Shortening Contractions Research Articles

MP31-09 HOW DO WE DIAGNOSE DETRUSOR UNDERACTIVITY? THE POTENTIAL OF PARAMETERS CALCULATED WITH WATTS FACTOR

INTRODUCTION AND OBJECTIVE: Detrusor underactivity (DU) increases with age. The International Continence Society definition of DU contains two different pathophysiological causes, namely weak detrusor contraction force and abnormally short detrusor contractions. Urodynamic studies have attempted to define DU, but no precise definition is widely accepted. Widely used measurements of bladder contractility are bladder contractility index (BCI) and the Watts factor (WF). Impaired bladder contraction represent not only decreased the peak of WF, but also poorly sustained contractions. From this point of view, the maximum height of the resulting curve (Wmax) and its pattern should be discussed separately. In the present study, we compare several contemporary urodynamic criteria and parameters calculated with WF. METHODS: Thirty five patients with pre and post radical prostatectomy were evaluated. The urodynamic parameters included the maximum flow rate (Qmax), postvoid residual volume (PVR), detrusor pressure at maximum flow (Pdet at Qmax), Wmax, W (Qmax), BCI (bladder contractility index; Pdet Qmax + 5Qmax), rV (Wmax), Line (W), W80-W20 were examined. (Each definition was shown in figure; see figure below) RESULTS: After radical prostatectomy, Qmax increased significantly (pre: 13.6 ± 6.7, post: 18.0 ± 7.0 ml/min; P < 0.01), whereas PdetQmax and PVR decreased significantly (50.6 ± 21.0 and 29.9 ± 16.9 cmH2O, 51.3 ± 61.9 and 15.1 ± 28.6 ml; P < 0.05). BCI, Wmax and W (Qmax) did not change significantly after radical prostatectomy (123.7 ± 36.0 and 114.0 ± 19.9, 11.5 ± 2.8 and 11.1 ± 3.0 W/m2, 8.8 ± 2.6 and 9.7 ± 2.9 W/m2), but rV(Wmax), Line(W), W80 – W20 decreased significantly (0.47 ± 0.3 and 0.20 ± 0.20, −0.0016 ± 0.01 and −0.010 ± 0.01 W/m2/mL, −0.4 ± 2.0 and −2.1 ± 2.6 W/m2; P < 0.05). Wmax and W (Qmax) represents the maximum power of bladder contraction at a particular point in time, whereas rV(Wmax) Line(W), W80 – W20 can be used to detect whole detrusor contraction pattern. The commonly used threshold values for the definition of DU are BCI < 100 or W max < 7 W/m2. In our current study, we couldn’t detect DU using these criteria. However, parameters calculated with WF detect subtle change or pattern of micturition. CONCLUSIONS: The parameters calculated with Watts factor provides some insight into the diagnosis of DU.Source of Funding: none

Effects of 21\xa0days of bed rest and whey protein supplementation on plantar flexor muscle fatigue resistance during repeated shortening contractions

PurposeSpace 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.MethodsTen 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.ResultsBR 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.ConclusionsTwenty-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.

111 Predicting Restoration of Joint Function in the Contracted Burned Hand: The Benefit of the Soft Tissue to Skeletal Ratio

