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PubMed2007

Assessment of the reliability of central and peripheral fatigue after sustained maximal voluntary contraction of the quadriceps muscle.

The aim of the present study was to further confirm the validity of measurements for characterizing neuromuscular alterations by establishing their reliability both before and after fatigue. Thirteen men (28 +/- 5 years) volunteered to participate in two separate identical sessions requiring the performance of a sustained maximal voluntary contraction (MVC) with the quadriceps muscle for 2 min. MVC and transcutaneous electrical stimulations were used before and immediately after the fatiguing contraction to investigate the reliability of MVC torque, central activation, and peripheral variables (M-wave properties, peak twitch, peak doublet) within and between sessions. Based on previous and present results, we advise the use of (1) voluntary activation level with potentiated doublet as a reference to describe central fatigue, (2) electromyographic activity of vastus lateralis muscle as a surrogate for quadriceps for both voluntary and evoked contraction, and (3) potentiated peak doublet amplitude to investigate contractile fatigue. These findings can be useful in the choice of the parameters describing central and peripheral fatigue of the quadriceps muscle in future studies.

Action Potentials↗

PubMed1993

Effect of knee angle and ligament insufficiency on anterior tibial translation during quadriceps muscle contraction: a preliminary report.

Additional information is needed regarding the effects of exercise protocols on the injured or reconstructed anterior cruciate ligament (ACL). The purpose of this investigation was to assess the effects of knee flexion angle and ACL insufficiency on anterior tibial translation (ATT) and patellar ligament insertion angle as subjects performed maximal isometric quadriceps muscle contractions. The subjects were two females and two males between the ages of 18 and 24 who had sustained injuries that resulted in unilateral ACL insufficiency. Each subject performed maximum isometric quadriceps muscle contractions with each leg on a Cybex II dynamometer at each of three positions: 15, 45, and 75 degrees knee flexion. A lateral knee roentgenogram was obtained as each subject maintained each isometric muscle contraction. A roentgenogram also was taken as subjects rested each knee in each of the three target positions. Anterior tibial translation for each isometric muscle contraction was assessed by measuring the anterior displacement of the tibial plateau on the isometric resisted roentgenogram relative to the resting roentgenogram. Patellar ligament insertion angle also was measured for each roentgenogram. Maximum ATT occurred at the 15 degrees knee flexion target angle for two subjects and at the 45 degrees target angle for the other two subjects. Patellar ligament insertion angle decreased as knee flexion angle increased. Appreciable stress may be imposed on the ACL as patients perform maximum quadriceps muscle contractions in positions of terminal knee extension and in midrange positions previously reported as being safe for maximal effort quadriceps exercise. Magnitude of stress imposed on the ACL is discussed as a function of the length-tension relationship of the quadriceps muscle-tendon unit and insertion angle of the patellar ligament. Suggestions are made for additional research regarding appropriate muscle strengthening protocols for patients who have undergone ACL reconstruction.

Adolescent↗

PubMed1999

Dynamic phosphorus-31 magnetic resonance spectroscopy of the quadriceps muscle: effects of age and sex on spectroscopic results.

RATIONALE AND OBJECTIVES: Phosphorus-31 (31p) magnetic resonance spectroscopy (MRS) is used to assess the influence of sex and age on quadriceps muscle metabolism before and after exercise. METHODS: Fifty-four healthy volunteers and 56 patients with an arterial occlusive disease were examined by dynamic 31p MRS. In the magnet, the quadriceps muscle was stressed by an isometric and an isotonic form of exercise until exhaustion. RESULTS: Older subjects showed a significantly larger ratio of inorganic phosphate (P(i)) to phosphocreatine (PCr) than younger subjects (r = 0.52, P = 8 x 10(-9)). With subjects' increasing age, the ratio of adenosine triphosphate (beta-ATP) to total phosphate decreased (r = -0.36, P = 5 x 10(-5)). The ratio of phosphomonoester to beta-ATP and phosphodiester (PDE) to beta-ATP showed a strong age dependence (r = 0.71 and 0.69, P = 3 x 10(-17) and 4 x 10(-15), respectively). The pH was the only one of the evaluated spectroscopic parameters that showed a sex dependence. Female subjects had a significantly lower pH (7.03+/-0.02) than male subjects (7.05+/-0.03) (P = 6 x 10(-4)). With increasing age, the maxima of P(i) to PCr were less extreme during both of the exercises (r = -0.51, P = 3 x 10(-16)). Likewise, the exercise-induced acidosis was less severe with increasing age (r = -0.51, P = 7 x 10(-16)). After the exercises ended, the times of half recovery of P(i) to PCr and the pH neither correlated with the subjects' age nor with sex or the cross-sectional area of the quadriceps muscle. CONCLUSIONS: The sex and age of volunteers or patients may affect spectroscopic results in a significant way. This influence has to be considered in the interpretation of spectroscopic studies. According to the recovery rates of P(i) to PCr and the pH, an age-related deterioration of muscular metabolism seems to be avoidable by appropriate physical activity.

