Force-time Parameters Research Articles

Upper-Limb Force-Time Characteristics Determine Serve Velocity in Competition Tennis Players.

(1)To analyze the associations between serve velocity (SV) and various single-joint upper-limb isometric force-time curve parameters, (2)to develop a prediction model based on the relationship between these variables, and (3)to determine whether these factors are capable of discriminating between tennis players with different SV performances. A total of 17 high-performance tennis players performed 8 isometric tests of joints and movements included in the serve kinetic chain (wrist and elbow flexion [EF] and extension; shoulder flexion [SHF] and extension [SHE], internal [SHIR] and external rotation). Isometric force (IF), rate of force development (RFD), and impulse (IMP) at different time intervals (0-250ms) were obtained for analysis. Significant (P < .05 to P < .01) and moderate to very large correlations were found between SV and isometric force (IF), RFD and impulse (IMP) at different time intervals in all joint positions tested (except for the EF). Stepwise multiple regression analysis highlighted the importance of RFD in the SHIR from 0 to 50milliseconds and isometric force (IF) in the SHF at 250milliseconds on SV performance. Moreover, the discriminant analyses established SHIR RFD from 0 to 30milliseconds as the most important factor discriminating players with different serve performances. Force-time parameters in upper-limb joints involved in the serve moderate to very largely influence SV. Findings suggest that the capability to develop force in short periods of time (<250ms), especially in the shoulder joint, seems relevant to develop high SV in competition tennis players.

Influence of force-time parameters of hip abductors on maintaining balance in frontal plane in young healthy females

Purpose: The aim of this study was to determine the impact of isometric and isokinetic hip abductor muscle strength on the quality of maintaining static balance in healthy subjects. Methods: The study enrolled 51 healthy women aged 18–25. Balance assessment was carried out according to the M-CTSIB protocol and Single Leg Stance Test (with eyes open and closed) using the Biodex Balance System. An isokinetic evaluation conducted at 30 °/s and an isometric evaluation of the hip abductors were performed with the Humac Norm system. Results: Regression analysis for Sway Index (SW) and Stability Index (ST) in bipedal standing showed a significant importance of the presence of visual feedback (RSW EO = –0.922; p &lt; 0.0001; RST EO = –0.613; p = 0.0493), and unstable surface (RSW US = 1.253; p &lt; 0.0001; RSW US = 2.547; p &lt; 0.0001). Regression analysis for single-leg stance showed correlations between the following indices: overall sway index (OR) in single-leg stance, the antero-posterior (AP) sway index and the medio-lateral (ML) sway index on the one hand and isometric abduction time to peak torque (ROR TPT = 0.769; p = 0.0005; RAP TPT = 0.565; p = 0.008; RML TPT = –1.74; p &lt; 0.05, respectively) as well as the test conditions on the other. Conclusions: Physiological activation of the hip abductors may be important for physiological maintenance of postural balance in young people, in both leg standing as well as in single-leg stance. The present results warrant prospective, randomized studies of larger groups that are diversified with regard to age and gender of the participants.

Differences in force-time parameters and electromyographic characteristics of two high-velocity, low-amplitude spinal manipulations following one another in quick succession

BackgroundSpinal manipulative therapy is an effective treatment for neck pain. However, the mechanisms underlying its clinical efficacy are not fully understood. Previous studies have not systematically compared force-time parameters and electromyographic responses associated with spinal manipulation. In this study, force-time parameters and electromyographic characteristics associated with multiple manual high-velocity, low-amplitude cervical and upper thoracic spinal manipulations were investigated. The purpose of this analysis was to compare the force-time parameters and electromyographic characteristics between two spinal manipulations delivered following one another in quick succession if the first thrust was not associated with an audible cavitation.MethodsNine asymptomatic and eighteen symptomatic participants received six Diversified-style spinal manipulations to the cervical and upper thoracic spines during data collected February 2018 to September 2019. Peak force, rate of force application and thrust duration were measured using a pressure pad. Bipolar surface electrodes were used to measure the electromyographic responses and reflex delay times in sixteen neck, back and limb outlet muscles bilaterally. Differences in force-time parameters and electromyographic data were analyzed between the first and second thrust.ResultsFifty-two spinal manipulations were included in this analysis. Peak force was greater (p < 0.001) and rate of force application faster (p < 0.001) in the second thrust. Furthermore, peak electromyographic responses were higher following the second thrust in asymptomatic (p < 0.001) and symptomatic (p < 0.001) subjects. Also, electromyographic delays were shorter in the symptomatic compared to the asymptomatic participants for the second thrust (p = 0.039). There were no adverse patient events.ConclusionWhen a second manipulation was delivered because there was not audible cavitation during the first thrust, the second thrust was associated with greater treatment forces and faster thrust rates. Peak electromyographic responses were greater following the second thrust.