Abstract Introduction Long-standing burn contractures of the hand produce marked soft tissue deficits across mobile joint surfaces. Optimum functional reconstruction requires attention to the ratio of soft tissue contracture to skeletal length. Maturation of burn scar and anticipated growth in children play a role in selecting procedures that maximize function and range. Methods Burn contractures of the hand show functional loss of motion by flexion/extension deformities of the fingers and wrist. Analysis of soft tissue shortening (contraction), measured against existing skeletal length was used to customize procedures to restore maximum function. Twenty six patients, aged 3 to 57 years underwent post burn reconstruction to maximize hand function. Each procedure selected was based on the ratio of existing soft tissue to skeletal length. Results Flexion/extension at all joints requires a balanced glide of soft tissue over existing bone length defining a 1:1 ratio between soft tissue and skeletal length. Burn contracture shortens soft tissue and restricts motion over joints. An objective ratio was applied to each joint contracture. A comparison of soft tissue quality to skeletal length (including growth in children) was used to determine the procedure for reconstruction associated with the best prognosis for maximum outcome with the least recurrence. Abnormalities included post burn boutonniere deformities, webspace contractures, flexion and extension contracture deformities of the hand, and digits. Burn scar hypertrophy and induration worsened the ratio. Eight patients with a soft tissue to skeletal ratio of 0.4:1 or less required bone shortening in the form of either joint arthrodesis, trapeziectomy, or wrist fusion. Twelve patients with a ratio of 0.7:1 or better were managed with skin grafts and/or adjacent tissue rearrangement. In the remaining group, four patients required a combination of procedures including composite soft tissue and tendon expansion restoring length of all soft tissue relative to bony length precluding the need for flap reconstruction and tendon lengthening. Two patients underwent bone shortening with prosthetic joint replacement. All patients restored to a 0.8:1 ratio or better regained optimum position of joint position and function. No secondary procedures were required for deterioration of function at two years. Conclusions A balanced dynamic soft tissue to skeletal ratio is essential in restoring maximum function to the burned hand. Understanding soft tissue contraction compared to existing bone length will permit the objective design of a reconstructive hand/finger procedure that will predict outcome and maximize hand function. Applicability of Research to Practice Predicable outcomes of functional hand reconstruction are possible based on the relationship of burn scar contracture to measurable skeletal length.

Experimental study of clear-water contraction scour

AbstractIn this paper, experimental results of clear-water scour on a sand bed under short contractions were studied. Sequences of test runs were performed under clear-water conditions for three different contraction ratios. The outcomes of the experiments were employed to define the effects of various parameters on equilibrium scour depth under clear-water scour conditions. In this work, the precision of three maximum scour depth equations was tested from previous studies for contraction scour cases. Two new analytical equations were proposed to calculate time-dependent scour depth and maximum scour at equilibrium conditions, respectively, from the study. The proposed equations were validated using measurements from the present study as well as from previous literature, and the equations show a reasonable agreement between measured and computed values of scour depth under clear-water conditions in short contraction. The presented equations can be used for studying protection of the submerged portion at a bridge abutment or any similar structure.

Differential contributions of fatigue-induced strength loss and slowing of angular velocity to power loss following repeated maximal shortening contractions.

The purpose of this study was to investigate the relationship between fatigue‐induced reductions in isometric torque and isotonic power and to quantify the extent to which the decreases in angular velocity and dynamic torque can explain the power loss immediately following an isotonic fatiguing task and throughout recovery in seven young males and six young females. All measurements were performed with both legs. For dorsiflexion, fatigue‐related time‐course changes in isometric maximal voluntary contraction (MVC) torque, angular velocity, dynamic torque, and power production following repeated maximal isotonic contractions (load: 20% MVC) were investigated before, immediately after, and 1, 2, 5 and 10 min after a fatiguing task. There were no relationships between the fatigue‐related reductions in isometric MVC torque and peak power at any timepoint, suggesting that fatigue‐induced reductions in isometric MVC torque does not entirely reflect fatigue‐induced changes in dynamic performance. The relative contribution of fatigue‐related reduction in dynamic torque on power loss was greater immediately following the task, and lower throughout recovery than the corresponding decrease in angular velocity. Thus, power loss immediately following the task was more strongly related to the decline in dynamic torque; however, this relationship shifted throughout recovery to a greater dependence on slowing of angular velocity for power loss.

