Influence of Forward Head Posture on Myotonometric Measurements of Superficial Neck Muscle Tone, Elasticity, and Stiffness in Asymptomatic Individuals With Sedentary Jobs.

OBJECTIVE: The objective of the study was to assess the influence of forward head posture on the mechanical parameters and pressure pain threshold of superficial neck muscles in clinically nonsymptomatic individuals with sedentary jobs. METHODS: Twenty-five office workers with forward head posture and 25 office workers with normal head posture were matched for sex, age, body mass index, and the nature and duration of their work and were compared at a single point. The study participants were divided into study groups on the basis of photometric craniovertebral angle measurements. The upper trapezius, sternocleidomastoid, and splenius capitis mechanical properties were assessed in the sitting position. Primary outcome measures were muscle stiffness (N/m), muscle tone (Hz), and muscle elasticity. The secondary variable was perceived pain threshold. RESULTS: No significant differences between the groups were found for biomechanical properties and perceived pain threshold in the studied muscles. CONCLUSION: Forward head posture has no impact on muscle stiffness, tone, and elasticity, nor does it increase the pressure sensitivity of superficial neck muscles in healthy, mildly symptomatic office workers. It is most likely that not incorrect posture of the cervical spine, but probably other factors combined with forward head posture, like comorbid acute and chronic cervical pain and musculoskeletal disorders or prolonged sitting, contribute to changes in active myofascial tone and tensegrity as well as increased pressure sensitivity of neck muscles.

Short-Term Effects of Kinesio Taping® on Electromyographic Characteristics of Paraspinal Muscles, Pain, and Disability in Patients With Lumbar Disk Herniation.

Context: Kinesio taping® (KT) is a therapeutic modality frequently used in the clinical practice for the treatment of various musculoskeletal disorders. It is often applied in patients with chronic low back pain to decrease pain and improve functional capacity. However, it is not known, whether thoracolumbar fascia KT technique can decrease back pain, restore normal activity of paraspinal muscles, and improve functional capacity in patients with lumbar disk herniation (LDH). Objective: To evaluate the impact of 7-day new KT stabilizing application on lumbar paraspinal muscles function, pain perception, and disability in patients with LDH. Design: A randomized controlled trial. Setting: Human Performance Laboratory. Patients: A number of 38 patients with LDH were randomized into KT (n = 19) and placebo taping (n = 19) groups. Interventions: Both groups received the same "x" type application running over the back along fibers of superficial lamina of the posterior layer of thoracolumbar fascia. Main Outcome Measures: The primary outcome measures were flexion-relaxation and extension-relaxation ratios calculated from electromyographic activity of lumbar multifidus and longissimus thoracic muscles. Pain intensity rating (Quadruple Visual Analogue Scale), pressure pain thresholds of the lower back, Roland-Morris Disability Questionnaire score, back extension force, and flexion range of motion (ROM) were among secondary outcomes. Results: KT application did not affect the lumbar multifidus and longissimus thoracic muscles flexion-relaxation and extension-relaxation ratios, lower back pressure pain thresholds, back flexion ROM, and back extension force (no group × time interaction [GTI]). KT and placebo taping comparably decreased disability level (time effect: F1,36 = 22.817, P < .001; GTI: F1,36 = 0.189, P = .67), average pain (time effect: F1,36 =39.648, P < .001; GTI: F1,36 = 2.553, P = .12), and the worst pain (time effect: F1,36 = 36.039, P < .001; GTI: F1,36 = 0.003, P = .96) intensity. Conclusion: Seven-day KT does not normalize lumbar paraspinal muscle function and is not superior to placebo in reducing disability and pain intensity in patients with LDH.

Differences in Isokinetic Strength of the Knee Extensors and Flexors in Men With Isolated and Combined Cruciate-Ligament Knee Injury.

