Electrically evoked eccentric and concentric torque-velocity relationships in human knee extensor muscles

The torque-velocity relationship, obtained during in situ conditions in humans, demonstrates a levelling-off of eccentric torque output at the isometric torque level, at least for knee extensor actions. In contrast, the in vitro force-velocity relationship for animal muscle preparations is characterized by a sharp rise in eccentric force from isometric maximum. A force-regulating 'protective' mechanism has been suggested during maximal voluntary high-tension eccentric muscle actions. To investigate this phenomenon, maximal voluntary and three different levels of submaximal, electrically induced torques were compared during isometric and low velocity (10, 20 and 30 degrees s-1) isokinetic eccentric and concentric knee extensor actions in 10 healthy, moderately trained subjects. Eccentric torque was higher than isometric during electrically evoked, but not during maximal voluntary muscle actions. In contrast, concentric torque was significantly lower than isometric for both maximal voluntary and submaximal, electrically evoked conditions. Comparisons of normalized torques (isometric value under each condition set to 100%) demonstrated that the maximal voluntary eccentric torque had to be increased by 20%, and the isometric by 10% in order for the maximal voluntary torque-velocity curve to coincide with the electrically stimulated submaximal ones. These results support the notion that a tension-regulating mechanism is present primarily during eccentric maximal voluntary knee extensor actions.     

Eccentric and concentric torque-velocity relationships during arm flexion and extension

Summary Forty men were tested with a computerized dynamometer for concentric and eccentric torques during arm flexion and extension at 0.52, 1.57, and 2.09 rad·s^–1. Based on the summed concentric and eccentric torque scores, subjects were placed into a high strength (HS) or low strength (LS) group. The eccentric and concentric segments of the torque-velocity curves (TVCs) were generated using peak torque and constant-angle torque (CAT) at 1.57 and 2.36 rad. Angle of peak torque was also recorded. Compared to LS, HS had significantly greater estimated lean body mass (+ 10.2 kg) and approximately 25% greater average torque output. Reliability of the peak torque scores on 2 days in 20 subjects wasr0.85. The difference between observed torques and the mathematically computed criterion torque scores averaged 1% for three validation loads that ranged from 11.4 to 90.4 kg. Statistical analysis revealed that torque output in LS plateaued at low concentric velocities and was also flattened with increasing eccentric velocities. Conversely, torque ouptput for HS increased with decreasing concentric velocities and increased with increasing eccentric velocities. The method of plotting the TVCs for peak or CAT did not influence the pattern of TVC. Eccentric flexion peak torque occurred at a significantly shorter muscle length (1.88 rad) than concentric torque (2.12 rad). This difference was also present for extension; it was 1.88 rad for eccentric and 2.03 rad for concentric torque. These findings are discussed in terms of study design, neural inhibition, activation history, muscle-tendon elasticity, muscle fiber types, muscle architecture, and methodological considerations. The present results illustrate the importance of strength level to explain individual differences in TVC.     

Muscle activation during maximal voluntary eccentric and concentric knee extension

Summary The aim of this investigation was to study the relationships among movement velocity, torque output and electromyographic (EMG) activity of the knee extensor muscles under eccentric and concentric loading. Fourteen male subjects performed maximal voluntary eccentric and concentric constant-velocity knee extensions at 45, 90, 180 and 360° · s^–1. Myoelectric signals were recorded, using surface electrodes, from the vastus medialis, vastus lateralis and rectus femoris muscles. For comparison, torque and full-wave rectified EMG signals were amplitude-averaged through the central half (30°–70°) of the range of motion. For each test velocity, eccentric torque was greater than concentric torque (range of mean differences: 20%–146%,P < 0.05). In contrast, EMG activity for all muscles was lower under eccentric loading than velocity-matched concentric loading (7%–31%,P < 0.05). Neither torque output nor EMG activity for the three muscles changed across eccentric test velocities (P > 0.05). While concentric torque increased with decreasing velocity, EMG activity for all muscles decreased with decreasing velocity (P < 0.05). These data suggest that under certain high-tension loading conditions (especially during eccentric muscle actions), the neural drive to the agonist muscles was reduced, despite maximal voluntary effort. This may protect the musculoskeletal system from an injury that could result if the muscle was to become fully activated under these conditions.     

