single WBV bout at four mechanical vibration frequencies. Methods: Thirteen healthy adults (37.1 ± 14.4 years old) randomly received WBV in 4 different frequencies (6, 12, 24, and 0 Hz control) for 5 min. Shear wave speed (SWS) in longitudinal and transverse projections, architecture, and electric muscle activity were recorded in the medial gastrocnemius (MG) and soleus (SOL) muscle during graded plantar flexor contraction. Subjective rating of perceived muscle stiffness was assessed via Likert-scale. Results: SWS of the MG at rest was enhanced in response to 5 min of 24 Hz WBV (p = 0.025), while a small reduction in SOL SWS was found during contraction (p = 0.005) in the longitudinal view. Subjective stiffness rating was increased following 12 Hz intervention. After 24 Hz WBV, pennation angle for MG was decreased (p = 0.011) during contraction. As a secondary finding, plantar flexor strength was significantly increased with each visit, which, however, did not affect the study's main outcome because of balanced sequence allocation. Conclusion: SWS effects were solely limited to 24 Hz mechanical vibration and in the longitudinal projection. The observed effects are compatible with an interpretation by post-activation potentiation, warm-up, and force-distribution within the triceps surae muscles following 5 min WBV. The outcome may suggest SWS as a useful tool for assessing acute changes in muscle stiffness.

Lower limb bone geometry in adult individuals with X-linked hypophosphatemia: an observational study.

We assessed lower-limb geometry in adults with X-linked hypophosphatemia (XLH) and controls. We found large differences in multiple measures including femoral and tibial torsion, bowing and cross-sectional area and acetabular version and coverage which may contribute to clinical problems such as osteoarthritis, fractures and altered gait common in XLH. PURPOSE: Individuals with X-linked hypophosphatemia (XLH) are at risk of lower-limb deformities and early onset of osteoarthritis. These two factors may be linked, as altered biomechanics is a risk factor for osteoarthritis. This exploratory evaluation aims at providing clues and concepts for this association to facilitate future larger-scale and longitudinal studies on that aspect. METHODS: For this observational study, 13 patients with XLH, aged 18-65 years (6 female), were compared with sex-, age- and weight-matched healthy individuals at a single German research centre. Femoral and hip joint geometry, including femoral and tibial torsion and femoral and tibial shaft bowing, bone cross-sectional area (CSA) and acetabular version and coverage were measured from magnetic resonance imaging (MRI) scans. RESULTS: Total femoral torsion was 29° lower in individuals with XLH than in controls (p < 0.001), mainly resulting from lower intertrochanteric torsion (ITT) (p < 0.001). Femoral lateral and frontal bowing, tibial frontal bowing, mechanical axis, femoral mechanical-anatomical angle, acetabular version and acetabular coverage were all greater and tibial torsion lower in individuals with XLH as compared to controls (all p < 0.05). Greater femoral total and marrow cavity CSA, greater tibial marrow cavity CSA and lower cortical CSA were observed in XLH (all p < 0.05). DISCUSSION: We observed large differences in clinically relevant measures of tibia and particularly femur bone geometry in individuals with XLH compared to controls. These differences may plausibly contribute to clinical manifestations of XLH such as early-onset osteoarthritis, pseudofractures and altered gait and therefore should be considered when planning corrective surgeries.

Daily 30-min exposure to artificial gravity during 60 days of bed rest does not maintain aerobic exercise capacity but mitigates some deteriorations of muscle function: results from the AGBRESA RCT.

