Voluntary activation of human knee extensors during isotonic shortening contractions.

PURPOSE: The interpolated twitch technique (ITT) is often used to assess voluntary activation during isometric contractions; however, this may have limited relevance to dynamic contractions. Although the ITT has been applied to relatively slow isokinetic contractions (< 150°/s), it has received limited consideration during unconstrained velocity (i.e., isotonic) contractions, despite their relevance to natural movements. Here, we explored the ITT during isotonic knee extension contractions using a modified dynamometer. METHODS: Young males (n = 6) and females (n = 4) performed isometric and isotonic knee extension contractions of sub-maximal and maximal intensities with doublet (150 Hz) muscle belly stimulations to assess voluntary activation. Following each voluntary isotonic contraction (velocity range ~ 35°/s to ~ 275°/s), resting potentiated doublets were evaluated during passive joint rotation at the same angular velocity achieved during voluntary efforts, to account for force-velocity characteristics. Correlations between voluntary activation and the proportion of maximal torque or power were evaluated for isometric and isotonic contractions, respectively. RESULTS: Isometric voluntary activation was strongly correlated with increasing torque output (r = 0.96, p < 0.001). Doublet torque during passive joint rotation displayed a hyperbolic relationship with increasing angular velocity (r = 0.98, p < 0.001). Isotonic voluntary activation was strongly correlated with increasing power output (r = 0.89, p < 0.001). During maximal effort contractions, no differences were observed in voluntary activation between isometric and isotonic conditions (89.4% vs. 89.2%, p = 0.904). CONCLUSIONS: The ITT is a valid approach to evaluate voluntary activation during an isotonic contraction using a modified dynamometer. Participants were able to achieve a similar high level of voluntary activation during isometric and isotonic contractions.

Three-Dimensional Ankle Exercise with Combined Isotonic Technique for an Obese Subject with Plantar Fasciitis: A Case Study.

Background and objectives: Obese people have many foot-related disorders and plantar fasciitis (PF) is the most common disorder among them. However, research on the role of therapeutic exercises in PF is lacking and there is no evidence to suggest its benefits. As such, a further insight into therapeutic exercises is needed within this group. This case study investigated the effect of three-dimensional (3D) ankle exercises using a combined isotonic (CI) technique on function and balance in an obese subject with PF. Material and methods: The subject in this study was a 28-year-old obese woman who was diagnosed with PF by an orthopedic surgeon. A 3D ankle exercise program was commenced three times a week for 15 min over 4 weeks. The evaluations were conducted at five intervals: pre-test, and at 1, 2, 3 and 4 weeks from the initiation of the intervention. The tests were conducted in the following order: the patient-specific functional scale test (PSFS), an ultrasound of the plantar fascia, the heel pressure and balance test, the pressure pain threshold (PPT), and the 4-way ankle strength test. Results: The mean score of the PSFS test reduced by 70.55% after 4 weeks of the intervention. The thickness of the plantar fascia and heel pressure measured during single-leg standing decreased by 6.67% and 10.37%, respectively, after 4 weeks of the intervention. The anteroposterior and medial-lateral balance ability showed improvements of 8.29% and 8.61%, respectively, after 4 weeks of the intervention. The PPT improved by 38.01% after 4 weeks of the intervention. In the 4-way ankle strength test, dorsiflexion, plantar flexion, inversion, and eversion increased by 14.46%, 9.63%, 4.3% and 13.25%, respectively, after 4 weeks of the intervention. Conclusion: 3D ankle exercises utilizing the CI technique were shown to be effective in improving foot function, pressure pain, and muscle strength in dorsiflexion and inversion in an obese patient with PF.

Regional variation in the mechanical properties and fibre-type composition of the rat extensor digitorum longus muscle.

