Gymnastik- och idrottshögskolan, GIH

This study investigated how a 6s maximal voluntary isometric conditioning contration (MVIC) affected plantar flexor twitch rate of torque development (RTDTW), as well as peak torque (PTCC) and rate of torque development (RTDCC) of maximal voluntary concentric contractions (MVCC) performed at 60°/s. RTDCC and normalized triceps surae electromyography signals (EMGTS) were measured during different phases of contraction. Additionally, muscle tendon unit passive stiffness index (SI) calculated from the torque-angle relation was measured following each MVCC.Enhancements were found in the RTDTW immediately (by 59.7%) and up to 480s (by 6.0%) post MVIC (p<0.05). RTDCC during the 100-200ms, 50-200ms, and 0-200ms phases and PTCC were enhanced (by 5.7-9.5%) from 90-300s post conditioning (p<0.05). Neuromuscular efficiency increased (decreased EMGTS/RTDCC) in the 50-200ms and 0-200ms phases by 8.8-12.4%, from 90-480s post MVIC (p<0.05). No significant changes were found in the SI or in RTDCC during the 50-100ms phase, suggesting that the enhancements reported, reflect mainly contractile rather than neural or tensile mechanisms.PAP effects on PTCC and RTDCC were significant, and more durable at a lower velocity than previously reported. Enhancement in RTDCC and neuromuscular efficiency were found to be more prominent in later phases (>100ms) of the MVCC. This suggests that enhanced contractile properties, attained via MVIC, benefit concentric contraction performance.

Electromechanical delay (EMD) presumably depends upon both contractile and tensile factors. It has recently been used as an indirect measure of muscle tendon stiffness to study adaptations to stretching and training. The aim of the present study was to investigate whether contractile properties induced by a 6 s maximum voluntary isometric contraction (MVIC) could affect EMD without altering passive muscle tendon stiffness or stiffness index. Plantar flexor twitches were evoked via electrical stimulation of the tibial nerve in eight highly trained male sprinters before and after a 6 s MVIC in passive isometric or passively shortening or lengthening muscles. For each twitch, EMD, twitch contractile properties and SOLM-Wave were measured. Passive muscle tendon stiffness was measured from the slope of the relation between torque and ankle angle during controlled passive dorsal flexion and stiffness index by curve-fitting the torque angle data using a second-order polynomial function. EMD did not differ between isometric, lengthening or shortening movements. EMD was reduced by up to 11.56 ± 5.64% immediately after the MVIC and stayed depressed for up to 60 s after conditioning. Peak twitch torque and rate of torque development were potentiated by up to 119.41 ± 37.15% and 116.06 ± 37.39%, respectively. Rising time was reduced by up to 14.46 ± 7.22%. No significant changes occurred in passive muscle tendon stiffness or stiffness index. Using a conditioning MVIC, it was shown that there was an acute enhancement of contractile muscle properties as well as a significant reduction in EMD with no corresponding changes in stiffness. Therefore, caution should be taken when using and interpreting EMD as a proxy for muscle tendon stiffness.

Flexing the knee to isolate the single joint soleus from the biarticular gastrocnemius is a strategy forinvestigating individual plantarflexor's post activation potentiation (PAP). We investigated the implications of testingplantarflexor PAP at different knee angles and provided indirect quantification of the contribution of gastrocnemiuspotentiation to the overall plantarflexor enhancements post conditioning. Plantarflexor supramaximal twitches weremeasured in ten male power athletes before and after a maximal isometric plantarflexion (MVIC) at both flexed andextended knee angles. Mean torque and soleus (SOLRMS) and medial gastrocnemius (MGRMS) activity were measuredduring the MVIC. The mean torque and MGRMS of the MVIC were lower (by 33.9 and 42.4%, respectively) in the flexedcompared to the extended position, with no significant difference in SOLRMS. After the MVIC, twitch peak torque (PT)and the rate of torque development (RTR) potentiated significantly more (by 17.4 and 14.7% respectively) in theextended as compared to the flexed knee position, but only immediately (5 s) after the MVIC. No significant differenceswere found in the twitch rate of torque development (RTD) potentiation between positions. It was concluded that kneejoint configuration should be taken into consideration when comparing studies of plantarflexor PAP. Furthermore,results reflect a rather brief contribution of the gastrocnemius potentiation to the overall plantarflexor twitchenhancements.

Introduction: Acute enhancements of muscle contractile properties and performance subsequent to a maximal or near maximal conditioning contraction are often termed post activation potentiation (PAP). Although still controversial, PAP is commonly linked to enhancements in the myosin regulatory light chain phosphorylation, leading to improvements in the excitation–contraction coupling. The PAP seen after a conditioning task often coexists with fatigue and is known to depend on strength level, muscle fiber type and age. Less is known about how factors such as static and dynamic changes in muscle length affect PAP, and on the relative contribution of contractile and tensile components to PAP.