The Acute Effect of Different Half Squat Set Configurations on Jump Potentiation

The aim of this study was to evaluate the acute effect of a half squat exercise performed with different set configurations on jump potentiation. Twelve resistance-trained men were evaluated on 3 occasions separated by 48-96 hours. First, they performed a 5 repetitions maximum (5RM) test. Subsequently, they performed in a randomized order 2 sessions: one session with 5RM until failure and the other with the same workload but with 30-second rest intervals between repetitions (i.e., cluster set [CS]). Countermovement jump performance was examined during the second and third sessions for jump height and force-time parameters using a force platform at the following time intervals: before and at 1, 3, 6, 9, and 12 minutes. Separate comparisons for each variable at the different time intervals were analyzed using analysis of variance, effect size, and qualitative inferences. The majority of the parameters improved independently of the time they occurred, except for peak force and vertical stiffness after a set until failure. For peak power, it appears that the cluster treatment resulted in superior potentiation at 1 minute, whereas the 5RM treatment resulted in greater potentiation at 9 minutes. Effect size analysis and qualitative outcomes revealed an improvement in vertical stiffness and a lowering in the depth of the countermovement in CS. There were significant correlations between participants' 5RM relative performance and various force-time parameters only in CS. It appears that a CS induces greater peak power than a 5RM set at 1 minute, although the reverse occurs at 9 minutes. Delayed potentiation associated with the 5RM may be attributed to greater fatigue versus the CS approach. Therefore, it follows that the optimal method for inducing peak power potentiation is dependent on the available time between heavy half squat exercise and the subsequent jump performance.

Is the handgrip strength performance better in judokas than in non-judokas?

The aim of this study was to compare the handgrip strength performance between judokas and non-judokas. Twenty-two highly trained Brazilian judokas, all black belts, participating in the Brazil Trophy of Judo, and 18 non-judokas, both men, participated in the study. The handgrip strength was measured in both hands (dominant and non-dominant) during 10 s using a handgrip dynamometer and force-time parameters were obtained. The effects of judo practice and hand dominance on the force curve parameters were calculated. No effects of judo practice and hand dominance on the peak grip force ( P > 0.05), time to peak ( P > 0.05) and impulse ( P > 0.05) were found. The effect of judo practice on the fatigue rate ( F = 4.253; P = 0.046; η 2 = 0.101) was found, but no effect of the dominance was detected. The results indicate that the judokas were not stronger than the non-judokas in absolute terms (peak grip force), but the judokas were more resistant to fatigue. This difference of fatigue resistance between judokas and non-judokas can be related to the characteristics of the judo practice, especially regarding the prolonged maintenance of the handgrip. Comparer la performance de la force de préhension entre les judokas et les non-judokas. Vingt-deux judokas et 18 non-judokas ont participé à l’étude. La force de préhension était mesurée sur les côtés dominant et non-dominant pendant 10 s à l’aide d’un dynamomètre. Les effets de la pratique du judo et de la prédominance de la main sur les paramètres de la courbe force ont été calculés. Aucun effet de la pratique du judo et de la prédominance de la main sur le pic de la courbe ( p > 0,05), ni sur le temps de montée ( p > 0,05) ni de l’impulsion ( p > 0,05) n’a été trouvé. Un effet de la pratique du judo sur la résistance à la fatigue musculaire a été retrouvé ( F = 4,253, p = 0,046 ; η 2 = 0,101), mais sans effet du côté dominant. Les résultats indiquent que les judokas ne sont pas plus forts que les non-judokas en termes absolus, mais les judokas étaient plus résistants à la fatigue. Cette différence de résistance à la fatigue entre les judokas et les non-judokas peut être liée aux caractéristiques de la pratique du judo, en particulier en ce qui concerne le maintien prolongé de la préhension.