CONTEXT: The extent of knee extensor and flexor weakness after disruption of knee ligaments affects a rehabilitation output and functional recovery and may give prognostic information on a possible risk of development of knee osteoarthritis. OBJECTIVE: The hypothesis tested was whether patients with a multiple-ligament tear would have larger abnormalities in strength of the knee extensors and flexors than patients with an isolated-ligament rupture. DESIGN: Cross-sectional study, level III. SETTING: Outpatient orthopedic clinic. PARTICIPANTS: 3 groups of recreationally active men: noninjured control (CON, n = 12), with an anterior cruciate ligament injury (ACLI, n = 10), and with combined anterior and posterior cruciate ligament injury (APCLI, n = 9), matched according to age, body mass, and height. INTERVENTION: All patients received conservative treatment and rehabilitation and awaited ligament reconstruction surgery. MAIN OUTCOME MEASURES: Isokinetic maximum-repetition peak torque per body mass (PT/BM) and total work (TW), PT and TW limb-symmetry index (LSI), and flexor-to- extensor PT ratio were evaluated during concentric knee extension-flexion movements at lower (60°/s) and higher (240°/s) isokinetic velocities. RESULTS: The main finding was that compared with the individuals with ACLI, patients with APCLI produced in their injured limbs lower mean TW (extension: 30.3%, flexion: 28.2%) and had lower mean TW LSI (extension 74% in APCLI vs. 91.6% in ACLI; flexion 61.3% in APCLI vs. 90.8% in ACLI) at the higher but not lower speed of isokinetic testing. However, at the lower velocity the quantified size of reduction in PT/BM and TW was greater in subjects with APCLI than ACLI as compared with the CON individuals. CONCLUSIONS: After bi-cruciate-ligament injury the capacity to produce torque by concentric muscle contractions throughout knee-extension and -flexion movements performed with high speed is lower in injured limbs than after isolated anterior cruciate ligament tear.

High-magnitude whole-body vibration effects on bone resorption in adult rats.

INTRODUCTION: Optimal parameters of whole-body vibration (WBV), effective in increasing bone mass or preventing bone loss, are always being sought. We investigated effects of a 6-mo WBV program, consisting of brief daily vibratory sessions, on the bone mass and bone metabolic markers. METHODS: There were 10 male Wistar rats (3 mo old) that were subjected to the 6-mo WBV (frequency 50 Hz, acceleration 4.92 g, peak-to-peak displacement 2.5 mm) composed of 4 30-s bouts per day, performed 5 d per week. There were 10 not vibrated, age-matched rats that served as controls. RESULTS: After completion of the WBV program the concentrations of C-terminal telopeptide of type I collagen in the WBV treated animals were significantly lower in comparison to the controls. There were no effects of the WBV on areal bone mineral density, osteocalcin, and sRANKL levels. DISCUSSION: High-frequency high-magnitude WBV applied for 6 mo decreases bone resorption, but does not affect bone formation and bone mineral density.

Effects of hand dominance on myoelectric signal of non­fatigued lumbar multifidus muscle during single arm lifts.

Some evidence indicates that lower back muscles located at the non­dominant side of the body are more fatigue resistant than their opposite counterparts presumably due to preferential use of the dominant hand. The aim of the study was to determine if any distinction exists in the surface electromyographic activity of corresponding contralateral non­fatigued lumbar multifidus (LM) muscles as a function of hand dominance. The relative to maximum root mean square, the median frequency (MdF) and spike shape parameters were computed from the surface myoelectric signals of ipsilateral and contralateral lumbar multifidus muscle of 46 adult healthy subjects (27 right­handed, 19 left­handed) during voluntary contractions evoked by the single arm lifts in prone position. Activation of LM as a contralateral muscle to lifted arm was greater than as ipsilateral muscle, independently of handedness. Regardless if LM performed ipsi­ or contralateral action to the lifted arm, the mean spike amplitude, slope, number of peaks per spike and spike duration were greater and mean spike frequency as well as MdF were smaller in the muscle of dominant than non­dominant side. Combined changes of spike shape measures indicate increased recruitment, lower firing rates and higher synchronization of motor units in the LM of dominant side as compared to its counterpart.