Eccentric and concentric torque-velocity characteristics of the quadriceps femoris in man

Summary The primary purpose of this investigation was to study the eccentric and concentric torque-velocity characteristics of the quadriceps femoris in man using a recently developed combined isometric, concentric and eccentric controlled velocity dynamometer (the SPARK System). A secondary purpose was to compare the method error associated with maximal voluntary concentric and eccentric torque output over a range of testing velocities. 21 males (21–32 years) performed on two separate days maximal voluntary isometric, concentric and eccentric contractions of the quadriceps femoris at 4 isokinetic lever arm velocities of 0° · s^–1 (isometric), 30° · s^–1 120° · s^–1 and 270° · s^–1. Eccentric peak torque and angle-specific torques (measured every 10° from 30° to 70°) did not significantly change from 0° · s^–1 to 270° · s^–1 (p>0.05) (with the exception of angle-specific 40° torque, which significantly increased;p<0.05). The mean method error was significantly higher for the eccentric tests (10.6%±1.6%) than for the concentric tests (8.1%±1.7%) (p<0.05). The mean method error decreased slightly with increasing concentric velocity (p>0.05), and increased slightly with increasing eccentric velocity (p>0.05). A tension restricting neural mechanism, if active during maximal eccentric contractions, could possibly account for the large difference seen between the present eccentric torque-velocity results and the classic results obtained from isolated animal muscle.     

Electrically evoked eccentric and concentric torque–velocity relationships in human knee extensor muscles

The torque–velocity relationship, obtained during in situ conditions in humans, demonstrates a levelling‐off of eccentric torque output at the isometric torque level, at least for knee extensor actions. In contrast, the in vitro force–velocity relationship for animal muscle preparations is characterized by a sharp rise in eccentric force from isometric maximum. A force‐regulating ‘protective’ mechanism has been suggested during maximal voluntary high‐tension eccentric muscle actions. To investigate this phenomenon, maximal voluntary and three different levels of submaximal, electrically induced torques were compared during isometric and low velocity (10, 20 and 30° s^–1) isokinetic eccentric and concentric knee extensor actions in 10 healthy, moderately trained subjects. Eccentric torque was higher than isometric during electrically evoked, but not during maximal voluntary muscle actions. In contrast, concentric torque was significantly lower than isometric for both maximal voluntary and submaximal, electrically evoked conditions. Comparisons of normalized torques (isometric value under each condition set to 100%) demonstrated that the maximal voluntary eccentric torque had to be increased by 20%, and the isometric by 10% in order for the maximal voluntary torque–velocity curve to coincide with the electrically stimulated submaximal ones. These results support the notion that a tension‐regulating mechanism is present primarily during eccentric maximal voluntary knee extensor actions.     

Knee extension torque and intramuscular pressure of the vastus lateralis muscle during eccentric and concentric activities

The objectives of this study were to determine whether the occurrence of delayed onset muscle soreness (DOMS) for the vastus lateralis muscle was associated with elevated intramuscular pressure (IMP); and to assess, whether high eccentric forces occurred at an increased muscle length (as determined by joint angle). Therefore, peak knee extension torque, peak IMP of the vastus lateralis muscle, and the joint angle at which peak torque (JAPT) occurred were determined in eight male subjects during repetitive eccentric and concentric activities until fatigue occurred. Peak torque was significantly higher for eccentric compared to concentric activity (P < 0.01) and declined significantly for both activities (P < 0.01) throughout the protocols. When comparing the start (prior to fatigue) to the end (fatigue state), mean torque for eccentric activity declined from 191 to 147 (N · m) and for concentric activity declined from 166 to 104 (N · m). In contrast, peak IMP was not significantly different between the types of activity and did not change significantly with time. At the start and the end, the mean IMP remained constant for eccentric activity at 54 mmHg (7.2 kPa) but for concentric activity was 78 mmHg (10.4 kPa) and 96 mmHg (12.8 kPa), respectively. All the subjects, however, experienced DOMS of the vastus lateralis muscle exclusively for the eccentric activity leg. The JAPT was not different between activity types and did not change significantly with time; however, a significant interaction between activity type and time was observed (P = 0.01). For eccentric activity JAPT (0° = fully extended leg) was 81° (1.38 rad) and 79° (1.34 rad) and for concentric activity was 76° (1.29 rad) and 83° (1.41 rad) at the start and the end, respectively. From our studies we concluded that during eccentric activity the magnitude of IMP for the vastus lateralis muscle did not reflect the high muscles forces; therefore it would appear that IMP was not an etiologic indicator of DOMS, and that JAPT measurements did not offer an explanation for the high forces which were associated with eccentric activity.     