PURPOSE: Spaceflight impairs physical capacity. Here we assessed the protective effect of artificial gravity (AG) on aerobic exercise capacity and muscle function during bed rest, a spaceflight analogue. METHODS: 24 participants (33 ± 9 years, 175 ± 9 cm, 74 ± 10 kg, 8 women) were randomly allocated to one of three groups: continuous AG (cAG), intermittent AG (iAG) or control (CTRL). All participants were subjected to 60 days of six-degree head-down tilt bed rest, and subjects of the intervention groups completed 30 min of centrifugation per day: cAG continuously and iAG for 6 × 5 min, with an acceleration of 1g at the center of mass. Physical capacity was assessed before and after bed rest via maximal voluntary contractions, cycling spiroergometry, and countermovement jumps. RESULTS: AG had no significant effect on aerobic exercise capacity, flexor muscle function and isometric knee extension strength or rate of force development (RFD). However, AG mitigated the effects of bed rest on jumping power (group * time interaction of the rmANOVA p < 0.001; iAG - 25%, cAG - 26%, CTRL - 33%), plantar flexion strength (group * time p = 0.003; iAG - 35%, cAG - 31%, CTRL - 48%) and plantar flexion RFD (group * time p = 0.020; iAG - 28%, cAG - 12%, CTRL - 40%). Women showed more pronounced losses than men in jumping power (p < 0.001) and knee extension strength (p = 0.010). CONCLUSION: The AG protocols were not suitable to maintain aerobic exercise capacity, probably due to the very low cardiorespiratory demand of this intervention. However, they mitigated some losses in muscle function, potentially due to the low-intensity muscle contractions during centrifugation used to avoid presyncope.

Effects of 21 days of bed rest and whey protein supplementation on plantar flexor muscle fatigue resistance during repeated shortening contractions.

PURPOSE: Space flight and bed rest (BR) lead to a rapid decline in exercise capacity. Whey protein plus potassium bicarbonate diet-supplementation (NUTR) could attenuate this effect by improving oxidative metabolism. We evaluated the impact of 21-day BR and NUTR on fatigue resistance of plantar flexor muscles (PF) during repeated shortening contractions, and whether any change was related to altered energy metabolism and muscle oxygenation. METHODS: Ten healthy men received a standardized isocaloric diet with (n = 5) or without (n = 5) NUTR. Eight bouts of 24 concentric plantar flexions (30 s each bout) with 20 s rest between bouts were employed. PF muscle size was assessed by means of peripheral quantitative computed tomography. PF muscle volume was assessed with magnetic resonance imaging. PF muscle force, contraction velocity, power and surface electromyogram signals were recorded during each contraction, as well as energy metabolism (31P nuclear magnetic resonance spectroscopy) and oxygenation (near-infrared spectroscopy). Cardiopulmonary parameters were measured during an incremental cycle exercise test. RESULTS: BR caused 10-15% loss of PF volume that was partly recovered 3 days after re-ambulation, as a consequence of fluid redistribution. Unexpectedly, PF fatigue resistance was not affected by BR or NUTR. BR induced a shift in muscle metabolism toward glycolysis and some signs of impaired muscle oxygen extraction. NUTR did not attenuate the BR-induced-shift in energy metabolism. CONCLUSIONS: Twenty-one days' BR did not impair PF fatigue resistance, but the shift to glycolytic metabolism and indications of impaired oxygen extraction may be early signs of developing reduced muscle fatigue resistance.

Changes in motor unit activity and respiratory oxygen uptake during 6 weeks of progressive whole-body vibration combined with progressive, high intensity resistance training.

OBJECTIVES: We hypothesized that the additional activation of motor units (MU) and the elevation of metabolic energy turnover resulting from whole-body vibration (WBV) superimposed to high intensity resistance training on a smith machine persist after 6 weeks of training with progressively increasing loads and vibration frequencies. METHODS: Two groups of healthy male subjects performed either 6 weeks of Resistive Vibration Exercise (RVE, squats and heel raises with WBV, n=13) or Resistive Exercise (RE using the same protocol, n=13). During the first (pre) and the last training session (post), we determined the oxygen uptake changes normalized to total training weight (∆V'O2/ttw) and the normalized MU activity from rectus femoris (squats) and gastrocnemius lateralis (heel raise) muscles filtered for vibration frequencies and harmonics (EMG/ttw). RESULTS: At pre measurement, RVE induced higher EMG/ttw (squats) than RE alone (group effect, P=0.006). At post measurement, EMG/ttw was reduced (time effects between P=0.087 and P<0.001 for both groups and exercises). At pre and post measurement, ∆V'O2/ttw was higher during RVE than during RE (group effects between P=0.005 and P=0.099 for both exercises). CONCLUSIONS: RVE permanently elevated metabolic energy turnover, although the initially observed additional MU activity by RVE could not be preserved in the working musculature.