NEW FINDINGS: What is the central question of this study? Mammalian muscle is typically heterogeneous in fibre-type distribution, with distinct regional variation in composition. The effects this might have on mechanical performance are largely unknown. What is the main finding and its importance? Contractile properties vary regionally within a heterogeneous muscle. The mixed extensor digitorum longus muscle has phenotypically distinct compartments that differ in their isometric twitch kinetics, the optimal cycle frequency for maximal power generation and fatigue resistance. The mechanisms underpinning the decline in performance during fatigue differ between compartments. Regional variation in mechanical performance suggests that regions of the extensor digitorum longus muscle might be differentially recruited during locomotion, depending upon functional demand. Fibre-type composition is heterogeneous, and distribution varies spatially in many muscles, indicating that there might be regional variation in recruitment and mechanical output. The rat extensor digitorum longus muscle is composed of predominantly fast-twitch fibres and exhibits a gradient in phenotype, resulting in oxidative medial (areal composition 24.3% type I/IIa) and glycolytic lateral (92.4% type IIx/IIb) compartments. Here, we investigated the variation in mechanical performance between the medial and lateral compartments during isometric, isotonic and cyclical contractions. Isometric tetanic stress and force-velocity relationships were similar in both compartments, but isometric twitch kinetics were slower in the medial compared with the lateral compartment. The medial compartment also had a lower optimal cycle frequency for maximal net power generation (11 versus 15 Hz; P < 0.05) attributable to slower isometric kinetics, resulting in a lower level of activation and reduced net work generation at higher cycle frequencies, compared with the lateral compartment. The more oxidative, medial compartment had higher fatigue resistance, maintaining net power 26% longer than the lateral compartment. The predominant mechanisms underpinning the decrease in net power varied between the compartments, resulting from an increase in the work to extend the muscle and from a reduction in work during shortening in the medial and lateral compartments, respectively. Regional variation in mechanical performance and resistance to fatigue within a mixed muscle suggests that a differential recruitment pattern is likely during locomotion, with the medial compartment being used during slow-speed locomotion and the lateral compartment during burst activities.

Utilizing Physiological Principles of Motor Unit Recruitment to Reduce Fatigability of Electrically-Evoked Contractions: A Narrative Review.

Neuromuscular electrical stimulation (NMES) is used to produce contractions to restore movement and reduce secondary complications for individuals experiencing motor impairment. NMES is conventionally delivered through a single pair of electrodes over a muscle belly or nerve trunk using short pulse durations and frequencies between 20 and 40Hz (conventional NMES). Unfortunately, the benefits and widespread use of conventional NMES are limited by contraction fatigability, which is in large part because of the nonphysiological way that contractions are generated. This review provides a summary of approaches designed to reduce fatigability during NMES, by using physiological principles that help minimize fatigability of voluntary contractions. First, relevant principles of the recruitment and discharge of motor units (MUs) inherent to voluntary contractions and conventional NMES are introduced, and the main mechanisms of fatigability for each contraction type are briefly discussed. A variety of NMES approaches are then described that were designed to reduce fatigability by generating contractions that more closely mimic voluntary contractions. These approaches include altering stimulation parameters, to recruit MUs in their physiological order, and stimulating through multiple electrodes, to reduce MU discharge rates. Although each approach has unique advantages and disadvantages, approaches that minimize MU discharge rates hold the most promise for imminent translation into rehabilitation practice. The way that NMES is currently delivered limits its utility as a rehabilitative tool. Reducing fatigability by delivering NMES in ways that better mimic voluntary contractions holds promise for optimizing the benefits and widespread use of NMES-based programs.

Go-activation endures following the presentation of a stop-signal: evidence from startle.

