Explosive Voluntary Contractions Research Articles

Reliability of Quadriceps Twitch Muscle Properties and Explosive Voluntary Contractions at Different Knee Joint Angles

ABSTRACT Background Measures that assess muscle strength and its development, either voluntarily or involuntarily, are important in the clinical and research context. The main aim of this study was to verify the interday reliability and the minimum detectable change (MDC) of the knee extensors muscles torque using evoked contractions and explosive voluntary contractions (EVC), across six different joint angles. Methods 20 participants, 16 men and 4 women (26.5 ± 6.6 years; 174.5 ± 11.1 cm; 73.7 ± 11.3 kg), participated in this study. Evoked twitch contractions and voluntary explosive contractions of the knee extensors were performed in 2 days (test-retest) at 20, 40, 70, 90, 105 and 115 degrees. Results The most reliable twitch parameters were peak torque (PT), contraction time (CT), half relaxation-time (½ time), total impulse and peak rate of torque development (RTDPeak), with coefficient of variation (CV) ranging from ~ 8% at 20° to 5% at 115° angle. It was verified that CT and ½ time become more reliable as the knee angle changed from 20 to 115 degrees, from shorter to longer muscle lengths (7.3 to 2.7% and 8.8% to 4.0%, respectively). The more reliable parameters for voluntary contraction were PT, RTD250 ms, RTDPeak and impulse250 ms (CV from around ~ 8% at 20°/115° and 5% at 70°/90°). For voluntary contractions, all measures performed at 70° angle were more reliable compared to the other angles. Conclusion It was concluded that for the knee extensor muscles, evoked twitch contractions presented greater reliability across knee angles, with lower CV values when compared to the voluntary measures for the same time windows. Specifically, for the evoked twitch contractions, angles from 70 to 115 degree proved to be highly reliable. On the other hand, for the voluntary contractions, the central angles (70 and 90 degree) were the most reliable.

Comparison of single and multiple joint muscle functions and neural drive of trained athletes and untrained individuals

BACKGROUND: There is insufficient knowledge about the rate of force development (RFD) characteristics over both single and multiple joint movements and the electromechanical delay (EMD) values obtained in athletes and untrained individuals. OBJECTIVE: To compare single and multiple joint functions and the neural drive of trained athletes and untrained individuals. METHODS: Eight trained athletes and 10 untrained individuals voluntarily participated to the study. The neuromuscular performance was assessed during explosive and maximum voluntary isometric contractions during leg press and knee extension related to single and multiple joint. Explosive force and surface electromyography of eight superficial lower limb muscles were measured in five 50-ms time windows from their onset, and normalized to peak force and electromyography activity at maximum voluntary force, respectively. The EMD was determined from explosive voluntary contractions (EVC’s). RESULTS: The results showed that there were significant differences in absolute forces during knee extension maximum voluntary force and EVC’s (p< 0.01) while trained athletes achieved greater relative forces than untrained individuals of EVC at all five time points (p< 0.05). CONCLUSIONS: The differences in explosive performance between trained athletes and untrained individuals in both movements may be explained by different levels of muscle activation within groups, attributed to variation in biarticular muscle function over both activities.

Explosive strength: effect of knee-joint angle on functional, neural, and intrinsic\xa0contractile properties

PurposeThe present study compared knee extension explosive isometric torque, neuromuscular activation, and intrinsic contractile properties at five different knee-joint angles (35°, 50°, 65°, 80°, and 95°; 0° = full knee extension).MethodsTwenty-eight young healthy males performed two experimental sessions each involving: 2 maximum, and 6–8 explosive voluntary contractions at each angle; to measure maximum voluntary torque (MVT), explosive voluntary torque (EVT; 50–150 ms after contraction onset) and quadriceps surface EMG (QEMG, 0–50, 0–100, and 0–150 ms after EMG onset during the explosive contractions). Maximum twitch and M-wave (MMAX) responses as well as octet contractions were evoked with femoral nerve stimulation at each angle.ResultsAbsolute MVT and EVT showed an inverted ‘U’ relationship with higher torque at intermediate angles. There were no differences between knee-joint angles for relative EVT (%MVT) during the early phase (≤ 75 ms) of contraction and only subtle differences during the late phase (≥ 75 ms) of contraction (≤ 11%). Neuromuscular activation during explosive contractions was greater at more flexed than extended positions, and this was also the case during MVT. Whilst relative twitch torque (%MVT) was higher at knee flexed positions (P ≤ 0.001), relative octet torque (%MVT) was higher at knee extended positions (P ≤ 0.001).ConclusionRelative EVT was broadly similar between joint angles, likely because neuromuscular activation during both explosive and plateau (maximum) phases of contraction changed proportionally, and due to the opposing changes in twitch and octet evoked responses with joint angle.

β-Alanine supplementation enhances human skeletal muscle relaxation speed but not force production capacity.