Lower body negative pressure enhances oxygen availability in the knee extensor muscles during intense resistive exercise in supine position.

PURPOSE: During exercise in supine posture or under microgravity in space, the gravity-dependent component of local blood pressure in leg muscles at upright posture can be simulated by lower body negative pressure (LBNP). We hypothesized that during resistive exercise LBNP favors oxygen availability in lower extremities, benefiting energy levels and performance of working muscles. METHODS: In permutated crossover design, nine subjects performed a series of fifteen slow-paced concentric (4 s) and eccentric contractions (4 s) without or with 40 mmHg LBNP and 4 s pause between repetitions. The force at knee flexion was 6% of the one repetition maximum (1-RM) and gradually increased to 60% 1RM in the first half of the individual range of motion, subsequently remaining constant until full extension. RESULTS: During the low force periods of continuous exercise, LBNP enhanced the refill of capillary blood measured by near infrared spectroscopy, amplifying the increase of total haemoglobin by about 20 µmol/l (p < 0.01) and oxyhaemoglobin by about 10 µmol/l (p < 0.01). During continuous exercise, LBNP induced a trend towards a lower EMG increment. This LBNP effect was not found when the periods of low forces at knee flexion were extended by 4 s pauses. Increased respiratory oxygen uptake (+ 0.1 l/min, p < 0.05) indicated overall enhanced muscle energy turn-over. CONCLUSIONS: Our results suggest stimulation of oxidative metabolism through LBNP enables working muscles to meet the energy demands of intense exercise. Further research is needed on the consequences for energy metabolism and the molecular control of growth and differentiation.

Vibration-related extrusion of capillary blood from the calf musculature depends upon directions of vibration of the leg and of the gravity vector.

PURPOSE: In this study, we investigated the effects of vibration of the whole lower leg on the content and the oxygenation of hemoglobin in the unloaded relaxed lateral gastrocnemius muscle. Vibration was applied orthogonal to and in parallel with leg axis to examine whether the extrusion of blood depends on an alignment of main vessel direction, axis of vibration and gravity. METHOD: The blood volume in the muscles was altered by horizontal and 30° upright body posture. Fifteen male subjects were exposed to 4 sets of experiments with both vibration directions and both tilt angles applied in permutated order. The absence of voluntary muscular activity and the potential occurrence of compound action potentials by stretch reflexes were monitored using electromyography. Total hemoglobin and tissue saturation index were measured with near infrared spectroscopy. Changes of lower leg circumference were measured with strain gauge system placed around the calf. RESULT: Vibration caused decrease in tHb and increase in TSI indicating extrusion of predominantly venous blood from the muscle. In 30° tilted position, muscles contained more blood at baseline and vibration ejected more blood from the muscle compared with horizontal posture (p < 0.01). At 30° tilting deeper drop in tHb and steeper increase in TSI (p < 0.01) were observed when vibration was applied in parallel with the length axis of muscle. CONCLUSION: It is concluded that the vibration extrudes more blood in 30° head up posture and the vibration applied in parallel with the length axis of the muscle is more effective than orthogonal vibration.

Short-arm centrifugation as a partially effective musculoskeletal countermeasure during 5-day head-down tilt bed rest--results from the BRAG1 study.