It has been proposed that, in a stop-signal task (SST), independent go- and stop-processes "race" to control behavior. If the go-process wins, an overt response is produced, whereas, if the stop-process wins, the response is withheld. One prediction that follows from this proposal is that, if the activation associated with one process is enhanced, it is more likely to win the race. We looked to determine whether these initiation and inhibition processes (and thus response outcomes) could be manipulated by using a startling acoustic stimulus (SAS), which has been shown to provide additional response activation. In the present study, participants were to respond to a visual go-stimulus; however, if a subsequent stop-signal appeared, they were to inhibit the response. The stop-signal was presented at a delay corresponding to a probability of responding of 0.4 (determined from a baseline block of trials). On stop-trials, a SAS was presented either simultaneously with the go-signal or stop-signal or 100, 150, or 200 ms following the stop-signal. Results showed that presenting a SAS during stop-trials led to an increase in probability of responding when presented with or following the stop-signal. The latency of SAS responses at the stop-signal + 150 ms and stop-signal + 200 ms probe times suggests that they would have been voluntarily inhibited but instead were involuntarily initiated by the SAS. Thus results demonstrate that go-activation endures even 200 ms following a stop-signal and remains accessible well after the response has been inhibited, providing evidence against a winner-take-all race between independent go- and stop-processes. NEW & NOTEWORTHY: In this study, a startling acoustic stimulus (SAS) was used to determine whether response outcome could be manipulated in a stop-signal task. Results revealed that presenting a SAS during stop-signal trials led to an increase in probability of responding even when presented 200 ms following the stop-signal. The latency of SAS responses indicates that go-activation remains accessible and modifiable well after the response is voluntarily inhibited, providing evidence against an irrevocable commitment to inhibition.

Effects of temperature and force requirements on muscle work and power output.

Performance of muscle-powered movements depends on temperature through its effects on muscle contractile properties. In vitro stimulation of Cuban treefrog (Osteopilus septentrionalis) plantaris muscles reveals that interactions between force and temperature affect the mechanical work of muscle. At low temperatures (9-17°C), muscle work depends on temperature when shortening at any force, and temperature effects are greater at higher forces. At warmer temperatures (13-21°C), muscle work depends on temperature when shortening with intermediate and high forces (≥30% peak isometric tetanic force). Shortening velocity is most strongly affected by temperature at low temperatures and high forces. Power is also most strongly affected at low temperature intervals, but this effect is minimized at intermediate forces. Effects of temperature on muscle force explain these interactions; force production decreases at lower temperatures, increasing the challenge of moving a constant force relative to the muscle's capacity. These results suggest that animal performance that requires muscles to do work with low forces relative to a muscle's maximum force production will be robust to temperature changes, and this effect should be true whether muscle acts directly or through elastic-recoil mechanisms and whether force is prescribed (i.e. internal) or variable (i.e. external). Conversely, performance requiring muscles to shorten with relatively large forces is expected to be more sensitive to temperature changes.

Mathematical model for isometric and isotonic muscle contractions.

A new mathematical model is presented to describe both the active and passive mechanics of muscles. In order to account for the active response, a two-layer kinematics that introduces both the visible and rest lengths of the muscle is presented within a rational mechanics framework. The formulation is based on an extended version of the principle of virtual power and the dissipation principle. By using an accurate constitutive description of muscle mobility under activation, details of microscopic processes that lead to muscle contraction are glossed over while macroscopic effects of chemical/electrical stimuli on muscle mechanics are retained. The model predictions are tested with isometric and isotonic experimental data collected from murine extensor digitorum muscle. It is shown that the proposed model captures experimental observations with only three scalar parameters.

Agonist muscle adaptation accompanied by antagonist muscle atrophy in the hindlimb of mice following stretch-shortening contraction training.

BACKGROUND: The vast majority of dynamometer-based animal models for investigation of the response to chronic muscle contraction exposure has been limited to analysis of isometric, lengthening, or shortening contractions in isolation. An exception to this has been the utilization of a rat model to study stretch-shortening contractions (SSCs), a sequence of consecutive isometric, lengthening, and shortening contractions common during daily activity and resistance-type exercise. However, the availability of diverse genetic strains of rats is limited. Therefore, the purpose of the present study was to develop a dynamometer-based SSC training protocol to induce increased muscle mass and performance in plantarflexor muscles of mice. METHODS: Young (3 months old) C57BL/6 mice were subjected to 1 month of plantarflexion SSC training. Hindlimb muscles were analyzed for muscle mass, quantitative morphology, myogenesis/myopathy relevant gene expression, and fiber type distribution. RESULTS: The main aim of the research was achieved when training induced a 2-fold increase in plantarflexion peak torque output and a 19% increase in muscle mass for the agonist plantaris (PLT) muscle. In establishing this model, several outcomes emerged which raised the value of the model past that of being a mere recapitulation of the rat model. An increase in the number of muscle fibers per transverse muscle section accounted for the PLT muscle mass gain while the antagonist tibialis anterior (TA) muscle atrophied by 30% with preferential atrophy of type IIb and IIx fibers. These alterations were accompanied by distinct gene expression profiles. CONCLUSIONS: The findings confirm the development of a stretch-shortening contraction training model for the PLT muscle of mice and demonstrate that increased cross-sectional fiber number can occur following high-intensity SSC training. Furthermore, the TA muscle atrophy provides direct evidence for the concept of muscle imbalance in phasic non-weight bearing muscles, a concept largely characterized based on clinical observation of patients. The susceptibility to this imbalance is demonstrated to be selective for the type IIb and IIx muscle fiber types. Overall, the study highlights the importance of considering muscle fiber number modulation and the effect of training on surrounding muscles in exercise comprised of SSCs.