Relationship between morphometric and mechanical properties of superficial lumbosacral soft tissue layers in healthy young adults.

Introduction: It is commonly considered that myotonometry is a non-invasive method capable of quantifying linear elastic and viscoelastic properties of the myofascial tissue through the application of a weak mechanical impulse to the surface of the skin. However, before the impulse can reach the myofascial tissue, it must cross more superficial tissues such as the skin and subcutaneous tissue (ST). All these superficial tissues have different distributions and organizations of structural components. Therefore, the study aimed to examine the potential relationships between the mechanical and morphometric properties of various superficial soft tissues surrounding the lumbar multifidus muscle (LM). Methods: Myotonometric measurements of dynamic stiffness, logarithmic decrement, and creep, and ultrasonographic measurements of thickness and echogenicity of cutaneous, subcutaneous, perimuscular tissue, and LM were obtained from 50 healthy individuals in the resting prone position and during contralateral arm lift. Results: The most important findings were that in both the relaxed and contracted LM state, the dynamic stiffness strongly negatively (r = -0.69; p < 0.001 in relaxation, r = -0.83; p < 0.001 in contraction) and creep strongly positively (r = 0.79; p < 0.001 in relaxation, r = 0.85; p < 0.001 in contraction) correlated with the thicknesses of the ST. Similar but weaker correlations were noticed between both these measures and the perimuscular tissue thickness. Elasticity was uncorrelated to the thicknesses of the tissues. With LM contraction (change from the relaxed to contracted state), the relative increase in dynamic stiffness was correlated with the relative decrease in dermis (r = -0.51; p < 0.001) and ST (r = -0.47; p = 0.001) thickness, and with the relative increase in LM (r = 0.36; p = 0.010) thickness. Moreover, the relative decrease (thinning) in the ST thickness was correlated with the relative increase in logarithmic decrement (i.e., decrease in soft tissue elasticity, r = -0.37, p = 0.011). The mechanical properties of the soft tissues were not related to their echogenicity. Discussion: In conclusion, the thicker the subcutaneous and perimuscular layers, the lesser the stiffness and the greater the time-dependent deformation to the external force of the tissues surrounding the LM during its relaxation and isometric contraction. Moreover, the greater the thinning of the ST and the thickening of the LM during its contraction, the higher the increase in lumbosacral tissue stiffness and the decrease in elasticity. Therefore, one should consider the thickness of the ST before planning or analyzing the outcomes of myotonometric or other external biomechanical measurements to avoid drawing the wrong conclusions about the mechanical properties of the myofascial tissue.

Motor Unit Fatigability following Chronic Carnosine Supplementation in Aged Rats.

Studies suggest that carnosine (beta-alanyl-L-histidine) is effective in treating neuromuscular diseases associated with aging, but there is still a need to clarify its role in motor units (MUs) function during aging. In this study, 40 male Wistar rats aged 15 months were randomly assigned to a control or to two experimental groups in which 0.1% carnosine supplementation was performed for 10 or 34 weeks. After 34 weeks, we examined fast fatigable (FF), fast fatigue-resistant (FR) and slow (S) MUs' force properties and fatigability, as well as antioxidant potential, advanced glycation end products, activity of enzymes, and histidyl dipeptides content in the medial gastrocnemius muscle. Short- and long-term carnosine supplementation maintained the force of FF MUs at a higher level during its rapid decline seen from the initial 10 to 70 s of the fatigue test. In FF, especially long-term, and in FR MUs, especially short-term, carnosine supplementation resulted in less rapid force decline during the initial 70 s of the second fatigue protocol. Carnosine supplementation did not change muscle antioxidant potential and mortality rate (~35% in all groups), nor muscle mass with aging. Moreover, instead of the expected increase, a decrease in histidyl dipeptides by ~30% in the red portion of medial gastrocnemius muscle after long-term supplementation was found. After chronic carnosine supplementation, the specific changes in fatigue resistance were observed in FF and FR units, but not in S MU types that were not accompanied by an improvement of antioxidant potential and activity of glycolytic or oxidative enzymes in aged rats. These observations indicate that carnosine supplementation during aging may generate different physiological adaptations which should be considered as an important factor when planning treatment strategies.

