Increase In Mechanical Work Research Articles

Is Altitude Training Bad for the Running Mechanics of Middle-Distance Runners?

It has been hypothesized that altitude training may alter running mechanics due to several factors such as the slower training velocity with associated alteration in muscle activation and coordination. This would lead to an altered running mechanics attested by an increase in mechanical work for a given intensity and to the need to "re-establish" the neuromuscular coordination and running biomechanics postaltitude. Therefore, the present study aimed to test the hypothesis that "live high-train high" would induce alteration in the running biomechanics (ie,longer contact time, higher vertical oscillations, decreased stiffness, higher external work). Before and 2 to 3days after 3 weeks of altitude training (1850-2200m), 9 national-level middle-distance (800-5000m) male runners performed 2 successive 5-minute bouts of running at moderate intensity on an instrumented treadmill with measured ground reaction forces and gas exchanges. Immediately after the running trials, peak knee extensor torque was assessed during isometric maximal voluntary contraction. Except for a slight (-3.0%; P = .04) decrease in vertical stiffness, no mechanical parameters (stride frequency and length, contact and flight times, ground reaction forces, and kinetic and potential work) were modified from prealtitude to postaltitude camp. Running oxygen cost was also unchanged. The present study is the first one to report that "live high-train high" did not change the main running mechanical parameters, even when measured immediately after the altitude camp. This result has an important practical implication: there is no need for a corrective period at sea level for "normalizing" the running mechanics after an altitude camp.

The stretch-shortening cycle effect is prominent in the inhibited force state

It has been suggested that residual force enhancement (RFE) contributes to the work enhancement observed in stretch-shortening cycles (SSC). Based on recent findings that RFE was preserved in the reduced force state, one may speculate that the SSC effect may be preserved in the reduced force state as well. The purpose of this study was to examine the magnitude of the SSC effect in inhibited skeletal muscle force states. Normal and inhibited force conditions were analyzed using skinned rabbit soleus fibres (N = 18). The inhibited force condition was achieved by adding 2,3-Butanedione monoxime into the activating solution. For both conditions, a SSC test and a pure shortening test were performed. In the SSC tests, fibres were activated at an average sarcomere length of 2.4 μm, and then stretched to 3.0 μm. Immediately after the end of the stretch, fibres were shortened to 2.4 μm. In the pure shortening tests, fibres were activated at an average sarcomere length of 3.0 μm and then shortened to 2.4 μm. The relative increase in mechanical work in the shortening phase of the SSC compared to the pure shortening condition was defined as the SSC effect index, and the magnitude of the SSC effect was compared between the normal and the inhibited force condition. The SSC effect was greater in the inhibited compared to the normal force condition (p < 0.001). We conclude that the SSC effect is at least in part preserved in the reduced force state.

Differences in stretch-shortening cycle and residual force enhancement between muscles

It has been suggested that cross bridge kinetics and residual force enhancement (RFE) affect force in the stretch-shortening cycle (SSC). Because cross bridge kinetics and titin isoforms, which are thought to be related to RFE, differ between muscles, the SSC effect may be also muscle-dependent. Thus, we compared the SSC effect between psoas and soleus muscles, which have a distinct fiber type distribution and different titin isoforms. Four tests (SSC, SSC control, RFE, RFE control) were conducted using isolated, skinned fibers of psoas and soleus. In the SSC tests, fibers were activated at an average sarcomere length of 2.4 μm, stretched to 3.0 μm, and shortened to 2.4 μm. In the SSC control tests, fibers were activated at an average sarcomere length of 3.0 μm and then shortened to 2.4 μm. The relative increase in mechanical work obtained during shortening between tests was defined as the SSC effect. In the RFE tests, fibers were activated at an average sarcomere length of 2.4 μm and then stretched to 3.0 μm, while the RFE control tests consisted of an isometric contraction at 3.0 μm. The difference in steady-state isometric force between tests was defined as RFE. The SSC effect was greater in soleus than in psoas, while the RFE was the same for both muscles. Since the SSC effect was greater in soleus, while the RFE was the same, the observed greater SSC effect is probably not directly caused by RFE, but may be related to differences in cross bridge kinetics.