Contractility of Right Ventricular Myocardium in Male and Female Rats during Physiological and Pathological Hypertrophy.

Sex differences in the morphogenesis and adaptation of the mechanisms controlling myocardium contractility during physiological and pathological hypertrophy of the right ventricle were demonstrated in mature rats. The study revealed sex-dependent effects of physiological and pathological cardiac hypertrophy on the coefficient of variation of the cardiomyocyte diameter, length-dependent control of the contractile force, and the maximum velocity of isotonic shortening.

Comparison of contractile responses of single human motor units in the toe extensors during unloaded and loaded isotonic and isometric conditions.

Much of the repertoire of muscle function performed in everyday life involves isotonic dynamic movements, either with or without an additional load, yet most studies of single motor units measure isometric forces. To assess the effects of muscle load on the contractile response, we measured the contractile properties of single motor units supplying the toe extensors, assessed by intraneural microstimulation of single human motor axons, in isotonic, loaded isotonic, and isometric conditions. Tungsten microelectrodes were inserted into the common peroneal nerve, and single motor axons (n = 10) supplying the long toe extensors were electrically stimulated through the microelectrode. Displacement was measured from the distal phalanx of the toe with either an angular displacement transducer for the unloaded (i.e., no additional load) and loaded (addition of a 4-g mass) isotonic conditions or a force transducer for the isometric conditions. Mean twitch profiles were measured at 1 Hz for all conditions: rise time, fall time, and duration were shortest for the unloaded isotonic conditions and longest for the isometric conditions. Peak displacements were lower in the loaded than unloaded isotonic conditions, and the half-maximal response in the loaded condition was achieved at lower frequencies than in the unloaded isotonic condition. We have shown that the contractile responses of single motor units supplying the human toe extensors are influenced by how they are measured: twitches are much slower when measured in loaded than unloaded isotonic conditions and slowest when measured in isometric conditions.

Isokinetic performance of hip muscles after revision total hip arthroplasty via previous anterolateral approach.

We investigated the isokinetic performance of hip muscles and clinical outcomes after revision total hip arthroplasty (THA) via same anterolateral approach used in primary surgery. Thirty patients who had undergone previous THA via an anterolateral approach underwent both acetabular and femoral component revision after aseptic loosening. The Harris Hip Score (HHS) was evaluated during a minimum 2-year follow-up. The isokinetic muscle strength of the operated and nonoperated hips was assessed 1 year after surgery. The HHS improved from 49.0 to 77.4. Operated and nonoperated hips exhibited similar isokinetic performance during all measurements (flexion, extension, and abduction) (p>0.05). This prospective study showed that the anterolateral approach preserves abductor strength after revision THA in aseptic cases with acceptable functional and clinical results. The main clinical relevance of this study is that the same anterolateral approach used in previous primary THA is also safe and viable for revision THA.

A common optimization principle for motor execution in healthy subjects and parkinsonian patients.