Motor Unit Force Potentiation and Calcium Handling Protein Concentration in Rat Fast Muscle After Resistance Training.

Post-tetanic potentiation (PTP) of force depends on intramuscular Ca2+ levels and sensitivity and may be affected by fatigue. The aim of this study was to determine the ability of isolated fast fatigue-resistant (FR) and fast-fatigable (FF) motor units (MUs) to potentiate force evoked with single and 40-Hz electrical stimulation after 5 weeks of voluntary weight-lifting training. Tetanic contractions evoked by gradually increasing (10-150 Hz) stimulation frequency served as conditioning stimulation. Additionally, the concentration of myosin light chain kinase and proteins engaged in calcium handling was measured in rat fast medial gastrocnemius muscle. After the training, the potentiation of twitch force and peak rate of force development was increased in FF but not FR MUs. Force potentiation of 40-Hz tetanic contractions was increased in both fast MU types. After the training, the twitch duration of FR MUs was decreased, and FF MUs were less prone to high-frequency fatigue during conditioning stimulation. Muscle concentration of triadin was increased, whereas concentrations of ryanodine receptor 1, junctin, FKBP12, sarcoplasmic reticulum calcium ATPase 1, parvalbumin, myosin light chain kinase, and actomyosin adenosine triphosphatase content were not modified. After short-term resistance training, the twitch contraction time and twitch:tetanus force ratio of FR MUs are decreased, and PTP ability is not changed. However, PTP capacity is increased in response to submaximal activation. In FF MUs increase in PTP ability coexists with lesser fatigability. Further work is required to find out if the increase in triadin concentration has any impact on the observed contractile response.

Tetanic depression and catch-like effect in fast motor units of the rat medial gastrocnemius at linearly increasing and decreasing stimulation frequencies.

The forces developed by fast resistant (FR) and fast fatigable (FF) motor units of the rat medial gastrocnemius during trains of electrical stimuli at linearly increasing or decreasing frequency were measured at the instantaneous frequency of 60 Hz and compared with the force evoked at a constant 60 Hz. In both motor unit types, the mean forces during stimulation at increasing frequency were depressed by 17%, while those recorded during stimulation at decreasing frequency were increased by 15% (FR) or 10% (FF) compared to values observed during constant-rate stimulation. During trains of stimuli at an increasing rate, the instantaneous frequency necessary to induce a force comparable to that produced at constant 60 Hz stimulation in FR and FF units was 84 and 88 Hz, respectively; whereas for the same units during stimulation at a decreasing rate these values were 45 and 47 Hz, respectively. When the stimulation frequency was increased up to 60 Hz and then held at this level, the force of both motor unit types was depressed by approximately 6% compared to 60 Hz constant-rate stimulation. From the available data it may be concluded that the phenomenon of tetanic depression is able to limit the development of force when the motoneuronal firing rate increases.

Female Office Workers With Moderate Neck Pain Have Increased Anterior Positioning of the Cervical Spine and Stiffness of Upper Trapezius Myofascial Tissue in Sitting Posture.