Adenosine Triphosphate↗

PubMed2000

Altered contractile properties of the quadriceps muscle in people with spinal cord injury following functional electrical stimulated cycle training.

STUDY DESIGN: A longitudinal training study. OBJECTIVES: To assess if contractile speed and fatigability of paralysed quadriceps muscles in individuals with spinal cord injury (SCI) can be altered by functional electrical stimulation leg cycle ergometry (FES-LCE) training. SETTINGS: The Sint Maartenskliniek rehabilitation centre and the University of Nijmegen, Nijmegen, the Netherlands. METHODS: Contractile properties of the quadriceps muscle were studied in seven people with motor-complete SCI who participated in a FES-LCE training program. Subjects trained for 30 min, three times per week for 6 weeks. Contractile speed and fatigue characteristics of electrically stimulated isometric contractions were compared before and after 6 weeks of FES-LCE. RESULTS: Fatigue resistance improved following FES-LCE training as indicated by the higher forces maintained in response to repetitive electrical stimulation. In contrast with an improved fatigue resistance, the maximal rate of force rise was unaffected, the speed of relaxation increased and the fusion of a 10 Hz force signal decreased. Furthermore, the force-frequency relationship shifted to the right at low stimulation frequencies, indicated by a decline in the ratio of 1 and 100 Hz force responses as well as the ratio of 10 and 100 Hz force responses. CONCLUSION: FES-LCE training can change the physiological properties of the quadriceps muscle in people with SCI. Even after a short period of training, the stimulated muscles become more resistant to fatigue. Furthermore, the increased speed of relaxation and associated decreased fusion and altered force-frequency relationship following training may be related to adaptations in the calcium handling processes, which reflect an early response of long-term disused muscles.

Adult↗

PubMed1990

[Isokinetic measurements of the stretch strength of the femoral quadriceps muscle in surfers in comparison to healthy, untrained persons].

The strength of the quadriceps muscle of twelve male surfers who practice their sport for more than 10 years was compared with results of 15 male untrained persons. The concentric peak torque of knee extensors was measured every 10 degrees in a range from 90 to 20 degrees isometrically and dynamically with a constant angle velocity of 10 degrees/sec. The difference in torque between the left and the right leg is not significant, but there is a slight dominance in favour of the standing leg. The body weight related strength of the surfers was 3.2 Nm/kg indicating a higher strength characteristic compared to 2.7 Nm/kg of untrained subjects. Moreover the strength values of our group of athletes were above the value of 3 Nm/kg, which is referred to in the literature as representing a good training status. In view of these results it can be concluded that a long-lasting specific training like surfing will change the strength characteristics of the quadriceps muscles, although none of the surfers had undertaken an extensive strength training for the lower extremity.

Adult↗

PubMedDate not supplied

Quadriceps muscles activation in anterior knee pain during isokinetic exercise.

The aim of this paper is to study the femoral quadriceps muscles activation in patients suffering from anterior knee pain (AKP) during an isokinetic exercise. AKP is a common pathological condition, associated to an abnormal motion of the patella. It possibly depends on a muscular or structural imbalance. A lack of synergy in the quadriceps muscles results in a dynamic misalignment of the patella, which in turn produces pain. A quantitative analysis of the muscle activation strategy is important for an objective measurement of the knee functionality in that it helps to diagnose and monitor the rehabilitative treatment. Surface electromyography (EMG) from the three superficial muscles of the femoral quadriceps during a concentric isokinetic exercise has been analysed. A group of AKP patients has been compared with a control group of healthy subjects. Ensemble average of the EMG linear envelopes reveals significant modifications in Vastus Medialis activity in AKP patients. In order to quantify the synergy of the muscles, different parameters have been associated to EMG linear envelopes significant features. A decomposition in gaussian pulses and an asymmetry coefficient have been utilised. The results relative to these concise parameters highlight an appreciable delay and a modification in the activation of the Vastus Medialis in AKP patients.

