Matthew W. Buckthorpe

Human capacity for explosive force production: Neural and contractile determinants

This study assessed the integrative neural and contractile determinants of human knee extension explosive force production. Forty untrained participants performed voluntary and involuntary (supramaximally evoked twitches and octets – eight pulses at 300 Hz that elicit the maximum possible rate of force development) explosive isometric contractions of the knee extensors. Explosive force (F0–150 ms) and sequential rate of force development (RFD, 50‐ms epochs) were measured. Surface electromyography (EMG) amplitude was recorded (superficial quadriceps and hamstrings, 50‐ms epochs) and normalized (quadriceps to Mmax, hamstrings to EMGmax). Maximum voluntary force (MVF) was also assessed. Multiple linear regressions assessed the significant neural and contractile determinants of absolute and relative (%MVF) explosive force and sequential RFD. Explosive force production exhibited substantial interindividual variability, particularly during the early phase of contraction [F50, 13‐fold (absolute); 7.5‐fold (relative)]. Multiple regression explained 59–93% (absolute) and 35–60% (relative) of the variance in explosive force production. The primary determinants of explosive force changed during the contraction (F0–50, quadriceps EMG and Twitch F; RFD50–100, Octet RFD0–50; F100–150, MVF). In conclusion, explosive force production was largely explained by predictor neural and contractile variables, but the specific determinants changed during the phase of contraction.

RELIABILITY OF NEUROMUSCULAR MEASUREMENTS DURING EXPLOSIVE ISOMETRIC CONTRACTIONS, WITH SPECIAL REFERENCE TO ELECTROMYOGRAPHY NORMALIZATION TECHNIQUES

Introduction: This study determined the between‐session reliability of neuromuscular measurements during explosive isometric contractions, with special consideration of electromyography (EMG) normalization. Methods: 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). Results: 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. Conclusions: 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. Muscle Nerve 46: 566–576, 2012

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.

Validity of vertical jump measurement devices.

Abstract Vertical jump height is thought to provide a valuable index of muscular power, which is an important factor in sports performance and for assessing the mobility and functional capacity of injured or aged individuals. The purpose of the present study was to investigate the criterion validity of four popular devices for measuring vertical jump height. A belt mat, contact mat, portable force plate, and Vertec were compared to a criterion device, a laboratory force plate. Forty participants performed three maximal countermovement jumps on each device in a counterbalanced order, using block randomization. The criterion device presented the highest mean value (50.3 cm). The portable force plate and belt mat devices recorded similar jump height values to the criterion device (within 1 cm). The contact mat and Vertec devices recorded significantly lower values than the criterion device (P < 0.001). The mean difference ± limits of agreement were: belt mat −0.1 ± 5.5 cm, contact mat −11.7 ± 6.4 cm, portable force plate −0.8 ± 3.9 cm, and Vertec −2.4 ± 6.6 cm. In conclusion, the portable force plate and belt mat devices provided valid measures of vertical jump height, whereas the Vertec and contact mat devices did not.

Central fatigue contributes to the greater reductions in explosive than maximal strength with high‐intensity fatigue

What is the central question of this study? Repeated high‐force contractions of skeletal muscle cause a decline in the force‐generating capacity, referred to as muscle fatigue. The influence of fatigue on explosive strength and the associated contractile and neural mechanisms responsible is not known. What is the main finding and its importance? Fatigue exerts a more pronounced influence on explosive force production than on maximal voluntary force production. Contractile and neural mechanisms were considered responsible.

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.

Task-specific neural adaptations to isoinertial resistance training

This study aimed to delineate the contribution of adaptations in agonist, antagonist, and stabilizer muscle activation to changes in isometric and isoinertial lifting strength after short‐term isoinertial resistance training (RT). Following familiarization, 45 men (23.2 ± 2.8 years) performed maximal isometric and isoinertial strength tests of the elbow flexors of their dominant arms before and after 3 weeks of isoinertial RT. During these tasks, surface electromyography (EMG) amplitude was recorded from the agonist (biceps brachii short and long heads), antagonist (triceps brachii lateral head), and stabilizer (anterior deltoid, pectoralis major) muscles and normalized to either Mmax (agonists) or to maximum EMG during relevant reference tasks (antagonist, stabilizers). After training, there was more than a twofold greater increase in training task‐specific isoinertial than isometric strength (17% vs 7%). There were also task‐specific adaptations in agonist EMG, with greater increases during the isoinertial than isometric strength task [analysis of variance (ANOVA), training × task, P = 0.005]. A novel finding of this study was that training increased stabilizer muscle activation during all the elbow flexion strength tasks (P < 0.001), although these were not task‐specific training effects. RT elicited specific neural adaptations to the training task that appeared to explain the greater increase in isoinertial than isometric strength.