Effect of Proprioceptive Neuromuscular Facilitation on Force-Time Curve and Electromyogram Parameters in Competitive Aerobics Athletes

Objective: to investigate the effect of proprioceptive neuromuscular facilitation stretching on force-time curve and EMG Parameters in competitive aerobics athletes. Methods: we take 36 competitive aerobics athletes as participants, and they were tested after bicycle exercise and bicycle exercise/PNF stretching protocol,then the data were compared and analyzed.Conclusion: PNF stretching for lower extremity will not cause the maximum muscular strength and force - time curve parameters (MVC,F100,IRF,and TMVC) significantly decreased in competitive aerobics athletes, however,EMG voltage of rectus femoris has undergone a significant decline,which is showed that PNF stretch will cause change in excited mode,but force - time curve parameters will not be affected significantly.EMG showed that neural inhibition did not affect the maximum isometric muscle strength and characteristics of time - the power curve parameters, indicating PNF stretching can improve capability of muscle coordination.To take full advantage of PNF stretching exercises, changes of muscular strength and force - time curve parameters caused by PNF stretching should be studied in a more comprehensive, in-depth detail.

Learning Spinal Manipulation Skills: Assessment of Biomechanical Parameters in a 5-Year Longitudinal Study

Objective Teaching spinal manipulation (SM) is a fundamental aspect of chiropractic training. Recent works have identified various biomechanical variables as indicators of SM performance and learning. However, only data from cross-sectional studies are available, limiting conclusions regarding the persistence of SM performance over the years. Therefore, the main objective of this investigation was to quantify the evolution of biomechanical parameters of SM over a 5-year learning period. Methods Thirty-three students enrolled in a chiropractic program participated in the present study. They were tested each year at the beginning of each fall semester by performing 10 SMs on an instrumented manikin while standing on a force plate. The procedure allowed us to measure various force-time parameters. Results Overall, significant time effects were noted for most dependent variables. The results indicated rapid improvement in the peak force applied and the rate of force production during the first 2 years. Time to peak force decreased drastically during the first year, whereas preload forces reached satisfactory levels during the third year. When various force-time requirements of bimanual task components were met, learners significantly reduced trial-to-trial variability of SM peak and preload forces, indicating automaticity of performance. Although global coordination improved in all learning processes, it was only in the later phase that learners reached values approaching those of experts. Conclusion Overall, these results highlighted the importance of considering learning principles in the development of didactic strategies related to SM motor skills.

Muscle endurance measurement using a progressive workload and a constant workload by maximal voluntary contraction

Muscle endurance measurement using a progressive workload method may reduce pain sensation in the subject. This study aimed to examine the relationships between force-time parameters during sustained static gripping as measured by maximal voluntary contraction (MVC) using either a progressive workload (PW) or a constant workload (CW). Sixteen subjects performed sustained static gripping with 7 gradually increasing relative demand values of 20% to 80% MVC and sustained static gripping by MVC. The staging of progressive workload was 10 s for 20% MVC, 20 s each for 30, 40, 50, 60, and 70% MVC, and 10 s for 80% MVC. The forces exerted at 120 s in the CW and PW methods were at around the 23-27% MVC level. Peak force, final force, and force during the last 30 s for the PW method evaluated muscle endurance after 1 min and showed high correlations (r = 0.746 ? 0.895). Significant correlations (r = 0.575 ? 0.605) were found between time to 40% MVC in the CW method and peak force, final force, and force in the last 30 s in the PW method group. The peak force in the PW method may be useful for evaluating muscle endurance with a short testing time and without high pain sensation.