β-Alanine (BA) supplementation improves human exercise performance. One possible explanation for this is an enhancement of muscle contractile properties, occurring via elevated intramuscular carnosine resulting in improved calcium sensitivity and handling. This study investigated the effect of BA supplementation on in vivo contractile properties and voluntary neuromuscular performance. Twenty-three men completed two experimental sessions, pre- and post-28 days supplementation with 6.4 g/day of BA (n = 12) or placebo (PLA; n = 11). During each session, force was recorded during a series of knee extensor contractions: resting and potentiated twitches and octet (8 pulses, 300 Hz) contractions elicited via femoral nerve stimulation; tetanic contractions (1 s, 1-100 Hz) via superficial muscle stimulation; and maximum and explosive voluntary contractions. BA supplementation had no effect on the force-frequency relationship, or the force responses (force at 25 and 50 ms from onset, peak force) of resting or potentiated twitches, and octet contractions (P > 0.05). Resting and potentiated twitch electromechanical delay and time-to-peak tension were unaffected by BA supplementation (P > 0.05), although half-relaxation time declined by 7-12% (P < 0.05). Maximum and explosive voluntary forces were unchanged after BA supplementation. BA supplementation had no effect on evoked force responses, implying that altered calcium sensitivity and/or release are not the mechanisms by which BA supplementation influences exercise performance. The reduced half-relaxation time with BA supplementation might, however, be explained by enhanced reuptake of calcium, which has implications for the efficiency of muscle contraction following BA supplementation.

Bilateral Deficit in Explosive Force Production Is Not Caused by Changes in Agonist Neural Drive

Bilateral deficit (BLD) describes the phenomenon of a reduction in performance during synchronous bilateral (BL) movements when compared to the sum of identical unilateral (UL) movements. Despite a large body of research investigating BLD of maximal voluntary force (MVF) there exist a paucity of research examining the BLD for explosive strength. Therefore, this study investigated the BLD in voluntary and electrically-evoked explosive isometric contractions of the knee extensors and assessed agonist and antagonist neuromuscular activation and measurement artefacts as potential mechanisms. Thirteen healthy untrained males performed a series of maximum and explosive voluntary contractions bilaterally (BL) and unilaterally (UL). UL and BL evoked twitch and octet contractions were also elicited. Two separate load cells were used to measure MVF and explosive force at 50, 100 and 150 ms after force onset. Surface EMG amplitude was measured from three superficial agonists and an antagonist. Rate of force development (RFD) and EMG were reported over consecutive 50 ms periods (0–50, 50–100 and 100–150 ms). Performance during UL contractions was compared to combined BL performance to measure BLD. Single limb performance during the BL contractions was assessed and potential measurement artefacts, including synchronisation of force onset from the two limbs, controlled for. MVF showed no BLD (P = 0.551), but there was a BLD for explosive force at 100 ms (11.2%, P = 0.007). There was a BLD in RFD 50–100 ms (14.9%, P = 0.004), but not for the other periods. Interestingly, there was a BLD in evoked force measures (6.3–9.0%, P<0.001). There was no difference in agonist or antagonist EMG for any condition (P≥0.233). Measurement artefacts contributed minimally to the observed BLD. The BLD in volitional explosive force found here could not be explained by measurement issues, or agonist and antagonist neuromuscular activation. The BLD in voluntary and evoked explosive force might indicate insufficient stabiliser muscle activation during BL explosive contractions.

Reliability of neuromuscular measurements during explosive isometric contractions, with special reference to electromyography normalization techniques

This study determined the between-session reliability of neuromuscular measurements during explosive isometric contractions, with special consideration of electromyography (EMG) normalization. Following familiarization, 13 men participated in 3 identical measurement sessions involving maximal and explosive voluntary contractions of the knee extensors, while force and surface EMG were recorded. Root mean square EMG amplitude was normalized to different reference measures: (evoked maximal M-wave peak-to-peak amplitude and area, maximum and sub-maximum voluntary contractions). Explosive voluntary force measurements were reliable on a group level, whereas within-subject reliability was low over the initial 50 ms and good from 100 ms onward. Normalization of EMG during explosive voluntary contractions, irrespective of the reference method, did not reduce the within-subject variability, but it did reduce substantially the variability between-subject. The high intra-individual variability of EMG and early phase explosive voluntary force production may limit their use to measuring group as opposed to individual responses to an intervention.

Short‐term training for explosive strength causes neural and mechanical adaptations

This study investigated the neural and peripheral adaptations to short-term training for explosive force production. Ten men trained the knee extensors with unilateral explosive isometric contractions (1 s 'fast and hard') for 4 weeks. Before and after training, force was recorded at 50-ms intervals from force onset (F(50), F(100) and F(150)) during both voluntary and involuntary (supramaximal evoked octet; eight pulses at 300 Hz) explosive isometric contractions. Neural drive during the explosive voluntary contractions was measured with the ratio of voluntary/octet force, and average EMG normalized to the peak-to-peak M-wave of the three superficial quadriceps. Maximal voluntary force (MVF) was also measured, and ultrasonic images of the vastus lateralis were recorded during ramped contractions to assess muscle-tendon unit stiffness between 50 and 90% MVF. There was an increase in voluntary F(50) (+54%), F(100) (+15%) and F(150) (+14%) and in octet F(50) (+7%) and F(100) (+10%). Voluntary F(100) and F(150), and octet F(50) and F(100) increased proportionally with MVF (+11%). However, the increase in voluntary F(50) was +37% even after normalization to MVF, and coincided with a 42% increase in both voluntary/octet force and agonist-normalized EMG over the first 50 ms. Muscle-tendon unit stiffness between 50 and 90% MVF also increased. In conclusion, enhanced agonist neural drive and MVF accounted for improved explosive voluntary force production in the early and late phases of the contraction, respectively. The increases in explosive octet force and muscle-tendon unit stiffness provide novel evidence of peripheral adaptations within merely 4 weeks of training for explosive force production.