Direct Observation of a Coil-to-Helix Contraction Triggered by Vinculin Binding to Talin

Vinculin binds unfolded talin domains in focal adhesions, which recruits actin filaments to reinforce the mechanical coupling of this organelle. However, it remains unknown how this interaction is regulated and its impact on the force transmission properties of this mechanotransduction pathway. Here, we use magnetic tweezers to measure the interaction between vinculin head and the talin R3 domain under physiological forces. For the first time, we resolve individual binding events as a short contraction of the unfolded talin polypeptide caused by the reformation of the vinculin-binding site helices, which dictates a biphasic mechanism that regulates this interaction. Force favors vinculin binding by unfolding talin and exposing the vinculin-binding sites; however, the coil-to-helix contraction introduces an energy penalty that increases with force, defining an optimal binding regime. This mechanism implies that the talin-vinculin-actin association could operate as a negative feedback mechanism to stabilize force on focal adhesions.

Short vs. long-term procurement contracts when supplier can invest in cost reduction

In dynamic markets where cost of components changes fast, buyers typically auction off regular short-term contracts to fully leverage supplier competition in each period to continuously source from the lowest-cost supplier. However, too much competition through short-term contracts does not incentivize the incumbent supplier to make relation-specific investments in reducing costs, as future business is not assured. We investigate this trade-off, between leveraging supplier competition in each period versus incentivizing incumbent’s investment, with a stylized two-period model in which the buyer decides whether to auction off short-term contracts in each period or auction off a single long-term contract spanning both periods. In both cases, we characterize the optimal incumbent supplier’s investment, the suppliers’ equilibrium bidding strategy and the buyer’s expected cost. Our analysis shows that the supplier always invests more in a long-term contract. However, the buyer’s cost depends on supply base size: it prefers short-term contracts for large supply base size, otherwise it prefers long-term contract. Moreover, we find that system cost is typically lower with short-term contracts and that the suppliers are always better off with short-term contracts. Finally, adding non-discriminatory or discriminatory reserve prices to our model does not fundamentally modify the trade-off, but we find that auctions with discriminatory reserve price are better at balancing this trade-off compared to long or short-term contracts.

Short contract student counselling in a neoliberal world

A research study using a Foucauldian discourse methodology was undertaken in a third-level student counselling service to explore the little researched but widespread practice of short contract counselling. Short contract counselling was defined as a standardised small number of counselling sessions, in this case six, which was offered to all clients. Discussion of psychotherapy literature relevant to this topic and its relationship to the findings from this study form the basis of this article. Consideration was given to some of the major changes in the higher education environment which link to the penetration of neoliberalism into education and health which then provides the conditions of possibility for the emergence of short contract counselling. This practice has developed and is described as a means of allocating scarce resources in the face of high demand for services. Although it is widespread it has not received much specific attention from researchers but tends to be seen as an inevitable and taken for granted background to the delivery of student counselling. The focus of this study is not to evaluate the short contract model but rather to explore the context which allowed it to come to such a position of prominence in a relatively short period of time. The study findings drew attention to a range of discourses drawn on by the study participants who were both student clients and counsellors working in a service which operated this model. These were the discourses of productivity, managerialism and risk management. These discourses illustrate the influence of neoliberalism and highlight how disciplinary power can operate within the short contract model.

Added mass in rat plantaris muscle causes a reduction in mechanical work.

Most of what we know about whole muscle behaviour comes from experiments on single fibres or small muscles that are scaled up in size without considering the effects of the additional muscle mass. Previous modelling studies have shown that tissue inertia acts to slow the rate of force development and maximum velocity of muscle during shortening contractions and decreases the work and power per cycle during cyclic contractions; however, these results have not yet been confirmed by experiments on living tissue. Therefore, in this study we conducted in situ work-loop experiments on rat plantaris muscle to determine the effects of increasing the mass of muscle on mechanical work during cyclic contractions. We additionally simulated these experimental contractions using a mass-enhanced Hill-type model to validate our previous modelling work. We found that greater added mass resulted in lower mechanical work per cycle relative to the unloaded trials in which no mass was added to the muscle (P=0.041 for both 85 and 123% increases in muscle mass). We additionally found that greater strain resulted in lower work per cycle relative to unloaded trials at the same strain to control for length change and velocity effects on the work output, possibly due to greater accelerations of the muscle mass at higher strains. These results confirm that tissue mass reduces muscle mechanical work at larger muscle sizes, and that this effect is likely amplified for lower activations.