Recent research on Parkinson's disease (PD) has emphasized that parkinsonian movement, although bradykinetic, shares many attributes with healthy behavior. This observation led to the suggestion that bradykinesia in PD could be due to a reduction in motor motivation. This hypothesis can be tested in the framework of optimal control theory, which accounts for many characteristics of healthy human movement while providing a link between the motor behavior and a cost/benefit trade-off. This approach offers the opportunity to interpret movement deficits of PD patients in the light of a computational theory of normal motor control. We studied 14 PD patients with bilateral subthalamic nucleus (STN) stimulation and 16 age-matched healthy controls, and tested whether reaching movements were governed by similar rules in these two groups. A single optimal control model accounted for the reaching movements of healthy subjects and PD patients, whatever the condition of STN stimulation (on or off). The choice of movement speed was explained in all subjects by the existence of a preset dynamic range for the motor signals. This range was idiosyncratic and applied to all movements regardless of their amplitude. In PD patients this dynamic range was abnormally narrow and correlated with bradykinesia. STN stimulation reduced bradykinesia and widened this range in all patients, but did not restore it to a normal value. These results, consistent with the motor motivation hypothesis, suggest that constrained optimization of motor effort is the main determinant of movement planning (choice of speed) and movement production, in both healthy and PD subjects.

Comparison of acute responses to isotonic or isokinetic eccentric muscle action: differential outcomes in skeletal muscle damage and implications for rehabilitation.

Both isotonic and isokinetic eccentric muscle contractions are commonly used in muscle research laboratories to induce muscle damage, yet, the muscle damage outcomes between these 2 modes of eccentric contraction have not been compared. The purpose of this study was to compare modes of contraction for differences in muscle damage. 16 men were placed in the isotonic (IT: 110% of maximal isometric torque) or the isokinetic (IK: 120°/s) group, with each group performing 200 eccentric muscle actions of the knee extensors. Isometric peak torque, perceived soreness and CK activity were measured immediately pre and post exercise, and 48-h post exercise. Mean total work (~1700 J) and peak torque per set (~265 Nm) decreased over the 200 repetitions (p<0.01), and was not different between groups. Damage markers changed 48-h post exercise (p<0.05): peak isometric torque (-13%), creatine kinase activity (+200%) and self-perceived muscular soreness (+4 unit change). Significant group×time interactions (p<0.01) indicated that peak isometric torque was 22% lower, and creatine kinase and self-perceived muscular soreness were 330% and 3 unit difference higher in the IT as compared to the IK groups, 48-h post exercise. When equating for total work, skeletal muscle damage markers are higher during IT vs. IK modes. This reflects differences inherent in contraction type and suggests that this should be taken into account during physical rehabilitation.

Isokinetic strength test and functional outcomes in proximal humeral fractures treated with a locking plate.

BACKGROUND: Despite the use of many shoulder outcome scales in subjects with rotator cuff pathology or instability symptoms, it can be problematic to select an instrumental evaluation in the shoulder trauma population. In this study we evaluated patients with proximal humeral fractures treated with internal fixation with a locking plate, analyzing the recovery of strength with an isokinetic test and its correlation with clinical and functional outcomes. METHODS: We enrolled 46 individuals (17 men, 29 women). The evaluation included a structured interview, measurement of ROM, isokinetic strength test and Constant-Murley and QuickDASH scores. The isokinetic test was performed in flexion/extension and external/internal rotation of the operated shoulder in comparison with the contralateral side and concentric contractions in all movements. The parameter tested was peak torque. RESULTS: In the operated shoulder values we noticed a statistically significant correlation between the QuickDASH and Constant-Murley score. QuickDASH showed a significant correlation with flexion isokinetic strength, partial correlation with extension isokinetic values and no correlation with external/internal rotation values. In addition, we found a correlation between the Constant-Murley score and all the isokinetic strength parameters. Comparing the operated shoulder and the contralateral, in Neer type 2 fractures there was no significant difference in all the isokinetic peak torque values; in Neer type 3 and type 4, there was a significant statistical difference in both flexion peak torque values and no significant difference in the other movements. CONCLUSIONS: The isokinetic test can give objective data on strength recovery and could help the surgeon's clinical evaluation to assess the functional recovery of the operated shoulder over time. We believe that the isokinetic test and Constant-Murley score could act as a reference in the evaluation of post-surgical outcome of proximal humeral fractures. Furthermore, the type of fracture could be a post-surgical limb recovery predictor and the shoulder flexion force could be the best functionality recovery indicator.