BACKGROUND: Work-related neck disorders are among the most common dysfunctions in office workers. Understanding cervical myofascial mechanical characteristics that differentiate symptomatic from asymptomatic office workers is crucial for the understanding of musculoskeletal dysfunctions in workers with neck pain. OBJECTIVE: To assess the effect of moderate work-related neck disorders on head posture and stiffness and perceived pain threshold of the upper trapezius (UT) and sternocleidomastoid (SCM) muscles. DESIGN: Cross-sectional study. SETTING: Work environment. PARTICIPANTS: Sixteen office workers with diagnosed moderate work-related neck disorders and 16 asymptomatic office workers matched for sex, age, body mass index, and the nature and duration of their work. INTERVENTION: Not applicable. OUTCOME: The primary outcome measure consisted of measurements of head posture using craniovertebral angle and myotonometric muscle stiffness (Newtons per meter) of the UT and SCM muscles. The secondary output variable was the perceived pain threshold algometry of the studied muscles. RESULTS: Compared with controls, subjects with work-related neck pain had a significantly smaller craniovertebral angle (by 8.3%; Cohen d = 0.88, P = .02) and greater stiffness of the UT myofascial tissue (by 11.3%; Cohen d = 1.05, P = .006). No significant differences in perceived pain threshold of the myofascial tissue of the UT or SCM muscles were found between groups. CONCLUSION: In this study female office workers with neck pain had increased anterior positioning of the head and stiffness of the UT muscle in the sitting posture, which was not associated with changes in the pressure pain threshold of this muscle. LEVEL OF EVIDENCE: III.

Jumping Height Does Not Increase in Well Trained Volleyball Players After Transcutaneous Spinal Direct Current Stimulation.

Transcutaneous spinal direct current stimulation (tsDCS) increases corticospinal and spinal reflex excitability, and may be a new tool for increasing muscle explosive performance in sports training. The aim of the study was to evaluate whether tsDCS can enhance jumping ability in trained humans practicing volleyball. Twenty eight participants completed the study, including 21 men and 7 women. We investigated the effects of a single 15-minute session of sham, anodal, and cathodal tsDCS over spine and shoulder on repeated counter movement jump (CMJ) and squat jump (SJ) performance at 0, 30 and 60 min post-stimulation. The order of SJs and CMJs sets in each session was randomized. Each SJ and CMJ set consisted of 3 jumps. The break between each attempt was 1 min and the interval between the sets was 3 min. Two-way repeated measures ANOVA did not show effect of time, nor stimulation method, nor stimulation method × time interactions on SJ (time: F ( 1 . 8,142 . 1 ) = 1.054; p = 0.346, stimulation: F ( 2,78 ) = 0.019; p = 0.981, stimulation × time: F ( 3 . 6,142 . 1 ) = 0.725; p = 0.564) or CMJ (time: F ( 1 . 8,140 . 9 ) = 2.092; p = 0.132, stimulation: F ( 2,78 ) = 0.005; p = 0.995, stimulation × time: F ( 3 . 6,140 . 9 ) = 0.517; p = 0.705) performance. Single session of tsDCS over spine and shoulder does not increase jumping height in well-trained volleyball players. This is an important finding for coaches and strength conditioning professionals for understanding the practical utility of tsDCS for enhancing muscular explosiveness.

Relationship between age, BMI, head posture and superficial neck muscle stiffness and elasticity in adult women.

This study determined relationships between age, BMI and cranio-vertebral angle (CVA) (independent variables) and stiffness and elasticity of sternocleidomasteoid [SCM] and upper trapezius [UT] (dependent variables) muscles in sitting posture in 95 women across adult life. Moreover, a stepwise regression was performed to determine to what extent the dependent variables are explained by age, BMI and CVA. Age was moderately correlated with BMI (r = 0.41), and both age and BMI were moderately negatively correlated with CVA (r = -0.54 and -0.55, respectively). High (r = 0.73) and moderate (r = 0.53) linear relationships were present between age and logarithmic decrement (inversely related to elasticity) and stiffness of SCM muscle, respectively. Low (r = 0.36) and moderate (r = 0.47) relationships were present between age and logarithmic decrement and stiffness of UT muscle, respectively. Age accounted for 53% variance in elasticity and 28.5% variance in stiffness of SCM, and for 13% variance in elasticity and 22% variance in stiffness of UT muscle. Introduction of BMI but not CVA to the model explained the variance of these parameters by additional 0-8%. Among the studied factors age is the major correlate of stiffness and elasticity of neck muscles across the adult life.

Postural Stability in Adolescent Girls with Progressive Idiopathic Scoliosis.