Effects of eccentric and concentric muscle actions in resistance training

The adaptive responses to two different resistance training regimens were compared. Healthy males performed five sets of either 12 maximum bilateral concentric (Grp CON; n = 11) or six pairs of maximum bilateral eccentric and concentric (Grp ECCON; n = 11) quadriceps muscle actions three times per week for 12 weeks. Uni- and bilateral eccentric and concentric peak torque at various angular velocities, vertical jump height and three-repetition maximum half-squat were measured before and after training. Muscle biopsies were obtained from m. vastus lateralis and analysed for fibre type composition and area using histochemical techniques. In contrast to a control group (n = 7), performing no training, Grps CON and ECCON demonstrated marked increases (P < 0.05) in overall eccentric (19 and 37% respectively) and concentric (15 and 26% respectively) peak torques. Grp ECCON, however, showed greater (P < 0.05) increases in peak torque, vertical jump height and three repetition maximum than Grp CON. The 7% increases in slow-twitch fibre area in Grps CON and ECCON and in fast-twitch fibre area in Grp CON were nonsignificant. This study suggests that increases in peak torque and strength-related performance parameters were greater following a programme consisting of maximum concentric and eccentric muscle actions than resistance training using concentric muscle actions only. Because increases in muscle fibre areas were small it is also suggested that the increased muscle strength shown subsequent to short-term accommodated resistance training is mainly due to neural adaptation.     

Human motor unit activity during concentric and eccentric movements

Single motor units (MUs) activity was investigated in human m. biceps brachii during movements against an elastic load. A total of sixty-five MUs were studied by means of subcutaneously placed fine-wire branched electrodes. Subjects were asked to perform active shortening and lengthening of the muscle with approximately constant velocities at two different speeds--slow and fast. Both recruitment (RT) and decruitment (DT) thresholds of MU were found to be lower in movement with higher velocity. The recruitment order of MUs was approximately one and the same during concentric movements with a different but constant velocity. The firing onset of MUs is organized so that the peak of the first twitch contraction occurs at approximately the same force level irrespective of how fast the movement is. In contrast, during the eccentric movements the peak of the last twitch contraction of MU occurs at different torque levels depending on the velocity. The decruitment of the MUs during eccentric movement was in a reverse order to their recruitment during concentric movements. Generally, at one and the same velocity the RT of a given MU was lower than DT. Nevertheless, the peaks of the first and the last twitch contractions during concentric and eccentric movements with one and the same velocity occurred at approximately one and the same torque level.     

Effects of blood flow restriction during moderate-intensity eccentric knee extensions

We investigated if blood flow restriction (BFR, cuff pressure 20 mmHG below individual occlusion pressure) increases metabolic stress, hormonal response, release of muscle damage markers, and muscle swelling induced by moderate-intensity eccentric contractions. In a randomized, matched-pair design, 20 male subjects (25.3 ± 3.3 years) performed four sets of unilateral eccentric knee extensions (75% 1RM) to volitional failure with (IG) or without (CG) femoral BFR. Despite significant differences of performed repetitions between IG (85.6 ± 15.4 repetitions) and CG (142.3 ± 44.1 repetitions), peak values of lactate (IG 7.0 ± 1.4 mmol l^?1, CG 6.9 ± 2.7 mmol l^?1), growth-hormone (IG 4.9 ± 4.8 ng ml^?1, CG 5.2 ± 3.5 ng ml^?1), insulin-like growth factor 1 (IG 172.1 ± 41.9 ng ml^?1, CG 178.7 ± 82.1 ng ml^?1), creatine-kinase (IG 625.5 ± 464.8 U l^?1, CG 510.7 ± 443.5 U l^?1), the absolute neutrophil count (IG 7.9 ± 1.3 10³ µl^?1, CG 8.7 ± 2.0 10³ µl^?1), induced muscle swelling of rectus femoris and vastus lateralis and perceived pain did not differ. The present data indicate that BFR is suitable to intensify eccentric exercises.     