Influence of stretch magnitude on the stretch-shortening cycle in skinned muscle fibres.

The mechanical work attained during muscle fibre shortening is increased by prior stretching. Recently, we suggested that residual force enhancement (RFE) may contribute to this enhanced work. RFE can be changed reliably by changing the stretch magnitude. Therefore, the purpose of this study was to examine the effect of stretch magnitude, and by association RFE, on the mechanics of the stretch-shortening cycle (SSC) in skinned skeletal muscle fibres. Three tests were performed using skinned rabbit soleus fibres (N=18). The first test was a pure shortening contraction in which fibres were activated and then shortened from an average sarcomere length of 3.3 μm to 3.0 μm. The second test was a SSC in which fibres were activated and stretched from 3.0 μm to 3.3 μm, and then shortened to 3.0 μm. The third test was a SSC in which fibres were activated and stretched from 2.4 μm to 3.3 μm, and then shortened to 3.0 μm. The mechanical work during shortening and the force maintained 15 s after the end of shortening were determined. The relative increase in mechanical work with respect to the pure shortening condition was greater for the large than for the small stretch condition (P<0.001). Similarly, the relative increase in force 15 s after the end of shortening was greater for the large than for the small stretch condition (P=0.043). We conclude that increasing the magnitude of stretch results in an increase in mechanical work and increased force at steady state following the stretch, probably because of the greater RFE.

Relationship between gait kinematics and walking energy expenditure during pregnancy in South African women

BackgroundVarious musculoskeletal changes occurring during pregnancy may lead to the change in gait and contribute to the increase in walking energy expenditure. Previous research indicates that changes in gait mechanics may lead to the increase in mechanical work required during walking. However, there is little information to indicate if changes in gait mechanics during pregnancy have impact on active or total energy expenditure. Therefore, the primary aim of this study was to investigate the relationship between changes in gait kinematics and walking energy expenditure in pregnant women.MethodsThirty-five women (mean age = 27.5 ± 6.1 years) volunteered for the study during various stages of pregnancy (1st trimester average = 12.1 ± 2.2 weeks; 2nd trimester = 22.3 ± 2.6 weeks; 3rd trimester = 31.4 ± 2.6 weeks). 3D motion analysis was used to assess changes in kinematic parameters during walking at self-selected pace. Resting metabolic rate, and walking energy expenditure expressed in terms of rate and cost of O2 were analysed with portable metabolic analyser.ResultsOnly medio-lateral deviation of centre of gravity (COGML) increased 13.6% between the 1st and 2nd, and 39.3% between 2nd and 3rd trimester (p ≤ 0.001). However, self-selected walking speed depicted strong significant positive linear relationship with net O2 rate (r = 0.70; p ≤ 0.001), and was strongly associated with the vertical excursion of the COG (r = 0.75, p ≤ 0.001).ConclusionsChanges in gait mechanics during pregnancy may lead to an increase in walking energy expenditure. However, the consequent increase in walking energy cost may not be sufficient to offset the natural energy sparing mechanism.

Influence of residual force enhancement and elongation of attached cross-bridges on stretch-shortening cycle in skinned muscle fibers.