Evaluating Fiscal Supports for the Public-Private Partnership: Determinants of Short Contract Survival

The marked increase in public-private partnerships (PPPs) has created both opportunities and fiscal challenges in developing countries. This paper examines how government's fiscal supports and the capacity of fiscal risk management explain the short survival of PPP contracts. The analysis shows that budget-constrained governments tend to adopt debt-hiding fiscal operation by structuring PPPs with guarantees when budget transparency is weak. After addressing contract endogeneity, the hazard estimation finds that guaranteed contracts face significantly high dispute risk due to fiscal manipulation and adverse selection. The paper quantifies how much infrastructure governance matters in determining the survival of guaranteed contracts.

The Effect of Shortening-induced Torque Depression on Fatigue-related Sex Differences.

This study investigated performance fatigability in males and females during ISO contractions and ISO contractions after active shortening (rTD). Fourteen females (22 ± 2 yr) and 14 males (23 ± 2 yr) performed three baseline maximal voluntary contractions (MVCs) of the dorsiflexors. The MVCs were used to determine a 30% submaximal torque target, which participants matched as steadily as possible until task failure. The ISO fatigue task was performed at 10° plantar flexion. In the rTD session, the participants' ankle was rotated from 40° to 10° plantar flexion before performing the same fatigue task. MVCs were performed immediately after task failure, 30 s, and 1, 2, 3, 4, 5, 10, 20, and 30 min after task failure to track recovery. The baseline MVC torque amplitude for males (32.1 ± 6.6 N·m) was 31% greater than that for females (22.3 ± 3.1 N·m; P < 0.001, ηp = 0.490). Females' time to task failure was 44% longer than that of males in the ISO state (P = 0.032, ηp = 0.164). However, there was no sex difference in the rTD state (P = 0.142). It seems that the sex differences in fatigue resistance observed in a low-intensity ISO task are abolished in the ISO state after an active shortening contraction.

Inhibitory tendon-evoked reflex is attenuated in the torque-depressed isometric steady-state following active shortening.

Residual torque depression (rTD) is the reduction in steady-state isometric torque following an active shortening contraction when compared with an isometric contraction at the same muscle length and activation level. We have shown that spinal excitability increases in the rTD state, yet the mechanisms remains unknown. Percutaneous electrical tendon stimulation was used to induce tendon-evoked inhibitory reflexes. We demonstrated that in the rTD state, reduced torque contributes to a reduction in inhibitory afferent feedback, which indicates that the history-dependent properties of muscle can alter spinal excitability and the voluntary control of submaximal contractions through changes in peripheral afferent feedback. Novelty Residual force depression is a basic property of skeletal muscle, which can influence spinal and supraspinal excitability via inhibitory reflex activity. Residual force depression alters the voluntary control of force.

Demand Visibility and Capacity Pooling With Temporal Commitments

Problem definition: We study the combined value of partial demand visibility and flexible capacity, two hedging mechanisms against demand uncertainty, when signing capacity contracts with temporal commitment. Academic/Practical Relevance: With new technological innovations, short commitment contracts are found in dynamic environments like distribution, processing and manufacturing; a trend likely to grow in the future. In contrast to classic procurement, where commitments are long, short commitments lead to new dynamics in which demand visibility allows companies to use flexible resources more efficiently by adapting to demand observations. Methodology: We incorporate flexible capacity and demand visibility simultaneously using a multi-period newsvendor network model with two nodes that are supplied using dedicated and flexible capacity contracts with temporal commitment. Results: The optimal commitment to capacity contracts adapts within bounds to the observed demand at each node. The ability to adapt to visible demand becomes more valuable when flexible capacity contracts are available. This allows us to show that demand visibility and flexible capacity can act as complements. Managerial Implications: In contrast to conventional wisdom, when contracts have short commitment, companies can enhance the value of demand visibility if flexible capacity is also available as an option.