Explosive neuromuscular performance of males versus females

The purpose of the study was to investigate sex-related differences in explosive muscular force production, as measured by electromechanical delay (EMD) and rate of force development (RFD), and to examine the physiological mechanisms responsible for any differences. The neuromuscular performance of untrained males (n = 20) and females (n = 20) was assessed during a series of isometric knee extension contractions; explosive and maximal voluntary efforts, as well as supramaximal evoked twitches and octets (eight pulses at 300 Hz). Evoked and voluntary EMD were determined from twitch and explosive contractions. The RFD was recorded over consecutive 50 ms time windows from force onset during evoked and explosive contractions, and normalized to maximal strength. Neuromuscular activity during explosive voluntary contractions was measured with EMG of the superficial knee extensors normalized to maximal M-wave. Muscle size (thickness) and muscle-tendon unit (MTU) stiffness were assessed using ultrasonic images of the vastus lateralis at rest and during ramped contractions. Males and females had similar evoked and voluntary EMD. Males were 33% stronger (P < 0.001) and their absolute RFD was 26-56% greater (all time points P < 0.05) compared with females. Muscle size (P < 0.001) and absolute MTU stiffness were also greater for males (P < 0.05). However, normalized RFD was similar for both sexes during the first 150 ms of the explosive voluntary contractions (P > 0.05). This was consistent with the similar normalized twitch and octet RFD, MTU stiffness and agonist EMG (all P > 0.05). When differences in maximal strength were accounted for, the evoked capacity of the knee extensors for explosive force production and the ability to utilize that capacity during explosive voluntary contractions was similar for males and females.

Contraction type influences the human ability to use the available torque capacity of skeletal muscle during explosive efforts

The influence of contraction type on the human ability to use the torque capacity of skeletal muscle during explosive efforts has not been documented. Fourteen male participants completed explosive voluntary contractions of the knee extensors in four separate conditions: concentric (CON) and eccentric (ECC); and isometric at two knee angles (101°, ISO101 and 155°, ISO155). In each condition, torque was measured at 25 ms intervals up to 150 ms from torque onset, and then normalized to the maximum voluntary torque (MVT) specific to that joint angle and angular velocity. Explosive voluntary torque after 50 ms in each condition was also expressed as a percentage of torque generated after 50 ms during a supramaximal 300 Hz electrically evoked octet in the same condition. Explosive voluntary torque normalized to MVT was more than 60 per cent larger in CON than any other condition after the initial 25 ms. The percentage of evoked torque expressed after 50 ms of the explosive voluntary contractions was also greatest in CON (ANOVA; p < 0.001), suggesting higher concentric volitional activation. This was confirmed by greater agonist electromyography normalized to M(max) (recorded during the explosive voluntary contractions) in CON. These results provide novel evidence that the ability to use the muscle's torque capacity explosively is influenced by contraction type, with concentric contractions being more conducive to explosive performance due to a more effective neural strategy.

Neuromuscular Performance of Explosive Power Athletes versus Untrained Individuals

Electromechanical delay (EMD) and rate of force development (RFD) are determinants of explosive neuromuscular performance. We may expect a contrast in EMD and RFD between explosive power athletes, who have a demonstrable ability for explosive contractions, and untrained individuals. However, comparison and the neuromuscular mechanisms for any differences have not been studied. The neuromuscular performance of explosive power athletes (n = 9) and untrained controls (n = 10) was assessed during a series of twitch, tetanic, explosive, and maximum voluntary isometric knee extensions. Knee extension force and EMG of the superficial quadriceps were measured in three 50-ms time windows from their onset and were normalized to strength and maximal M-wave (Mmax), respectively. Involuntary and voluntary EMD were determined from twitch and explosive voluntary contractions, respectively, and were similar for both groups. The athletes were 28% stronger, and their absolute RFD in the first 50 ms was twofold that of controls. Athletes had greater normalized RFD (4.86 ± 1.46 vs 2.81 ± 1.20 MVC·s(-1)) and neural activation (mean quadriceps, 0.26 ± 0.07 vs 0.15 ± 0.06 Mmax) during the first 50 ms of explosive voluntary contractions. Surprisingly, the controls had a greater normalized RFD in the second 50 ms (6.68 ± 0.92 vs 7.93 ± 1.11 MVC·s-1) and a greater change in EMG preceding this period. However, there were no differences in the twitch response or normalized tetanic RFD between groups. The differences in voluntary normalized RFD between athletes and controls were explained by agonist muscle neural activation and not by the similar intrinsic contractile properties of the groups.