Internal work and physiological responses during concentric and eccentric cycle ergometry

Summary Internal mechanical work during cycling, required to raise and lower the legs and change their velocities, is shown to be an important factor when interpreting physiological responses to cycle ergometer exercise. The internal work required to move the legs during concentric and eccentric cycle ergometry at different speeds and workloads was calculated from segmental energy changes determined using cinematography and directly using an eccentric ergometer. The mean internal work rates obtained at pedal frequencies of 30, 60 and 90 min⁻¹ were 11.5, 20 and 62 W respectively. When these estimates were added to the external work rates, they increased concentric and decreased eccentric work rates. The largest differences were seen at low work rates and high pedal frequencies during which concentric work rates increased by 51% and eccentric decreased 60% by the inclusion of internal work. When comparisons of concentric and eccentric cycling at equal uncorrected work rates were made, neglecting to include internal work introduced errors ranging from 12 to 97%. The calculated estimates of internal work agreed well with the power supplied by the eccentric ergometer to move the legs passively. The investigations show that the inclusion of internal work is important when comparing physiological responses during concentric and eccentric ergometry, especially when pedal frequences exceed 60 min⁻¹ and when work rates are small.     

Skeletal muscle glucolysis, glycogenolysis and glycogen phosphorylase during electrical stimulation in man

Phosphorylase activity, glycogenolytic and glucolytic rates were estimated in human quadriceps muscle during electrical stimulation at 20 Hz. Two stimulation periods of 10 s duration were separated by a pause of 60 s. The blood circulation to the leg was intact or occluded during the experiment. ATP turnover rates and force production were of the same order during the two contraction periods both with and without intact blood flow. Also the increase in phosphorylase a activity (from approximately 30% to approximately 65%) was the same during the contraction periods. Glycogenolytic and glucolytic rates were however about 30% higher (P less than 0.05) during the second contraction compared with the first when circulation was occluded, but similar when the circulation was intact. During the 60 s rest period, the phosphocreatine (PCr) was maintained at a low level and inorganic phosphate (Pi) remained increased under occluded circulation while PCr was resynthesized in the rest period with intact circulation. We conclude that the increased glycogenolytic rate observed during the second contraction with occluded blood circulation was due to the high [Pi] in the muscle and that the increased glucolytic rate was caused by high [Pi] and low [PCr]. In the rest period with anoxia the glycogenolysis was completely inhibited and glucolysis was inhibited by 95% in spite of the changes in [PCr] and [Pi].     

Age-related fatigability of the ankle dorsiflexor muscles during concentric and eccentric contractions

This study compares the fatigability of the ankle dorsiflexors during five sets of 30 maximal concentric and eccentric contractions in young and elderly adults. The torque produced by the ankle dorsiflexors and the average surface electromyogram (aEMG) of the tibialis anterior were continuously recorded. The contribution of central and peripheral mechanisms to muscle fatigue was tested before, after each set of contractions, and during a 30 min recovery period by the superimposed electrical stimulation method. The compound muscle action potential (M-wave), the mechanical response to single (twitch) and paired (doublet) stimulation, and the postactivation potentiation were also recorded. Compared with young subjects, elderly adults exhibited a greater loss of torque for concentric (50.2 vs. 40.9%; P<0.05) and eccentric (42.1 vs. 27.1%; P < 0.01) contractions. Although young subjects showed a lesser decrease in torque during the eccentric compared with concentric contractions, elderly adults experienced similar fatigability for the two types of contractions despite a comparable depression in the EMG activity of both groups and contraction types (10–20%). As tested by the interpolated-twitch method and aEMG/M-wave ratio, voluntary activation was not altered during either type of contraction or for either age group. During the two fatigue tasks, only elderly adults experienced a decrease in M-wave area (26.4–35.4%; P < 0.05). All together, our results suggest that the fatigue exhibited by both young and elderly adults during maximal concentric and eccentric contractions mainly involved peripheral alterations and that elderly adults may also have experienced a decline in neuromuscular propagation.     