Isotonic resistance jaw exercise alters jaw muscle coordination during jaw movements.

The aim was to investigate the effects of isotonic resistance exercise on the electro-myographic (EMG) activity of the jaw muscles during standardised jaw movements. In 12 asymptomatic adults surface EMG activity was recorded from the anterior temporalis and masseter muscles bilaterally and the right anterior digastric muscle during right lateral jaw movements that tracked a target. Participants were randomly assigned to a Control group or an Exercise group. Jaw movement and EMG activity were collected (i) at baseline, before the exercise task (pre-exercise); (ii) immediately after the exercise task (isotonic resistance at 60% MVC against right lateral jaw movements); (iii) after 4 weeks of a home-based exercise programme; and, (iv) at 8-weeks follow-up. There were no significant within-subject or between-group differences in the velocity and amplitude of the right lateral jaw movements either within or between data collection sessions (P > 0.05). However, over the 8 weeks of the study, three of the tested EMG variables (EMG Duration, Time to Peak EMG from EMG Onset, and Time to Peak EMG activity relative to Movement Onset) showed significant (P < 0.05) differences in the five tested muscles. Many of the significant changes occurred in the Control group, while the Exercise group tended to maintain the majority of the tested variables at pre-exercise baseline values. The data suggest a level of variability between recording sessions in the recruitment patterns of some of the muscles of mastication for the production of the same right lateral jaw movement and that isotonic resistance exercise may reduce this variability.

Comparison of muscle activity between two adult groups according to the number of Shaker exercise.

The purpose of this study was to investigate the muscle activity of the suprahyoid and infrahyoid muscles according to the number of Shaker exercise. The 19 experimental subjects were recruited and randomly assigned to the two experimental groups. The 1st experimental group performed Shaker exercise once a day, and 2nd experimental group performed Shaker exercise three times a day for 6 weeks. Shaker exercise consists of isometric and isotonic contraction movement, enhancing the strength of suprahyoid muscle and increasing the opening of UES. After performing Shaker exercise for 6 weeks, the muscle activity of experimental groups was measured and analysed by surface electromyography. As a result, muscle activity of the suprahyoid and infrahyoid muscles showed significant improvement in both groups (P < 0·05). Comparing muscle activity of both groups, there was no significant difference. This means the Shaker exercise shows similar exercise effects on suprahyoid muscle which is the primary target muscle irrespective of the number of Shaker exercise. The activity of the infrahyoid muscle, which takes a supportive role, also showed significant difference between the two groups. In the results of a follow-up test after 4 weeks, muscle activity was higher than the initial status and the value of the EMG activity was statistically significant (P < 0·05). These results by EMG study may suggest that Shaker exercise performed once a day, like performing three times a day, has enough exercise effect and there is similar effect on the suprahyoid muscle between the 2 groups. In addition, the exercise effect lasts for 4 weeks after completing exercise.

Fatigue and recovery from dynamic contractions in men and women differ for arm and leg muscles.

INTRODUCTION: Whether there is a gender difference in fatigue and recovery from maximal velocity fatiguing contractions and across muscles is not understood. METHODS: Sixteen men and 19 women performed 90 isotonic contractions at maximal voluntary shortening velocity (maximal velocity concentric contractions, MVCC) with the elbow flexor and knee extensor muscles (separate days) at a load equivalent to 20% maximal voluntary isometric contraction (MVIC). RESULTS: Power (from MVCCs) decreased similarly for men and women for both muscles (P > 0.05). Men and women had similar declines in MVIC of elbow flexors, but men had greater reductions in knee extensor MVIC force and MVIC electromyogram activity than women (P < 0.05). The decline in MVIC and power was greater, and force recovery was slower for the elbow flexors compared with knee extensors. CONCLUSIONS: The gender difference in muscle fatigue often observed during isometric tasks was diminished during fast dynamic contractions for upper and lower limb muscles.