The aim of this work was to analyze postural stability of girls having progressive form of idiopathic scoliosis and undergoing specific period of the adolescent growth spurt. Twenty-seven girls, aged 13.4 ± 1.2 years, presenting structural idiopathic scoliosis, thoracic or thoracolumbar pattern, radiological Cobb angle 41.7 ± 17.4° (study group) and 37 healthy girls (control group) were included. The groups were sex, age, height, weight, and BMI matched. Postural stability examination was performed using two stabilometric platforms with visual control (eyes open) at three stages: (1) both legs' stance, (2) left leg stance, and (3) right leg stance. The Center of Pressure (COP) sway path length, the area and the displacement amplitude were compared. For the double stance, no difference in postural stability parameters between the groups was found. However, for the right leg stance, the total sway path length was longer (p = 0.04) and the mean amplitude of the lateral COP displacement was increased (p = 0.03) in the scoliotic group. In conclusion, for double stance, the adolescent girls with progressive form of idiopathic scoliosis revealed fair postural stability compared to control group. An impaired postural control was observed during right leg stance.

Effect of ageing on the regulation of motor unit force in rat medial gastrocnemius muscle.

The influence of ageing on the regulation of force through the firing rate (force-frequency relationship) and motor unit contractile output were investigated in three types of motor unit (MU): FF, FR and S, in the medial gastrocnemius muscle. A control group of young (5-10 months) Wistar rats was compared to three groups of older (20-21, 24-25 and 28-30 months) animals. The optimal tetanus characterized by the maximum contractile output (force-time area - FTA - per single pulse) was determined. During ageing, the steep part of the force-frequency relationship of medial gastrocnemius MUs shifted towards lower stimulation rates. However, in all MU types of the oldest rats, the opposite shift (towards higher rates) was observed. Ageing induced a substantial increase in the maximal FTA per pulse, particularly in S and FF units, but only subtly altered the fusion index of the optimal tetanus of MUs. Moreover, a transient increase in the mean forces of FF MUs was revealed in the groups of 20-21 and 24-25 months rats, and a significant decrease in the fatigue resistance of FR MUs accompanied ageing. These findings increase our understanding of the functional mechanisms responsible for changes in rate coding and alterations in muscle fatigability during ageing.

Adaptation of motor unit contractile properties in rat medial gastrocnemius to treadmill endurance training: Relationship to muscle mitochondrial biogenesis.

This study aimed at investigating the effects of 2, 4 and 8 weeks of endurance training on the contractile properties of slow (S), fast fatigue resistant (FR) and fast fatigable (FF) motor units (MUs) in rat medial gastrocnemius (MG) in relation to the changes in muscle mitochondrial biogenesis. The properties of functionally isolated MUs were examined in vivo. Mitochondrial biogenesis was judged based on the changes in mitochondrial DNA copy number (mtDNA), the content of the electron transport chain (ETC) proteins and PGC-1α in the MG. Moreover, the markers of mitochondria remodeling mitofusins (Mfn1, Mfn2) and dynamin-like protein (Opa1) were studied using qPCR. A proportion of FR MUs increased from 37.9% to 50.8% and a proportion of FF units decreased from 44.7% to 26.6% after 8 weeks of training. The increased fatigue resistance, shortened twitch duration, and increased ability to potentiate force were found as early as after 2 weeks of endurance training, predominantly in FR MUs. Moreover, just after 2 weeks of the training an enhancement of the mitochondrial network remodeling was present as judged by an increase in expression of Mfn1, Opa1 and an increase in PGC-1α in the slow part of MG. Interestingly, no signs of intensification of mitochondrial biogenesis assessed by ETC proteins content and mtDNA in slow and fast parts of gastrocnemius were found at this stage of the training. Nevertheless, after 8 weeks of training an increase in the ETC protein content was observed, but mainly in the slow part of gastrocnemius. Concluding, the functional changes in MUs' contractile properties leading to the enhancement of muscle performance accompanied by an activation of signalling that controls the muscle mitochondrial network reorganisation and mitochondrial biogenesis belong to an early muscle adaptive responses that precede an increase in mitochondrial ETC protein content.