Adolescent↗

PubMed1999

Atrophy of the quadriceps muscle in children with a painful hip.

The objective of this study was to determine the degree of muscle wasting of various components of the quadriceps muscle in children with a painful hip. Between January 1994 and September 1997, 327 consecutive children with a unilateral painful hip and/or limping were evaluated prospectively with ultrasonography. Quadriceps thickness was measured on both sides. Moreover, muscle thickness was measured in 59 control subjects. The patients were divided into eight groups; transient synovitis (n = 134), Perthes' disease (n = 35), slipped capital femoral epiphysis (n = 5), osteomyelitis (n = 4), aspecific synovitis (n = 5), rheumatoid arthritis (n = 3) and miscellaneous (n = 16). In 125 patients, no sonographic and radiological abnormalities were found and during follow-up the symptoms disappeared ('no pathology' group). Ipsilateral muscle wasting was present in all patient groups, whereas the control subjects showed no significant difference in muscle thickness between legs. The degree of muscle wasting was compared between transient synovitis, the 'no pathology' group, Perthes' disease and control subjects. For both quadriceps and vastus intermedius muscles, there was a significant difference between these groups, except between control subjects and the 'no pathology' group. For the rectus femoris muscle, there was a significant difference between these groups, except between transient synovitis and 'no pathology'. Muscle wasting showed a positive correlation with duration of symptoms and pre-existing muscle mass. In conclusion, different diseases show different degrees of muscle wasting, and there are different patterns of muscle wasting of various components of the quadriceps femoris muscle.

Adolescent↗

PubMed2006

Effective therapy of transected quadriceps muscle in rat: Gastric pentadecapeptide BPC 157.

We report complete transection of major muscle and the systemic peptide treatment that induces healing of quadriceps muscle promptly and then maintains the healing with functional restoration. Initially, stable gastric pentadecapeptide BPC 157 (GEPPPGKPADDAGLV, M.W. 1419, PL-10, PLD-116, PL 14736 Pliva, Croatia; in trials for inflammatory bowel disease; wound treatment; no toxicity reported; effective alone without carrier) also superiorly accelerates the healing of transected Achilles tendon. Regularly, quadriceps muscle completely transected transversely 1.0 cm proximal to patella presents a definitive defect that cannot be compensated in rat. BPC 157 (10 microg, 10 ng, 10 pg/kg) is given intraperitoneally, once daily; the first application 30 min posttransection, the final 24 h before sacrifice. It consistently improves muscle healing throughout the whole 72-day period. Improved are: (i) biomechanic (load of failure increased); (ii) function (walking recovery and extensor postural thrust/motor function index returned toward normal healthy values); (iii) microscopy/immunochemistry [i.e., mostly muscle fibers connect muscle segments; absent gap; significant desmin positivity for ongoing regeneration of muscle; larger myofibril diameters on both sides, distal and proximal (normal healthy rat-values reached)]; (iv) macroscopic presentation (stumps connected; subsequently, atrophy markedly attenuated; finally, presentation close to normal noninjured muscle, no postsurgery leg contracture). Thus, posttransection healing-consistently improved-may suggest this peptide therapeutic application in muscle disorders.

Amino Acid Sequence↗

PubMed1984

Development of human fetal muscles: a comparative histochemical analysis of the psoas and the quadriceps muscles.

The development of human muscle can be divided into three distinct phases on the basis of the enzyme histochemistry (Dubowitz 1965). On the other hand, muscles can be classified into trunk muscles innervated by the motoneurons in the anteromedial nucleus of the anterior horn and limb muscles innervated by those of the lateral nucleus. We compared histochemically the development of the psoas muscle as a trunk muscle with the quadriceps. Four legally aborted fetuses and sixteen autopsied preterm or full-term infants with conceptional ages ranging from sixteen to fourty-two weeks, served as subjects. Serial crossly cut frozen sections of the psoas and the quadriceps were stained with HE, modified Gomori-trichrome, NADH-TR, myofibrillar ATPase (routine, preincubation at pH 4.6, 4.3), and analyzed quantitatively. In the psoas and the quadriceps, all muscles consisted of undifferentiated type IIC fibers before the eighteenth week of gestation. Type I fiber began to appear around the eighteenth week in both muscles. Before the thirtieth week, both muscles comprised a great number of smaller type IIC fibers and scattered large type I fibers. The psoas had already a little more type I fibers than the quadriceps in this phase. After the thirtieth week, the percentage of type I fibers increased greatly to about 50% at term in the psoas, gradually to 20-23% at term in the quadriceps. Type IIA and IIB fibers appeared suddenly around the thirty-seventh week. Total percentage of these two type fibers increased at term to about 50% and 70% in the psoas and the quadriceps, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)