PURPOSE: Human centrifugation, also called artificial gravity (AG), is proposed as a combined strategy against detrimental effects of microgravity in long-term space missions. This study scrutinized human short-arm centrifugation as countermeasure against musculoskeletal de-conditioning. METHOD: Eleven healthy male subjects [mean age of 34 (SD 7) years] completed the cross-over trial, including three campaigns of -6° head-down tilt bed rest (HDT) for 5 days, with preceding baseline data collection and recovery phases. Bed rest without AG was used as control condition (Ctrl), and AG with 1 g at the center of mass applied once per day for 30 min in one bout (AG1×30) and in 6 bouts of 5 min (AG6×5, 3-min rest between bouts) as experimental conditions. End-points were muscle strength, vertical jump performance, and biomarkers of bone and protein metabolism. RESULT: AG6×5 was better tolerated than AG1×30. Bone resorption markers CTX, NTX, and DPD all increased by approximately 25 % toward the end of bed rest (P < 0.001), and nitrogen balance decreased by approximately 3 g/day (P < 0.001), without any protection by AG (P > 0.4). Decreases in vertical jump height by 2.1 (SE 0.6) cm after Ctrl bed rest was prevented by either of the AG protocols (P = 0.039). CONCLUSION: The present study yielded succinct catabolic effects upon muscle and bone metabolism that were un-prevented by AG. The preservation of vertical jump performance by AG in this study is likely caused by central nervous rather than by peripheral musculoskeletal effects.

Muscular forces affect the glycosaminoglycan content of joint cartilage: unloading in human volunteers with the HEPHAISTOS lower leg orthosis.

BACKGROUND AND PURPOSE: Unloading alters the thickness of joint cartilage. It is unknown, however, to what extent unloading leads to a loss of glycosaminoglycans (GAGs) in the cartilage tissue. We hypothesized that muscle forces, in addition to axial loading, are necessary to maintain the joint cartilage GAG content of the knee and the upper and lower ankle. PATIENTS AND METHODS: The HEPHAISTOS orthosis was worn unilaterally by 11 men (mean age 31 (23-50) years old) for 56 days. The orthosis reduces activation and force production of the calf muscles while it permits full gravitational loading of the lower leg. MRI measurements of the knee and ankle were taken before the intervention, during the intervention (on day 49), and 14 days after the end of the intervention. Cartilage segmentation was conducted semiautomatically for the knee joint (4 segments) and for the upper (tibio-talar) and lower (subtalar) ankle joints (2 segments each). Linear mixed-effects (LME) models were used for statistical analysis. RESULTS: 8 volunteers completed the MRI experiment. In the lower ankle joint, differences in ΔT1 were found between the end of the intervention and 14 days after (p = 0.004), indicating a decrease in GAG content after reloading. There were no statistically significant differences in ΔT1 values in the knee and upper ankle joints. INTERPRETATION: Our findings suggest that in addition to gravitational load, muscular forces affect cartilage composition depending on the local distribution of forces in the joints affected by muscle contraction.

In the unloaded lower leg, vibration extrudes venous blood out of the calf muscles probably by direct acceleration and without arterial vasodilation.

PURPOSE: During vibration of the whole unloaded lower leg, effects on capillary blood content and blood oxygenation were measured in the calf muscle. The hypotheses predicted extrusion of venous blood by a tonic reflex contraction and that reactive hyperaemia could be observed after vibration. METHODS: Twelve male subjects sat in front of a vibration platform with their right foot affixed to the platform. In four intervals of 3-min duration vibration was applied with a peak-to-peak displacement of 5 mm at frequencies 15 or 25 Hz, and two foot positions, respectively. Near infrared spectroscopy was used for measuring haemoglobin oxygen saturation (SmO2) and the concentration of total haemoglobin (tHb) in the medial gastrocnemius muscle. RESULTS: Within 30 s of vibration SmO2 increased from 55 ± 1 to 66 ± 1 % (mean ± SE). Within 1.5 min afterwards SmO2 decreased to a steady state (62 ± 1 %). During the following 3 min of recovery SmO2 slowly decreased back to base line. THb decreased within the first 30 s of vibration, remained almost constant until the end of vibration, and slowly recovered to baseline afterwards. No significant differences were found for the two vibration frequencies and the two foot positions. CONCLUSIONS: The relaxed and unloaded calf muscles did not respond to vibration with a remarkable reflex contraction. The acceleration by vibration apparently ejected capillary venous blood from the muscle. Subsequent recovery did not match with a reactive hyperaemia indicating that the mere mechanical stress did not cause vasodilation.