Matthew T.G. Pain

Neuromuscular Performance of Explosive Power Athletes versus Untrained Individuals

PURPOSE 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. METHODS 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. RESULTS 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. CONCLUSIONS 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.

The role of the heel pad and shank soft tissue during impacts: a further resolution of a paradox

The aim of this study was to test the hypothesis that the motion of the soft tissue of the lower leg contributes significantly to the attenuation of the forces during heel impacts. To examine this, a two-dimensional model of the shank and heel pad was developed using DADS. The model contained a heel pad element and a rigid skeleton to which was connected soft tissue which could move relative to the bone. Simulations permitted estimation of heel pad properties directly from heel pad deformations, and from the kinematics of an impacting pendulum. These two approaches paralleled those used in vitro and in vivo, respectively. Measurements from the pendulum indicated that heel pad properties changed from those found in vitro to those found in vivo as relative motion of the bone and soft tissue was allowed. This would indicate that pendulum measures of the in vivo heel pad properties are also measuring the properties of the whole lower leg. The ability of the wobbling mass of the shank to dissipate energy during an impact was found to be significant. These results demonstrate the important role of both the heel pad and soft tissue of the shank to the dissipation of mechanical energy during impacts. These results provide a further clarification of the paradox between the measurements of heel pad properties made in vivo and in vitro.

Sprint starts and the minimum auditory reaction time

Abstract The simple auditory reaction time is one of the fastest reaction times and is thought to be rarely less than 100 ms. The current false start criterion in a sprint used by the International Association of Athletics Federations is based on this assumed auditory reaction time of 100 ms. However, there is evidence, both anecdotal and from reflex research, that simple auditory reaction times of less than 100 ms can be achieved. Reaction time in nine athletes performing sprint starts in four conditions was measured using starting blocks instrumented with piezoelectric force transducers in each footplate that were synchronized with the starting signal. Only three conditions were used to calculate reaction times. The pre-motor and pseudo-motor time for two athletes were also measured across 13 muscles using surface electromyography (EMG) synchronized with the rest of the system. Five of the athletes had mean reaction times of less than 100 ms in at least one condition and 20% of all starts in the first two conditions had a reaction time of less than 100 ms. The results demonstrate that the neuromuscular-physiological component of simple auditory reaction times can be under 85 ms and that EMG latencies can be under 60 ms.

Explosive force production during isometric squats correlates with athletic performance in rugby union players

Abstract This study investigated the association between explosive force production during isometric squats and athletic performance (sprint time and countermovement jump height). Sprint time (5 and 20 m) and jump height were recorded in 18 male elite-standard varsity rugby union players. Participants also completed a series of maximal- and explosive-isometric squats to measure maximal force and explosive force at 50-ms intervals up to 250 ms from force onset. Sprint performance was related to early phase (≤100 ms) explosive force normalised to maximal force (5 m, r = −0.63, P = 0.005; and 20 m, r = −0.54, P = 0.020), but jump height was related to later phase (>100 ms) absolute explosive force (0.51 < r < 0.61; 0.006 < P < 0.035). When participants were separated for 5-m sprint time (< or ≥ 1s), the faster group had greater normalised explosive force in the first 150 ms of explosive-isometric squats (33–67%; 0.001 < P < 0.017). The results suggest that explosive force production during isometric squats was associated with athletic performance. Specifically, sprint performance was most strongly related to the proportion of maximal force achieved in the initial phase of explosive-isometric squats, whilst jump height was most strongly related to absolute force in the later phase of the explosive-isometric squats.

Short-term unilateral resistance training affects the agonist–antagonist but not the force–agonist activation relationship

In this study we investigated the contribution of neural adaptations to strength changes after 4 weeks of unilateral isometric resistance training. Maximal and submaximal isometric knee extension contractions were assessed before and after training. Surface electromyography (EMG) data were collected from the agonist and antagonist muscles and normalized to evoked maximal M‐wave and maximal knee flexor EMG, respectively. The interpolated twitch technique (ITT) was also used to determine activation at maximum voluntary force (MVF). MVF increased in the trained (+20%) and untrained (+8%) legs. Agonist EMG at MVF increased in the trained leg (+26%), although activation determined via the ITT was unchanged. In both legs the position of the force–agonist EMG relationship was unchanged, but antagonist coactivation was lower for all levels of agonist activation. Strength gains in the trained leg were due to enhanced agonist activation, whereas decreased coactivation may have affected strength changes in both legs. Muscle Nerve, 2011

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 (F50, F100 and F150) 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 F50 (+54%), F100 (+15%) and F150 (+14%) and in octet F50 (+7%) and F100 (+10%). Voluntary F100 and F150, and octet F50 and F100 increased proportionally with MVF (+11%). However, the increase in voluntary F50 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.