Adenosine Triphosphatases↗

PubMed1988

Time course and magnitude of blood flow changes in the human quadriceps muscles during and following rhythmic exercise.

1. Pulsed bidirectional Doppler-ultrasound equipment was used to measure changes in blood velocities in the femoral artery on a beat to beat basis for consecutive contraction and relaxation phases during voluntary rhythmic exercise of the quadriceps muscle group in man. 2. Rapid and large fluctuations of blood velocities were found, being high during relaxation and low during contraction phases. At the onset of contraction phase, negative velocities were present, indicating retrograde flow. During the rest of the contraction phase, forward flow occurred comparable to the resting flow level even at high loads. 3. Estimated maximal flow to the whole leg during relaxation phase, calculated from these blood velocity measurements and vessel diameter (measured with echo-ultrasound equipment with high resolution) was in two of the subjects 3.32 l min-1 (female) and 5.97 l min-1 (male). When using computer tomography to estimate the volume of the quadriceps muscle group, the calculated maximum flow to this muscle group was 243 (female) and 257 (male) ml min-1 100 ml muscle-1. The time-averaged flow during exercise to the whole leg was 1.51 l min-1 (female) and 2.47 l min-1 (male). The calculated time-averaged flow to the quadriceps muscle group was 101 (female) and 98 (male) ml min-1 100 ml muscle-1. 4. The duration of post-contraction hyperaemia following such rhythmic exercise of up to 6 min duration and up to 75% maximum voluntary contraction was never in excess of 150 s.

Adult↗

PubMed2007

Histopathologic study of minced and injected expanded polytetrafluoroethylene (Gore-Tex) in quadriceps muscle of rats for injection laryngoplasty.

OBJECTIVE: Gore-Tex has not yet been used as an injection material, and no animal study has been performed. The purpose of this study was to investigate the histopathological and volumetric changes of minced and injected expanded polytetrafluoroethylene (Gore-Tex) in the quadriceps muscle of rats to find out its possibility as an injection material in future injection laryngoplasty. DESIGN: A prospective study was performed in 13 Wistar rats. METHODS: The 50+/-5 mg Gore-Tex vascular graft was minced until it was fine enough to pass through a 16G injection cannula. To create viscosity in order to avoid precipitation, minced Gore-Tex was mixed with sodium hyaluronate (SH) gel. 0.5 ml of this mixture was injected into the middle part of the right quadriceps muscle of each subject. Then, pure SH (0.5 ml) was injected into the middle part of the left quadriceps muscle of each subject. To compare the volumetric and histopathological changes of the injection area occupied by the materials over the time, two rats were sacrificed 1 day after the procedure, four rats after 1 month, four rats after 3 months and three rats after 6 months. RESULTS: In pure SH: SH was observed as an effusion in the muscle of one subject on the first day only. We did not detect SH in the muscle in the first, third and sixth months and, as a result, there was no augmentation. Inflammation and fibrosis was not detected in any of these applications of SH. In minced Gore-Tex+SH gel mixture: SH was again eliminated from the muscle in a very short period of time, but the remaining Gore-Tex created a 15.8% volumetric gain. Fibrosis and capsule formation began in the first month and, in the sixth month; we observed connective tissue surrounding the Gore-Tex material. Ingrowth of connective tissue into the Gore-Tex, though not present in the first month, occurred partially in the third and significantly in the sixth month. In addition, we observed foreign body granuloma formation after the third month. CONCLUSIONS: Pure SH, though easily applicable, was immediately eliminated leaving no trace. In the relatively difficult applications of Gore-Tex mixed with SH, however, there was residual Gore-Tex present in all subjects throughout the study. Gore-Tex may be an applicable material for injection laryngoplasty.

Animals↗

PubMed2003

Functional changes of human quadriceps muscle injured by eccentric exercise.