Increased muscle force during stretch‐shortening cycles (SSCs) has been widely examined. However, the mechanisms causing increased muscle force in SSCs remain unknown. The purpose of this study was to determine the influence of residual force enhancement and elongation of attached cross‐bridges on the work enhancement in SSCs. For the Control condition, skinned rabbit soleus fibers were elongated passively from an average sarcomere length of 2.4 to 3.0 μm, activated and then actively shortened to 2.4 μm. For the Transition condition, fibers were elongated actively from an average sarcomere length of 2.4 to 3.0 μm. Two seconds after the end of active lengthening, fibers were actively shortened to 2.4 μm. In the SSC condition, fibers were lengthened actively from an average sarcomere length of 2.4 to 3.0 μm, and then immediately shortened actively to 2.4 μm. Increased muscle force in the SSCs was quantified by the increase in mechanical work during active shortening compared to the mechanical work measured during the purely active shortening contractions. Work enhancement was significantly greater in the SSC compared to the Transition conditions. This difference was associated with the pause given between the active lengthening and shortening phase in the Transition test, which likely resulted in a reduction of the average elongation of the attached cross‐bridges caused by active stretching. Since some work enhancement was still observed in the Transition condition, another factor, for example the stretch‐induced residual force enhancement, must also have contributed to the work enhancement in SSCs.

A cross-bridge based model of force depression: Can a single modification address both transient and steady-state behaviors?

Force depression (FD), the reduction of isometric force following active shortening, is a phenomenon of skeletal muscle that has received significant attention in biomechanical and physiological literature, yet the mechanisms underlying FD remain unknown. Recent experiments identified a slower rate of force redevelopment with increasing amounts of steady-state FD, suggesting that FD may be caused, at least in part, by a decrease in cross-bridge binding rate (Corr and Herzog, 2005; Koppes et al., 2014). Herein, we develop a cross-bridge based model of FD in which the binding rate function, f, decreases with the mechanical work performed during shortening. This modification incorporates a direct relationship between steady-state FD and muscle mechanical work (Corr and Herzog, 2005; Herzog et al., 2000; Kosterina et al., 2008), and is consistent with a proposed mechanism attributing FD to stress-induced inhibition of cross-bridge attachments (Herzog, 1998; Maréchal and Plaghki, 1979). Thus, for an increase in mechanical work, the model should predict a slower force redevelopment (decreased attachment rate) to a more depressed steady-state force (fewer attached cross-bridges), and a reduction in contractile element stiffness (Ford et al., 1981). We hypothesized that since this modification affects the cross-bridge kinetics, a corresponding model would be able to account for both transient and steady-state FD behaviors. Comparisons to prior experiments (Corr and Herzog, 2005; Herzog et al., 2000; Kosterina et al., 2008) show that both steady-state and transient aspects of FD, as well as the relationship of FD with respect to speed and amplitude of shortening, are well captured by this model. Thus, this relatively simple cross-bridge based model of FD lends support to a mechanism involving the inhibition of cross-bridge binding, and indicates that cross-bridge kinetics may play a critical role in FD.

A modeling study of mechanical energetic optimality in incline walking

To maintain steady level walking, collision loss is predominantly compensated for with push-off propulsion, and negligible additional work is performed during the single support phase. The observed energy balance during the double support phase is energetically optimal. However, unlike level walking, significant work proportional to the incline slope was observed during the single support phase, which raises the question of whether energetic optimality applies to incline walking. In this study, we examined the energetic optimality of incline walking using a simple work-energy relationship. Work performed by the leading and trailing leg over a gait cycle was estimated for various incline slopes, and the optimal push-off impulse that minimized the total work performed was calculated. The model prediction for least costly gait occurred when push-off propulsion provided all of the necessary work for raising or lowering the body center of mass (CoM) and collision compensation. When we assumed that the generation of optimal propulsion is gradually scaled to obey a feasible push-off constraint, which was estimated based on the allowable plantar flexor torque and the weight support of the trailing leg, the predicted slope-proportional increase in mechanical work done by the ground reaction force (GRF) during the single support phase was consistent with the empirical data. This result implies that the energetic optimality of incline walking can be described from a mechanical perspective and is subject to a feasible push-off propulsion constraint. However, the implication of the mechanical perspective of energetic optimality on the metabolic cost should be further examined and compared using empirical data.