Changes of motor unit contractile output during repeated activity.

The aim of the study was to evaluate changes in the motor unit output and to determine changes in the optimal stimulation frequency (i.e., giving the maximal output per one pulse) during prolonged contractile activity when, successively, potentiation of force and fatigue developed. The influence of these phenomena was studied on three types of motor units: fast fatigable (FF), fast resistant (FR) and slow (S) in the rat medial gastrocnemius muscle. The motor units were isolated by a method of splitting of L5 ventral root into very thin bundles of axons which were electrically stimulated 10 times with repeated series of 10 trains of stimuli at duration of 500 ms and progressively increasing (1-150 Hz) frequency. The initial (the first series of stimulating trains), potentiated (the second series), as well as fatigued (the tenths series) force recordings were compared. The motor unit output was expressed as the area under the force-time record in response to one stimulus measured at a plateau phase of the tetanic force. The stimulation frequency when the force-time area per one pulse was maximal was accepted as the optimal frequency. In fast motor units, the maximal contractile output increased with potentiation and was reduced with fatigue, and the optimal frequency decreased and increased, respectively. Nevertheless, the fusion degrees of the optimal tetanic contractions were similar in initial state, potentiation and fatigue independently of the changes in force. The applied stimulation protocol had almost no influence on the mechanical activity of slow motor units. The study highlights the physiological importance of force potentiation induced by preceding contractile activity for the economy of motor performance. The observed changes of the optimal stimulation frequency are consistent with the known changes in the motor unit firing rates during voluntary activity when the two phenomena develop.

Adaptations of motoneuron properties after weight-lifting training in rats.

Resistance training, with repeated short-term and high-intensity exercises, is responsible for an increase in muscle mass and force. The aim of this study was to determine whether such training induces adaptations in the electrophysiological properties of motoneurons innervating the trained muscles and to relate these adaptive changes to previous observations made on motor unit contractile properties. The study was performed on adult male Wistar rats. Animals from the training group were subjected to a 5-wk voluntary progressive weight-lifting program, whereas control rats were restricted to standard cage activity. Intracellular recordings from lumbar spinal motoneurons were made under pentobarbital anesthesia. Membrane properties were measured, and rhythmic firing of motoneurons was analyzed. Strength training evoked adaptive changes in both slow- and fast-type motoneurons, indicating their increased excitability. A shorter spike duration, a higher input resistance, a lower rheobase, a decrease in the minimum current required to evoke rhythmic firing, an increase in the maximum frequencies of the early-state firing (ESF) and the steady-state firing (SSF), and an increase in the respective slopes of the frequency-current (f/I) relationship were observed in fast motoneurons of the trained group. The increase in the maximum ESF and SSF frequencies and an increase in the SSF f/I slope were also present in slow motoneurons. Higher maximum firing rates of motoneurons as well as higher discharge frequencies evoked at the same level of intracellular depolarization current imply higher levels of tetanic forces developed by motor units over the operating range of force production after strength training.NEW & NOTEWORTHY Neuronal responses to weight-lifting training can be observed in altered properties of both slow and fast motoneurons. Motoneurons of trained animals are more excitable, require lower intracellular currents to evoke rhythmic firing, and have the ability to evoke higher maximum discharge frequencies during repetitive firing.

Effects of aging on mechanical properties of sternocleidomastoid and trapezius muscles during transition from lying to sitting position-A cross-sectional study.