The present study evaluated functional changes of quadriceps muscle after injury induced by eccentric exercise. Maximal isometric torque of quadriceps and the surface electromyography (root mean square, RMS, and median frequency, MDF) of the vastus medialis oblique (VMO) and vastus lateralis (VL) muscles were examined before, immediately after and during the first 7 days after injury. Serum creatine kinase (CK) levels and magnetic resonance imaging (MRI) were used to identify muscle injury. The subject was used as her own control and percent refers to pre-injury data. Experiments were carried out with a sedentary 23-year-old female. Injury was induced by 4 bouts of 15 maximal isokinetic eccentric contractions (angular velocity of 5 /s; range of motion from 40 to 110 of knee flexion). The isometric torque of the quadriceps (knee at 90 flexion) decreased 52% immediately after eccentric exercise and recovered on the 5th day. The highest reduction of RMS occurred on the 2nd day after injury in both VL (63%) and VMO (66%) and only VL recovered to the pre-injury level on the 7th day. Immediately after injury, the MDF decreased by 5 and 3% (VMO and VL, respectively) and recovered one day later. Serum CK levels increased by 109% on the 2nd day and were still increased by 32% on the 7th day. MRI showed large areas of injury especially in the deep region of quadriceps. In conclusion, eccentric exercise decreased the isometric torque and electromyographic signals of quadriceps muscle, which were recovered in one week, despite the muscle regeneration signals.

Adult↗

PubMed1998

Isokinetic aerobic power output testing of the quadriceps muscle.

A stepwise increasing exercise protocol to measure the maximal aerobic power output of the quadriceps muscle is proposed for use in rehabilitation medicine. Sixteen healthy volunteers performed isokinetic knee extension/flexion exercises at an angular velocity of 180 degrees x s(-1). The protocol consisted of stages of 200 knee movements from 100 flexion to full extension, starting at 10% peak torque (PT) and increasing by 10% PT each following stage until exhaustion. Quadriceps work, oxygen uptake, heart rate, plasma lactate concentration and surface electromyography were monitored. Quadriceps power output (Pext) was highly related (r = 0.95) to the extension torque at which the subjects were instructed to exercise. The test-retest (r = 0.82) and left to right (r = 0.94) correlations of maximum quadriceps power output (Pmax) were high. Both sexes (males 43+/-9W, females 36+/-8W) achieved their Pmax at 47% PT. At submaximal power output stages oxygen uptake (r = 0.85) and EMG-amplitude (r = 0.88) were linearly related to Pext. Mechanical efficiency (optimum at 60% Pmax) showed a large variation between power output stages and between subjects. The relationship between relative oxygen uptake and heart rate or plasma lactate concentration was similar to that of whole body exercise, e.g. running. We conclude that the proposed test is reliable for assessing the maximal aerobic power output of the quadriceps muscle. This parameter of the muscle function may add useful information for assessing the rehabilitation process after knee immobilization.

Adult↗

PubMed1985

The effect of intensive interval training on the anaerobic power of the rat quadriceps muscle.

The effect of intensive interval training on the maximal anaerobic power of the rat quadriceps muscle was investigated. The anaerobic energy production was estimated from the changes in the concentrations of phosphocreatine, adenine nucleotides, inosine monophosphate and lactate in freeze-clamped muscle tissue after electrical stimulation for 2-30 s. The results showed that the maximal running speed of rats tested increased by 24%, the maximal force exerted increased by 13%, and the succinate dehydrogenase activity by 48%, while the adenylate kinase activity was the same before and after training. No difference could be observed between the maximal anaerobic power of the quadriceps muscles of trained and sedentary animals. It seems that trained muscles may be able to work with a higher degree of economy than untrained muscles.

Anaerobiosis↗

PubMed2001

Reproducibility of contractile properties of the human paralysed and non-paralysed quadriceps muscle.