Impact of multiple joint impairments on the energetics and mechanics of walking in patients with haemophilia

Few studies have assessed the changes produced by multiple joint impairments (MJI) of the lower limbs on gait in patients with haemophilia (PWH). In patients with MJI, quantifiable outcome measures are necessary if treatment benefits are to be compared. This study was aimed at observing the metabolic cost, mechanical work and efficiency of walking among PWH with MJI and to investigate the relationship between joint damage and any changes in mechanical and energetic variables. This study used three-dimensional gait analysis to investigate the kinematics, cost, mechanical work and efficiency of walking in 31 PWH with MJI, with the results being compared with speed-matched values from a database of healthy subjects. Regarding energetics, the mass-specific net cost of transport (C(net)) was significantly higher for PWH with MJI compared with control and directly related to a loss in dynamic joint range of motion. Surprisingly, however, there was no substantial increase in mechanical work, with PWH being able to adopt a walking strategy to improve energy recovery via the pendulum mechanism. This probable compensatory mechanism to economize energy likely counterbalances the supplementary work associated with an increased vertical excursion of centre of mass (CoM) and lower muscle efficiency of locomotion. Metabolic variables were probably the most representative variables of gait disability for these subjects with complex orthopaedic degenerative disorders.

Assessment of potential locomotor functionality in chronic stroke patients

The assessment of gait functionality in chronic stroke patients is fundamental in the design of the rehabilitation programme: the resources available must be evidenced in order to exploit them in the functional recovery. A potential increase of speed [1,2] appears to be a main feature to be identified, but it is relevant also to understand how speed increase is obtained: an increase of mechanical work is expected, but also timing of joints’ dynamics and emg patterns are potential contributors to speed gain. When considering the hemiplegic side in the rehabilitation context, an increase of mechanical work requires strengthening, while a modulation of timings requires a time-oriented activity: in this latter case, a biofeedback technique may be particularly fitting. In the present work a group of chronic stroke patients were required to walk at preferred and peak velocity, the gait analysis results evidenced the underlying factors and provided evidences for a rehabilitation program.

Force recovery after activated shortening in whole skeletal muscle: transient and steady-state aspects of force depression

The depression of isometric force after active shortening is a well-accepted characteristic of skeletal muscle, yet its mechanisms remain unknown. Although traditionally analyzed at steady state, transient phenomena caused, at least in part, by cross-bridge kinetics may provide novel insight into the mechanisms associated with force depression (FD). To identify the transient aspects of FD and its relation to shortening speed, shortening amplitude, and muscle mechanical work, in situ experiments were conducted in soleus muscle-tendon units of anesthetized cats. The period immediately after shortening, in which force recovers toward steady state, was fit by using an exponential recovery function (R2 > 0.99). Statistical analyses revealed that steady-state FD (FD(ss)) increased with shortening amplitude and mechanical work. This FD(ss) increase was always accompanied by a significant decrease in force recovery rate. Furthermore, a significant reduction in stiffness was observed after all activated shortenings, presumably because of a reduced proportion of attached cross bridges. These results were interpreted with respect to the two most prominent proposed mechanisms of force depression: sarcomere length nonuniformity theory (7, 32) and a stress-induced inhibition of cross-bridge binding in the newly formed actin-myosin overlap zone (14, 28). We hypothesized that the latter could describe both steady-state and transient aspects of FD using a single scalar variable, the mechanical work done during shortening. As either excursion (overlap) or force (stress) is increased, mechanical work increases, and cross-bridge attachment would become more inhibited, as supported by this study in which an increase in mechanical work resulted in a slower recovery to a more depressed steady-state force.

Insights into amputee running. A muscle work analysis.