AIM: The aim of this study was to analyze the effects of aging on the viscoelastic properties of the upper trapezius (UT) and the sternocleidomastoid (SCM) muscle during transition from lying to sitting position. MATERIALS AND METHODS: The study included 39 older (mean age 67±5.9years) and 36 younger (21.1±1.8years) women. Tone, stiffness and elasticity of the UT and the SCM were measured by means of myotonometry (MyotonPRO) in lying and then, in sitting position. The results were compared using two-way analysis of variance. RESULTS: Irrespective of the position, older women presented with significantly higher muscle tone, stiffness and elasticity than younger subjects (P<0.05). In both groups, the transition from lying to sitting position resulted in a decrease (P<0.05) in the tone and stiffness, but not the elasticity (P>0.05) of the SCM, and stimulated an increase in the tone, stiffness and elasticity of the UT (P<0.05). The degree of changes in both study groups was similar, except from the absolute value of the UT elasticity, significantly higher increase in older women than in younger subjects (P<0.05). CONCLUSION: Age contributes to an increase in the stiffness and tone of the UT and the SCM, as well as to a decrease in the elasticity of these muscles in female subjects. In contrast, age exerts only a slight effect on the mechanical properties of both muscles during transition from lying to sitting position.

Role of histidyl dipeptides in contractile function of fast and slow motor units in rat skeletal muscle.

The physiological role of the muscle histidyl dipeptides carnosine and anserine in contractile function of various types of muscle fibers in vivo is poorly understood. Ten adult male Wistar rats were randomly assigned to two groups: control and supplemented for 10 wk with beta-alanine, the precursor of carnosine (∼640 mg·kg body wt(-1)·day(-1)). Thereafter, contractile properties and fatigability of isolated fast fatigable (FF), fast resistant to fatigue (FR), and slow motor units (MUs) from the medial gastrocnemius were determined in deeply anaesthetized animals. The fatigue resistance was tested with a 40-Hz fatigue protocol followed by a second protocol at 40 Hz in fast and 20 Hz in slow units. In the supplemented rats, histidyl dipeptide concentrations significantly increased (P < 0.05) by 25% in the red portion of the gastrocnemius, and carnosine increased by 94% in the white portion. The twitch force of FF units and maximum tetanic force of FR units were significantly increased (P < 0.05), and the half-relaxation time was prolonged in slow units (P < 0.05). FF units showed less fatigue during the first 10 s, and FR units showed higher forces between 10 and 60 s during the 40-Hz fatigue test. In slow units, forces declined less during the first 60 s of the 20-Hz test. In conclusion, this in vivo experiment demonstrates that an elevation in muscle histidyl dipeptide content elicits beneficial changes in MU contractile characteristics and fatigue resistance. Carnosine and anserine seem to play an important yet divergent role in various MUs.

Contractile properties of motor units and expression of myosin heavy chain isoforms in rat fast-type muscle after volitional weight-lifting training.

Dynamic resistance training increases the force and speed of muscle contraction, but little is known about modifications to the contractile properties of the main physiological types of motor units (MUs) that contribute to these muscle adaptations. Although the contractile profile of MU muscle fibers is tightly coupled to myosin heavy chain (MyHC) protein expression, it is not well understood if MyHC transition is a prerequisite for modifications to the contractile characteristics of MUs. In this study, we examined MU contractile properties, the mRNA expression of MyHC, parvalbumin, and sarcoendoplasmic reticulum Ca2+ pump isoforms, as well as the MyHC protein content after 5 wk of volitional progressive weight-lifting training in the medial gastrocnemius muscle in rats. The training had no effect on MyHC profiling or Ca2+-handling protein gene expression. Maximum force increased in slow (by 49%) and fast (by 21%) MUs. Within fast MUs, the maximum force increased in most fatigue-resistant and intermediate but not most fatigable MUs. Twitch contraction time was shortened in slow and fast fatigue-resistant MUs. Twitch half-relaxation was shortened in fast most fatigue-resistant and intermediate MUs. The force-frequency curve shifted rightward in fast fatigue-resistant MUs. Fast fatigable MUs fatigued less within the initial 15 s while fast fatigue-resistant units increased the ability to potentiate the force within the first minute of the standard fatigue test. In conclusion, at the early stage of resistance training, modifications to the contractile characteristics of MUs appear in the absence of MyHC transition and the upregulation of Ca2+-handling genes.