Neuromuscular function in healthy occlusion

This study aimed to measure neuromuscular function for the masticatory muscles under a range of occlusal conditions in healthy, dentate adults. Forty-one subjects conducted maximum voluntary clenches under nine different occlusal loading conditions encompassing bilateral posterior teeth contacts with the mandible in different positions, anterior teeth contacts and unilateral posterior teeth contacts. Surface electromyography was recorded bilaterally from the anterior temporalis, superficial masseter, sternocleidomastoid, anterior digastric and trapezius muscles. Clench condition had a significant effect on muscle function (P = 0.0000) with the maximum function obtained for occlusions with bilateral posterior contacts and the mandible in a stable centric position. The remaining contact points and moving the mandible to a protruded position, whilst keeping posterior contacts, resulted in significantly lower muscle activities. Clench condition also had a significant effect on the per cent overlap, anterior-posterior and torque coefficients (P = 0.0000-0.0024), which describe the degree of symmetry in these muscle activities. Bilateral posterior contact conditions had significantly greater symmetry in muscle activities than anterior contact conditions. Activity in the sternocleidomastoid, anterior digastric and trapezius was consistently low for all clench conditions, i.e. <20% of the maximum voluntary contraction level. In conclusion, during maximum voluntary clenches in a healthy population, maximum masticatory muscle activity requires bilateral posterior contacts and the mandible to be in a stable centric position, whilst with anterior teeth contacts, both the muscle activity and the degree of symmetry in muscle activity are significantly reduced.

Essay Risk taking in sport

S33 Extreme sports have never been so popular. Mountain climbing is one of the fastest growing outdoor pursuits, and interest in extreme skiing—where skiers drop from ledge to ledge on sheer cliffs—is also burgeoning. Paragliding, skydiving, whitewater kayaking, and bungee jumping are fast becoming de rigueur. Full contact martial arts clubs are exploding and no holds barred fighting is becoming the number one pay per view draw. Meanwhile golf courses in the USA and UK struggle to retain members. According to the Freudian interpretation, risk taking individuals like Jim Wickwire (panel) have a death fulfilment wish; as such, the repetition of lifethreatening behaviours is classified as expressing suicidal tendencies. Wickwire himself is still unsure why he pursues his dangerous hobbies, stating that “The people who engage in this [sport] are probably driven to it in a psychological fashion that they may not even understand themselves”. Most seasoned climbers would, however, baulk at this Freudian interpretation, and indeed results of research studies into the mental health of risk takers indicate no differences from the general population. Furthermore, engaging in risky sports leads to an increase in confidence and selfesteem, much like people who take financial risks in the workplace tend to be more successful. Risk taking cannot, therefore, simply be explained away as a selfdefeating psychosis. In fact, strong evidence suggests that the inclination to take risks is hard-wired into the brain and intimately bound to arousal and pleasure mechanisms. Such behaviour might even have ensured our survival as a species and underpinned our rapid population of the earth. Early man first came out of Africa about 100 000 years ago. Confronted by new and hazardous environments our ancestors were forced to take great risks and travel large distances to find food, shelter, and sexual partners. So-called risky genes were therefore adaptive and became more common through natural selection. Although our brains have continued to evolve, primitive instincts still exert a strong influence over us. Genetically, we are evolved for exploration and risk, not for an urbanised and sedentary lifestyle. This evolutionary explanation of risk taking certainly seems

In vivo determination of the effect of shoulder pads on tackling forces in rugby

Abstract Tackling in rugby is now a major cause of injury. The use of rugby shoulder pads is intended to reduce injury from front-on tackles, although the pads ability to reduce injury has not been examined. This paper strives to present a novel method, using Tekscan sensors, for measuring in vivo impact intensities during a front-on tackle to assess the effectiveness of rugby shoulder padding in reducing peak force during impact. It was hypothesized that padding would not significantly reduce peak impact force. Rugby pads were instrumented with thin film force sensors to measure impact intensities during tackles with and without pads. Sensors were first statically then dynamically calibrated using force plate data. Results showed that the pad significantly reduced peak impact force by up to 35% when impacted with an object and by 40% overall for all tackles. The hypothesis that the shoulder pad could not significantly reduce peak force at impact was rejected, since the pad reduced peak force by 41% in tackles with a run-up and 40% overall for all tackles. However, this reduction in force was localized directly above the acromioclavicular joint, while forces in the surrounding areas were not reduced.

The need for muscle co-contraction prior to a landing

In landings from a flight phase the mass centre of an athlete experiences rapid decelerations. This study investigated the extent to which co-contraction is beneficial or necessary in drop landings, using both experimental data and computer simulations. High speed video and force recordings were made of an elite martial artist performing drop landings onto a force plate from heights of 1.2, 1.5 and 1.8m. Matching simulations of these landings were produced using a planar 8-segment torque-driven subject-specific computer simulation model. It was found that there was substantial co-activation of joint flexor and extensor torques at touchdown in all three landings. Optimisations were carried out to determine whether landings could be effected without any co-contraction at touchdown. The model was not capable of landing from higher than 1.05m with no initial flexor or extensor activations. Due to the force-velocity properties of muscle, co-contraction with net zero joint torque at touchdown leads to increased extensor torque and decreased flexor torque as joint flexion velocity increases. The same considerations apply in any activity where rapid changes in net joint torque are required, as for example in jumps from a running approach.