Five young, active, unilateral below knee amputees wearing the SACH prosthetic foot, and six normal subjects participated in the study. Subjects ran at a controlled velocity of 2.8 m/s +/- 10% over a ground reaction force plate while being filmed with a video camera. Joint moments, power outputs and mechanical work characteristics were then calculated. During stance phase the amputee prosthetic limb exhibited a marked reduction in total work. There was a reduction in the mechanical work at the knee and the prosthetic foot/ankle with a compensatory increase in mechanical work by the hip musculature. The intact stance phase limb mechanical work characteristics were not significantly different from normal. The hip flexors were the only muscle group in the swing phase prosthetic limb with a significant increase in muscle work compared with normal subjects. The intact swing phase limb in contrast exhibited a marked increase in concentric muscle work by the hip flexors and eccentric muscle work by the knee flexors in early swing phase, and an increase in concentric hip extensor and eccentric knee flexor muscle work in late swing phase. The major compensatory patterns, therefore, that allow below knee amputees to run appear to be an increase in stance phase hip muscle work on the prosthetic limb and increased hip and knee muscle work on the intact limb during swing phase.

Acute volume loading studied in cat myocardium with 31P nuclear magnetic resonance.

To study the effects of acute volume loading on myocardial metabolic and mechanical function, seven cats were volume loaded via anastomosis of the abdominal aorta to the vena cava (AV shunt). Metabolic effects were evaluated with 31P nuclear magnetic resonance (NMR). Mechanical function was evaluated with heart rate X systolic blood pressure product (HR X SBP). Shunts were opened for 1-2 h during which time phosphocreatine (PCr), adenosine triphosphate (ATP), inorganic phosphate (Pi), and HR X SBP were monitored. High-energy phosphate energetics as determined by Pi/PCr and PCr/ATP ratios were correlated with HR X SBP. Opening of the AV shunts was associated with an increase (four cats) or a decrease (three cats) in HR X SBP. Pi/PCr ratios increased and PCr/ATP ratios decreased in cats with an increase in HR X SBP. In cats with a decrease in HR X SBP, Pi/PCr and PCr/ATP generally did not change significantly. In summary, acute volume loading could be associated with an increase or decrease in myocardial external work as evaluated by HR X SBP, accompanied by metabolic changes suggestive of appropriate induction of state 3 metabolism (active metabolic state: ADP + Pi----ATP) in those cats with increased mechanical work, and minimal change in bioenergetics in cats with no or minimal increase in mechanical work. These induced metabolic responses to myocardial mechanical loading can be evaluated with 31P NMR techniques and may provide insight into in vivo metabolic control mechanisms.

Relation between cytosolic free Ca2+ concentration and the control of pyruvate dehydrogenase in isolated cardiac myocytes

The proportion of pyruvate dehydrogenase existing in the active form (PDHA) in suspensions of unstimulated cardiac myocytes oxidizing glucose is approx. 30%. Depolarization of the cells with concentrations of K+ above physiological values leads to an increase in the content of PDHA. Overloading of the cells with Na+ by treatment with veratridine and ouabain gives the same result. Each of these interventions is shown in experiments with Quin 2-loaded myocytes to lead to an increase in cytosolic free Ca2+ concentration ([Ca2+]c). Treatment of the cells with Ruthenium Red, an inhibitor of Ca2+ transport into mitochondria, largely prevents an increase in PDHA in response to addition of KCl or of veratridine plus ouabain. Ruthenium Red does not attenuate the increase in [Ca2+]c that occurs under these conditions. By contrast, treatment of the cells with ryanodine, an inhibitor of sarcoplasmic-reticulum Ca2+ transport and therefore of contraction, does not diminish the response of PDHA content to agents which raise [Ca2+]c; nor does loading of the cells with the Ca2+-chelating agent Quin 2, which also prevents contraction, at appropriate concentrations. It is concluded that an increase in [Ca2+]c causes an increase in PDHA content of cardiac myocytes independently of an increase in mechanical work. In the normal physiological situation the activation of dehydrogenases by Ca2+ is thought to help to maintain the balance of energy supply and demand during periods of increased work-load, which are associated with an increased myoplasmic [Ca2+]c.