Viscosity of the elbow flexor muscles during maximal eccentric and concentric actions

The aim of the present study was to estimate the damping coefficient (B factor) of the elbow flexor muscles during both eccentric and concentric muscle actions. We used a muscle model consisting of a viscous damper associated in parallel with a contractile component, both in series with an elastic component. The viscous damper allowed the concentric loss and the eccentric gain of force to be modelled. Eight volunteer subjects performed maximal eccentric and concentric elbow movements on an isokinetic dynamometer at angular velocities of 0.52, 1.04 and 2.09 rad·s^–1. Torques at an elbow joint angle of 90° were recorded. Electromyogram (EMG) signals from the belly of the right elbow flexor and from the long head of the triceps brachia muscles were recorded using two pairs of bipolar surface electrodes. The root mean square (rms) of the EMG was determined. Eccentric and concentric rms were not significantly different (P>0.05). The B factor was higher in the concentric than in the eccentric conditions (P<0.05), and, whatever the muscle action type it decreased as the velocity increased. These results indicated that the concentric loss and the eccentric gain of force were attributable to the behaviour of the contractile machinery. Furthermore, whatever the exact cause of loss and gain of tension, our study showed that the total effect can be modelled by the viscous damper of a three-component muscle model.     

Effects of detraining following short term resistance training on eccentric and concentric muscle strength

Healthy males were examined before and after 12 weeks of accommodated resistance training (three week^-1) and after 12 weeks of detraining. Training consisted of four to five sets of six coupled maximum voluntary bilateral concentric and eccentric (Grp ECCON; n= 10) or 12 concentric (Grp CON; n= 8) quadriceps muscle actions. Concentric and eccentric peak torque at various constant angular velocities and three repetition maximum half-squat and vertical jump height were measured. Grp ECCON showed greater (P < 0.05) overall increase in peak torque after training and detraining than Grp CON. Thus, concentric peak torque (0.52 rad s^-1) increased more (P < 0.05) over the experimental period in Grp ECCON and increases in eccentric peak torque were preserved in Grp ECCON only. Increases in peak torque in response to training were greater (P < 0.05) at 0.52 than at 2.62 rad s^-1. Alterations in the torque-velocity patterns induced by training remained after detraining in Grp ECCON but not in Grp CON. The retained increases (P < 0.05) in half-squat were 12 and 18% in Grps CON and ECCON, respectively. Neither group showed increased vertical jump height after detraining. This study showed greater preservation of concentric and eccentric peak torque after detraining following coupled concentric and eccentric than concentric resistance training. Only the former regime induced a change in the shape of torque-velocity curves that was manifest after detraining. These results suggest that the performance of eccentric muscle actions is critical to optimize increases in muscular strength in response to heavy resistance training, because it probably induce greater and more long-lived neural adaptations than the performance of concentric actions.     

Effects of waveform parameters on comfort during transcutaneous neuromuscular electrical stimulation

Twenty-three females between the ages of 19 and 35 were studied in order to compare the effects of variations in pulse duration, waveform symmetry, and source regulation on comfort during quadriceps surface stimulation at amplitudes necessary to produce 27 Nm torque. Stimulation parameters compared were: 1) 50 and 300 μs pulse durations, 2) asymmetrical and symmetrical biphasic waveforms, and 3) current and voltage source regulation. Subjects overwhelmingly preferred the 300 μs pulse duration regardless of waveform or source regulation, strongly preferred the symmetrical biphasic waveform, and had inconsistent preference for either regulated voltage or regulated current sources.     

Modeling energy expenditure associated with isometric, concentric, and eccentric muscle action at the knee

This study was designed to derive and test a model of energy expenditure (dE/dt) during different modes of human muscle actionin vivo dE/dt for the quadriceps muscle was expressed as: dE_quad/dt=K1(ω)+K2(Tiso)+dW/dt, where K1 and K2 are constants, (ω) is joint angular velocity (^o/sec), (Tiso) represents the knee extension torque that could be developed by the active muscle if the muscle action were isometric (N-m), and (dW/dt) is the rate of work performed (W). Volunteers performed a series of repetitive 2- to 4-min knee extension exercises, while varying either the knee extension torque or velocity. The average joint torque, angular velocity, rate of work performed, and net energy expenditure (E) above resting and dE/dt were determined for each muscle action. The best fit values for K1 and K2 for concentric, eccentric, and isometric muscle actions were 0.044, −0.55, and 0 W/^c/sec, and 4.14, 5.28, and 2.17 W/N-m, respectively. The coefficients of determination (r ²) for the model predictions of dE_quad/dt for the three modes of muscle action were 0.78, 0.71, and 0.71, respectively. The correlation coefficient between predicted and experimental dE/dt for all modes of muscle action combined was 0.93. These findings indicate the model provides a useful tool for predicting the rate of energy expenditure associated with cyclic knee extension efforts.     