CEO Turnover and Volatility under Long-Term Employment Contracts

We study the role of the contractual time horizon of chief executive officers (CEOs) for CEO turnover and corporate policies. Using hand-collected data on 3,954 fixed-term CEO contracts, we show that remaining time under contract predicts CEO turnover. When contracts are close to expiration, turnover is more likely and is more sensitive to performance. We also show a positive within-CEO relation between remaining time under contract and firm risk. Our results are similar across short and long contracts and are driven neither by firm or CEO survival, nor technological cycles. They are consistent with incentives to take long-term projects with interim volatility.

Potentiation of the first and second phases of the M wave after maximal voluntary contractions in the biceps brachii muscle.

The study was undertaken to examine separately the potentiation of the first and second phases of the M wave in biceps brachii after conditioning maximal voluntary contractions (MVCs) of different durations. M waves were evoked in the biceps brachii muscle before and after isometric MVCs of 1, 3, 6, 10, 30, and 60s. The amplitude, duration, and area of the first and second phases of monopolar M waves were measured during the 10-min period following each contraction. Our results indicated that the amplitude and area of the M-wave first phase increased after MVCs of long (≥30s) duration (P< 0.05), while it decreased after MVCs of short (≤ 10s) duration (P< 0.05). The enlargement after the long MVCs persisted for 5min, whereas the depression after the short contractions lasted only for 15s. The amplitude of the second phase increased immediately (1s) after all MVCs tested (P< 0.05), regardless of their duration, and then returned rapidly (10s) to control levels. Unexpectedly, the amplitude of the second phase decreased below control values between 15s and 1min after the MVCs lasting ≥6s (P< 0.05). Our results reinforce the idea that the presence of fatigue is a necessary condition to induce an enlargement of the M-wave first phase and that this enlargement would be greater (and occur sooner) in muscles with a predominance of type II fibers (quadriceps and biceps brachii) compared to type-I predominant muscles (tibialis anterior). The unique findings observed for the M-wave second phase indicate that changes in this phase are highly muscle dependent. Graphical abstract Left panel-Representative examples of M waves recorded in one participant before (control) and at various times after conditioning maximal voluntary contractions (MVCs) of short (a1) and long (a2) duration. Left panel-Time course of recovery of the amplitude of the first (b1) and second (b2) phases of the M wave after conditioning MVCs of different durations.

Force depression following a stretch-shortening cycle depends on the amount of residual force enhancement established in the initial stretch phase.

Studies on residual force enhancement (rFE) and residual force depression (rFD) of the muscle‐tendon unit (MTU) have typically been conducted independent of each other, with little information available on how stretch‐induced rFE affects the shortening phase and the steady‐state MTU isometric force at the end of stretch‐shortening cycles (SSCs). We showed previously that when rFE is kept constant, but the force at the end of the stretch is varied by changing the stretch speed, the steady‐state forces at the end of SSCs were the same. These results led to the hypothesis that the amount of rFE of the MTU established in the initial stretch phase of SSCs determines the steady‐state force following the shortening phase of SSCs. This study was aimed at testing this hypothesis. Steady‐state MTU isometric thumb adduction forces were measured for pure isometric contractions, following pure shortening contractions, following pure stretch contractions, and following SSCs with constant shortening speed and magnitude. However, two stretch magnitudes (30° and 10° thumb abduction) and stretch speeds (15°/sec and ~ 60°/sec, respectively) were chosen such that forces at the end of the stretch phase of the SSCs were the same, while rFE differed substantially. As hypothesized, the steady‐state isometric MTU forces following SSCs were positively related to the stretch‐magnitude dependent amount of rFE established in the stretch phase and were independent of the force reached at the end of the stretch phase in SSCs. Among many competing theories, these results can potentially be explained with the idea that there is a length‐specific engagement of a passive structural element at the initial length of muscle activation.