This study assessed the reproducibility of electrically evoked, isometric quadriceps contractile properties in eight people with spinal cord injury (SCI) and eight able-bodied (AB) individuals. Over all, the pooled coefficients of variation (CVps) in the SCI group were significantly lower (ranging from 0.03 to 0.15) than in the AB group (ranging from 0.08 to 0.21) (P<0.05). Furthermore, in all subjects, the variability of force production increased as stimulation frequency decreased (P<0.01). In subjects with SCI, variables of contractile speed are clearly less reproducible than tetanic tension or resistance to fatigue. Contractile properties of quadriceps muscles of SCI subjects were significantly different from that of AB subjects. Muscles of people with SCI were less fatigue resistant (P<0.05) and produced force-frequency relationships that were shifted to the left, compared with AB controls (P<.01). In addition, fusion of force responses resulting from 10 Hz stimulation was reduced (P<.05) and speed of contraction (but not relaxation) was increased (P<0.05), indicating an increased contractile speed in paralysed muscles compared with non-paralysed muscles. These results correspond with an expected predominance of fast glycolytic muscle fibres in paralysed muscles. It is concluded that quadriceps dynamometry is a useful technique to study muscle function in non-paralysed as well as in paralysed muscles. Furthermore, these techniques can be reliably used, for example, to assess therapeutic interventions on paralysed muscles provided that expected differences in relative tetanic tension and fatigue resistance are larger than approximately 5% and differences in contractile speed are larger than approximately 15%.

Adult↗

PubMed1986

Morphometric evaluation of capillary basement membrane thickness in the quadriceps muscle of diabetic and nondiabetic Chinese hamsters.

Quadriceps muscle capillaries from 19-23 month old genetically diabetic (XA and AC) and nondiabetic (M) subline Chinese hamsters were morphometrically evaluated to determine if capillary basement membrane thickening (CBMT) is a quantifiable complication of diabetes. Significant CBMT was present in the diabetic XA Chinese hamsters (49.37 nm +/- 17.81, p less than 0.007) in comparison with the nondiabetic M hamsters (34.08 nm +/- 9.98). Although there was a trend towards expansion of the muscle capillary basement membranes in the diabetic AC Chinese hamsters, the value was not statistically significant. A nested analysis of variance showed that the greatest source of variation in basement membrane thickness occurred among capillaries within each animal. In addition, a positive correlation (r = 0.62; p less than 0.002) existed between blood glucose levels and CBMT in the XA subline. These data should serve as guidelines for evaluation of antimicrovascular disease compounds which will be tested to determine if they prevent or retard microangiopathy in the diabetic Chinese hamster.

Animals↗

PubMed1995

The twitch interpolation technique for study of fatigue of human quadriceps muscle.

The aim of the study was to examine if the twitch interpolation technique could be used to objectively measure fatigue in the quadriceps muscle in subjects performing submaximally. The 'true' maximum isometric quadriceps torque was determined in 21 healthy subject using the twitch interpolation technique. Then an endurance test was performed in which the subjects made repeated isometric contractions at 50% of the 'true' maximum torque for 4 s, separated by 6 s rest periods. During the test, the force response to single electrical stimulation (twitch amplitude) was measured at 50% and 25% of the estimated maximum torque. In 10 subjects, the test was repeated 2-4 weeks later. Twitch amplitudes at 50% of maximum torque declined exponentially with time in 20 of 21 subjects. The distribution of the exponential rate constant was skewed with a mean of 4.6 h-1 and range of 0.3-21.5 h-1. After logarithmical transformation, the distribution of the exponential rate constant fitted closely to a normal distribution, and the inter-individual variation was SD = 1.15 compared to an intra-individual variation of 0.29. The coefficient of correlation for repeated determination was 0.91 (P < 0.001, n = 10). In conclusion, the twitch technique can be used for objectively measuring fatigue of the quadriceps muscle.

Adult↗

PubMed1990

Contraction characteristics of the human quadriceps muscle during percutaneous electrical stimulation.

Percutaneous electrical stimulation was used to study the force response of the quadriceps muscle. The normal frequency dependence of force was investigated in muscles at rest and after fatiguing contractions. A comparison between force response during fatigue induced by electrical stimulation at different frequencies and by voluntary work suggested equal changes in contractility, irrespective of the fatigue-inducing procedure. In fresh muscle we found a linear relation between stimulation period (10-100 ms) and force. At fatigue the relation changes with maximal deviation from linearity at a 50-ms period (20 Hz). There is a rapid recovery of high frequency force whereas the low frequency response remains low even after 30 min rest. At very low frequencies there is initially unexpectedly high force in fatigued muscle. This could be a result of increased fusion of twitches with initially prolonged relaxation time. To study the twitch summation we compared experimental results in a wide frequency range with computer-simulated twitch summations and present the frequency dependence of summation processes in human quadriceps muscle.

Adult↗