Children with heavy prenatal alcohol exposure experience reduced control of isotonic force.

BACKGROUND: Heavy prenatal alcohol exposure can result in diverse and extensive damage to the central nervous system, including the cerebellum, basal ganglia, and cerebral cortex. Given that these brain regions are involved in the generation and maintenance of motor force, we predicted that prenatal alcohol exposure would adversely affect this parameter of motor control. We previously reported that children with gestational alcohol exposure experience significant deficits in regulating isometric (i.e., constant) force. The purpose of this study was to determine whether these children exhibit similar deficits when producing isotonic (i.e., graded) force. METHODS: Children with heavy prenatal alcohol exposure and typically developing children completed a series of isotonic force contractions by exerting force on a load cell to match a criterion target force displayed on a computer monitor. Two levels of target force (5 or 20% of maximum voluntary force) were investigated in combination with varying levels of visual feedback. RESULTS: Compared with control children, children with heavy prenatal alcohol exposure generated isotonic force signals that were less accurate, more variable, and less complex in the time domain. Specifically, interactions were found between group and visual feedback for response accuracy and signal complexity, suggesting that these children have greater difficulty altering their motor output when visual feedback is low. CONCLUSIONS: These data suggest that prenatal alcohol exposure produces deficits in regulating isotonic force, which presumably result from alcohol-related damage to developing brain regions involved in motor control. These children will most likely experience difficulty performing basic motor skills and daily functional skills that require coordination of finely graded force. Therapeutic strategies designed to increase feedback and, consequently, facilitate visual-motor integration could improve isotonic force production in these children.

Critical damping conditions for third order muscle models: implications for force control.

Experimental results presented in the literature suggest that humans use a position control strategy to indirectly control force rather than direct force control. Modeling the muscle-tendon system as a third-order linear model, we provide an explanation of why an indirect force control strategy is preferred. We analyzed a third-order muscle system and verified that it is required for a faithful representation of muscle-tendon mechanics, especially when investigating critical damping conditions. We provided numerical examples using biomechanical properties of muscles and tendons reported in the literature. We demonstrated that at maximum isotonic contraction, for muscle and tendon stiffness within physiologically compatible ranges, a third-order muscle-tendon system can be under-damped. Over-damping occurs for values of the damping coefficient included within a finite interval defined by two separate critical limits (such interval is a semi-infinite region in second-order models). An increase in damping beyond the larger critical value would lead the system to mechanical instability. We proved the existence of a theoretical threshold for the ratio between tendon and muscle stiffness above which critical damping can never be achieved; thus resulting in an oscillatory free response of the system, independently of the value of the damping. Under such condition, combined with high muscle activation, oscillation of the system can be compensated only by active control.

Sex differences in blood flow restricted isotonic knee extensions to fatigue.

AIM: It has been shown that females have greater muscular endurance than males and that this advantage is eliminated when blood flow is restricted. It is unknown if sex differences in dynamic endurance exist during low-load blood flow restricted (BFR) resistance exercise. The purpose of this study was to investigate sex differences in quadriceps femoris fatigability during isotonic knee extension exercise coupled with a blood flow restriction. METHODS: Ten males and ten females completed three sets of low-load isotonic knee extension exercises (20% of peak torque) to volitional failure under two conditions: blood flow restricted (BFR) and non-restricted free flow (FF). The number of repetitions, exercise volume, post-exercise strength loss and surface electromyography (EMG) were measured. RESULTS: Females performed more repetitions than males in the FF (252±37 vs. 112±17 repetitions; P<0.01) and BFR conditions (165±29 vs. 79±8 repetitions; P<0.01). Both sexes performed ~30% fewer repetitions during the BFR condition. MVC torque decreased approximately 37% following both conditions (P<0.01) and EMG activity increased (P<0.05) during the exercise bouts. CONCLUSION: Similar fatigue characteristics were evident in FF and BFR conditions for both sexes, and females demonstrated greater endurance, as determined by the number of repetitions completed, in both conditions. It may be beneficial to increase the relative exercise load for females in order to decrease the time under BFR.