Muscle performance during isokinetic concentric and eccentric abduction in subjects with subacromial impingement syndrome

Peak torque (PT), total work (TW) and acceleration time (AT) were measured during isokinetic concentric and eccentric abduction of the shoulder in subjects with subacromial impingement syndrome (SIS) and healthy subjects. The SIS group consisted of 27 subjects (33.48 ± 9.94 years) with unilateral SIS and it was divided into two groups: (1) SIS with the dominant involved side, (2) SIS with the nondominant involved side. The control group consisted of 23 healthy subjects (32.26 ± 9.04 years). PT, TW and AT were measured bilaterally at 60 and 180°/s. No significant interactions were found between group and side (P > 0.05), as well as no significant main effects of group and side (P > 0.05) for all variables during concentric abduction of the shoulder at both tested speeds. During the eccentric contractions, lower TW (P < 0.05) was demonstrated by the nondominant uninvolved side of SIS group at 60°/s, and by the uninvolved sides of the SIS groups at 180°/s compared to the dominant side of the controls. At 180°/s, the nondominant uninvolved side of the SIS group demonstrated slower AT (P < 0.05) compared to the dominant side of the control group, as well as the nondominant involved side of the SIS group compared to the nondominant side of the controls. The results showed no alterations in the measured parameters during concentric and eccentric phases of isokinetic abduction in subjects with SIS when compared to a control group. However, alterations may be observed in the contralateral side during the eccentric phase of abduction in subjects with unilateral conditions of SIS.     

Effects of detraining following short term resistance training on eccentric and concentric muscle strength.

Healthy males were examined before and after 12 weeks of accommodated resistance training (three week-1) and after 12 weeks of detraining. Training consisted of four to five sets of six coupled maximum voluntary bilateral concentric and eccentric (Grp ECCON; n = 10) or 12 concentric (Grp CON; n = 8) quadriceps muscle actions. Concentric and eccentric peak torque at various constant angular velocities and three repetition maximum half-squat and vertical jump height were measured. Grp ECCON showed greater (P less than 0.05) overall increase in peak torque after training and detraining than Grp CON. Thus, concentric peak torque (0.52 rad s-1) increased more (P less than 0.05) over the experimental period in Grp ECCON and increases in eccentric peak torque were preserved in Grp ECCON only. Increases in peak torque in response to training were greater (P less than 0.05) at 0.52 than at 2.62 rad s-1. Alterations in the torque-velocity patterns induced by training remained after detraining in Grp ECCON but not in Grp CON. The retained increases (P less than 0.05) in half-squat were 12 and 18% in Grps CON and ECCON, respectively. Neither group showed increased vertical jump height after detraining. This study showed greater preservation of concentric and eccentric peak torque after detraining following coupled concentric and eccentric than concentric resistance training. Only the former regime induced a change in the shape of torque-velocity curves that was manifest after detraining.(ABSTRACT TRUNCATED AT 250 WORDS)     

Descending volley after electrical and magnetic transcranial stimulation in man

The descending volley evoked by electrical and magnetic transcranial stimulation was recorded with spinal electrodes in 3 subjects undergoing spinal surgery. The descending volley evoked by electrical stimulation, as previously described, was composed by a short-latency initial wave followed by later waves. In two subjects magnetic stimulation evoked an initial wave of slightly longer latency (0.2-0.3 ms), smaller amplitude and higher threshold than the initial wave evoked by electrical stimulation. In these two subjects, magnetic stimuli probably activated the pyramidal axons directly. In the third subject the initial wave evoked by magnetic stimulation had a latency of 1.4 ms longer and a considerably smaller amplitude than that evoked by electrical stimulation. In this case magnetic stimulation may activate the pyramidal axons indirectly.