Évaluation des programmes d’entraînement musculaire du plancher pelvien et incontinence urinaire chez la femme : revue de la littérature

Pelvic floor rehabilitation is efficient for the first line treatment of urinary incontinence in women. However, several questions remain as regards the best pelvic floor muscle training program to recommend. The objective was perform a literature review regarding the muscular strengthening program to recommend to treat stress or mixed urinary incontinence in women. We performed a systematic review of studies on the topic using Medline's database covering the 10last years. Among the 1130articles that were identified, we retained 6for our analysis. Given the heterogeneity of the exercise programs, it was impossible to exhaustively and comparatively analyze their efficacies. However, a significant improvement is noted when the exercises are based on the muscular training principles of the American College of Sports Medicine. They combine long and short contractions with the practice of the knack in situations of pressure exertion and are part of a self-rehabilitation program based on the needs of the patient and the individual's progression. Current knowledge does not allow us to recommend an optimal muscle training program to treat female urinary incontinence. Research must be conducted to evaluate different muscular training regimens but also their integration into a program based on personal needs as well as factors of adherence to the treatment of patients.

On the Shape of the Force-Velocity Relationship in Skeletal Muscles: The Linear, the Hyperbolic, and the Double-Hyperbolic.

The shape of the force-velocity (F-V) relationship has important implications for different aspects of muscle physiology, such as muscle efficiency and fatigue, the understanding of the pathophysiology of several myopathies or the mechanisms of muscle contraction per se, and may be of relevance for other fields, such as the development of robotics and prosthetic applications featuring natural muscle-like properties. However, different opinions regarding the shape of the F-V relationship and the underlying mechanisms exist in the literature. In this review, we summarize relevant evidence on the shape of the F-V relationship obtained over the last century. Studies performed at multiple scales ranging from the sarcomere to the organism level have described the concentric F-V relationship as linear, hyperbolic or double-hyperbolic. While the F-V relationship has most frequently been described as a rectangular hyperbola, a large number of studies have found deviations from the hyperbolic function at both ends of the F-V relation. Indeed, current evidence suggests that the F-V relation in skeletal muscles follows a double-hyperbolic pattern, with a breakpoint located at very high forces/low velocities, which may be a direct consequence of the kinetic properties of myofilament cross-bridge formation. Deviations at low forces/high velocities, by contrast, may be related to a recently discovered, calcium-independent regulatory mechanism of muscle contraction, which may also explain the low metabolic cost of very fast muscle shortening contractions. Controversial results have also been reported regarding the eccentric F-V relationship, with studies in prepared muscle specimens suggesting that maximum eccentric force is substantially greater than isometric force, whereas in vivo studies in humans show only a modest increase, no change, or even a decrease in force in lengthening contractions. This review discusses possible reasons reported in the literature for these discrepant findings, including the testing procedures (familiarization, pre-load condition, and temperature) and a potential neural inhibition at higher lengthening velocities. Finally, some unresolved questions and recommendations for F-V testing in humans are reported at the end of this document.