Aim: To enhance our understanding of how, and under what conditions, a single maximal isometric contraction affects plantar flexor muscle contractile performance, and other muscle tendon properties, in power athletes.

Methods: Supramaximal twitches were evoked via electrical stimulation of the tibial nerve of athletes before and on several occasions after a 6-second maximal voluntary isometric contraction (6-s MVIC) in both static muscle, and during passive muscle lengthening and shorting at different angular velocities. Several contractile variables were measured from the twitches. The effects of a 6-s MVIC on Achilles tendon stiffness was calculated from torque and ultrasonography based measurements of tendon length at two submaximal contraction intensities. Overall stiffness index was calculated by analyzing the passive lengthening torque/angle curve.Results: A single MVIC enhanced muscle contractile properties and electromechanical delay for up to 5 minutes. Plantar flexor twitch variables such as peak twitch, rate of torque development and rate of torque relaxation were enhanced during shortening compared to lengthening muscle actions, and in an extended as compared to a flexed knee position. Achilles tendon stiffness and overall stiffness index were not significantly modulated by a single 6-s MVIC.

Conclusion: The results of this thesis imply that functional enhancements from a 6-s conditioning MVIC would mainly come from improvements in contractile rather than tensile components. Stiffness changes should be monitored in future PAP-related studies since they may still occur after more extensive conditioning protocols than the current one. Improvements in contractile components subserving muscle strength after a conditioning MVIC suggests that enhancements in muscle power after a conditioning task should be greatest in fast concentric muscle actions, though still present in muscle lengthening. Conditioning should be performed in a position where full activation is easy to achieve and tailored to mach an athlete or group of athlete’s current status and characteristics, maximizing performance in a specific sport event.

PURPOSE: This study aimed to investigate conditioning effects from a single 6-s plantar flexion maximal voluntary isometric contraction (MVIC) on Achilles tendon stiffness (ATS) and twitch properties of the triceps surae in athletes.

METHODS: Peak twitch (PT), rate of torque development (RTD), rising time (RT10-90%) and half relaxation time (HRT) were measured from supramaximal twitches evoked in the plantar flexors of 10 highly trained athletes. Twitches were evoked before and at seven occasions during 10 min of recovery after a 6-s MVIC. In a second session, but at identical post-conditioning time points, ATS was measured at 30 and 50 % of MVIC (ATS30% and ATS50%) using an ultrasonography-based method.

RESULTS: The magnitude and duration of the conditioning MVIC on muscle contractile properties were in accordance with previous literature on post activation potentiation (PAP), i.e., high potentiation immediately after MVIC, with significant PAP for up to 3 min after the MVIC. While PT and RTD were significantly enhanced (by 60.6 ± 19.3 and 90.1 ± 22.5 %, respectively) and RT10-90% and HRT were reduced (by 10.1 ± 7.7 and 18.7 ± 5.6 %, respectively) after conditioning, ATS remained unaffected.

CONCLUSIONS: Previous studies have suggested that changes in stiffness after conditioning may interfere with the enhancements in twitch contractile properties. The present study, however, provided some evidence that twitch enhancements after a standard PAP can be induced without changes in ATS. This result may suggest that athletes can use this protocol to enhance muscle contractile properties without performance deficits due to changes in ATS.

INTRODUCTION: A conditioning maximal voluntary muscle action (MVC) has been shown to induce post-activation potentiation, i.e. improved contractile muscle properties, when muscles are contracted isometrically. It is still uncertain how the contractile properties are affected during ongoing muscle length changes. The purpose of this study was to investigate the effects of a 6 s conditioning MVC on twitch properties of the plantar flexors during ongoing muscle length changes.

METHODS: Peak twitch, rate of torque development (RTD) and relaxation (RTR), rising time and half relaxation time (HRT) were measured from supramaximal twitches evoked in the plantar flexors of 11 highly trained athletes. Twitches were evoked prior to a 6 s MVC and subsequently on 8 different occasions during a 10-minute recovery, for five different modes: fast lengthening, slow lengthening, isometric, fast shortening and slow shortening of the plantar flexors.

RESULTS: The magnitude and duration of effects from the conditioning MVC were significantly different between modes. Peak twitch, RTD and RTR significantly increased for all modes but more so for twitches evoked during fast and slow shortening as compared to lengthening. Rising time was reduced in the lengthening modes, but slightly prolonged in the shortening modes. HRT was significantly reduced for all modes except fast lengthening.

CONCLUSION: The findings show that the effects of a conditioning MVC on twitch contractile properties are dependent on direction and velocity of ongoing muscle length changes. This may imply that functional enhancements from a conditioning MVC might be expected to be greatest for concentric muscle actions, but are still present in isometric and eccentric parts of a movement.