Doublets Initiating Low Frequency Stimulation Improve Dynamic Contractility Without Exacerbating Fatigue

PURPOSE: Doublets, two action potentials (AP) with a very short inter-impulse interval, frequently occur initially in impulse trains from motor nerves during muscle activation. Such doublets improve dynamic contractile parameters such as rate of force development (RFD), velocity and power. In addition, previous studies have shown delayed fatigue development from repeated isometric contractions when a doublet initiates a sub-tetanic train compared with constant frequency stimulation. However, these comparisons were not matched for impulse number, train duration and work, which may have confounded the interpretations. Furthermore, it is unknown how doublets would affect endurance during dynamic contractions. We, therefore, compared the development of fatigue from repeated shortening contractions activated with doublet-initiated sub-tetanic train (DT) or constant-frequency sub-tetanic train (CFT) using protocols matched for work, AP and train duration. METHODS: EDL muscles were isolated from 4 week old rats and incubated at 30°C in Krebs Ringer buffer. To induce fatigue, 100 shortening contractions were elicited either by a doublet-initiated sub-tetanic train (DT; 60 Hz) or a constant-frequency sub-tetanic train (CFT; 71 Hz). Contractions in the two groups were matched for work, train duration and pulse number. Isometric contractility was tested before and immediately after fatigue. RESULTS: Work of each contraction was similar in DT- and CFT-stimulated muscles both initially (368 ± 10 vs 369 ± 13 μJ, respectively) and after 100 fatiguing contractions (202 ± 9 vs 193 ± 8 μJ, respectively). However the contraction force and shortening developed faster in the DT-fatigued than the CFT-fatigued muscles: Initial values (RFD 20 ± 0.7 vs 13 ± 0.3 N/s and shortening velocity 48 ± 2 vs 41 ± 1); fatigued values (RFD 16 ± 0.7 vs 10 ± 0.4 N/s, velocity 29 ± 2 vs 23 ±1 mm/s ). Isometric tetanic force was equally affected in both groups; DT-fatigued muscles decreased to 73 ± 2 % and CFT-fatigued muscles to 75 ± 1 % of initial force. CONCLUSION: The results indicate that during repeated contractions matched for total work, stimulation duration and pulse number, an initiation by a doublet does not exacerbate fatigue development, but provides an activation strategy for muscles to produce a higher RFD and shortening velocity.

Flexor Hallucis Brevis Morphology is Associated with Visual Reliance While Balancing in Previously Injured Runners

Running related injuries to the foot and lower leg are pervasive and thought to be due to poor plantar intrinsic muscle (foot core) function. Previously injured runners have decreased cross sectional area (CSA) and thickness of the flexor hallucis brevis (FHB) and increased reliance on visual information while balancing relative to uninjured runners. However, it remains unknown if FHB morphology is associated with an increased reliance on visual information. PURPOSE: To determine if visual reliance while balancing is associated with FHB morphology in those with a history of running related injuries. METHODS: Twenty-four runners with foot and/or leg running injuries within the past three years but were currently asymptomatic (age: 21.66±2.44 years, mass: 66.84±10.03kg, height: 169.21±19.34cm, runs per week: 4.37±1.30, miles per week: 14.16±8.88) volunteered. Three, 10-second eyes open and eyes closed single leg stance trials on the injured limb were recorded. Center of pressure velocity (cm/s) for each condition was used to calculate %-modulation (%-modulation=[(eyes open-eyes closed)/eyes open]. Larger negative scores indicates greater reliance on visual information. While standing in a weight-bearing, subtalar neutral position, diagnostic ultrasound was used to image the FHB in a relaxed state and contracted state (while holding a short foot contraction). Images were processed to calculate CSA and FHB thickness. CSA was defined as the area (cm2) within the fascial borders of the muscle. Thickness was the distance (cm) between superior to inferior fascia perpendicular to the muscle fibers. Pearson product moment correlations determined the strength of associations and an a priori alpha level of 0.05 was used for all analyses. RESULTS: Larger negative %-modulation was associated with the less contracted FHB thickness (r=0.451, p=0.027) and trended towards being associated with less relaxed FHB thickness (r=0.368, p=0.077). No association was noted with the FHB CSA (r=0.273, p=0.198). CONCLUSIONS: In previously injured runners decreased FHB thickness is associated with an increased reliance on visual information while balancing. Short foot exercises, aimed at increased FHB strength may decrease reliance on visual information but future research is needed to confirm this hypothesis.