Changes in Maximal and Explosive Strength, Electromyography, and Muscle Thickness of Lower and Upper Extremities Induced by Combined Strength and Endurance Training in Soldiers

The purpose of present study was to examine to what extent an 8-week endurance-based military training period interferes with muscle strength development in the conscripts (n = 72) compared with that caused by sport-related military training with added strength training (ST) or endurance training (ET). More specifically, we examined the effects of these 3 training modes on maximal isometric force, maximal rate of force development (RFD), electromyography (EMG), and muscle thickness of the lower and upper extremities. The measurements included isometric force-time parameters of leg and arm extensors and EMG activity from the vastus lateralis, vastus medialis, rectus femoris, and triceps brachii muscles. The 8-week basic training period combined with added ST and ET significantly improved maximal bilateral isometric force of the arm extensors in ST by 11.8% (p < 0.001), ET by 13.9% (p < 0.001), and normal training (NT) by 7.8% (p < 0.05). Strength training and ET showed significant increases in maximal EMG activity of the trained arm muscles. A significant increase was observed in maximal RFD of the upper extremities only in ST by 28.1% (p < 0.05). Both ST and ET increased their maximal leg extension strength by 12.9% (p < 0.01) and 9.1% (p < 0.05), respectively, whereas no significant change occurred in NT (5.2%, p = 0.45). No significant changes were observed in the shape of the force-time curves of leg extensors. No increases occurred in muscle thickness either in the lower or upper extremities. The present BT training with a large amount of endurance-based military training interfered with strength development, and especially, explosive power development of the lower extremities in the ST group. The optimal improvements in neuromuscular characteristics may not be possible without some decreases in the amount of the endurance-based military training and/or some increases in the amount of the maximal/explosive strength training during the BT.

Relationships between Force-Time Parameters and Muscle Oxygenation Kinetics during Maximal Sustained Isometric Grip and Maximal Repeated Rhythmic Grip with Different Contraction Frequencies

The purposes of this study were to examine the relationships between various force-time parameters and muscle oxygenation kinetics during maximal sustained isometric grip (SIG) and maximal repeated rhythmic grips (RRG) with different grip intervals (interval times: 5, 4, 3, and 2 s). Subjects were 10 healthy young males, aged 20-26 years (height 173.9+/-7.3 cm, body mass 71.5+/-11.2 kg). After measuring maximal grip force, each subject performed the SIG and RRG tests with a target frequency of 12, 15, 20, and 30 grips.min(-1) (interval times: 5, 4, 3, and 2 s, respectively) for 6 min. The decreasing time until 80% MVC showed significant and high correlations with final force values in RRGs with over 3 s intervals (r=0.866-0.941), but not in the SIG and RRG with a 2 s interval. The time at the lowest Oxy-Hb/Mb value showed a significant and high correlation with the time at the highest Deoxy-Hb/Mb value only in the SIG and RRG with a 2 s interval (r=0.825-0.916). Oxy-Hb/Mb decreases markedly and deoxy-Hb/Mb increases after the onset of SIG due to the obstruction of blood flow caused by the increase in intramuscular pressure. A similar physiological response to that of SIG occurs also in RRG with a 2 s interval, but RRGs with intervals over 3 s achieve more resumption of blood flow in the muscular relaxation phase. Hence, in spite of the same RRGs, it was determined that RRGs with intervals over 3 s differ significantly in a changing pattern of grip force and muscle oxygen kinetics from RRGs with a 2 s interval.

Force-time parameters during explosive isometric grip correlate with muscle power

Although explosive isometric contraction provides little work toward the outside, force-time parameters of the rising phase of the force-time curve may be able to predict muscle power. The purpose of this study was to examine the relationship between muscle power with work (power grip) and force-time parameters during the rising phase in explosive isometric grip. Fifteen healthy young adult males participated in this study. Power grip was measured using loads of 20%–50% of maximal voluntary contraction (MVC) (peak isometric force). Subjects pulled explosively on a grip bar held with the second digital joints without the thumb. Peak power was calculated from peak velocity and load. Explosive isometric grip was measured using a hand dynamometer. Time-series data of both tests were sampled by an analog-to-digital interface. Both tests were performed with the subjects seated with a sagittal and horizontal position of the arm supported by an armrest. Peak power in the power grip test tended to be larger with an increase of the load, but there was no significant difference between 40% and 50% MVC. Only the peak power in 50% MVC significantly correlated with peak grip force (r=0.52, p<0.05). The force-time parameters related to the peak rate of the rising force phase in explosive isometric grip significantly correlated with the peak powers (30%–50% MVC, r=|0.58−0.78|). Peak rate of the rising force phase in explosive isometric grip may be useful for predicting muscle power with loads between 30%–50% MVC.

The Properties and Interrelationships of Various Force-time Parameters during Maximal Repeated Rhythmic Grip

The purpose of this study was to examine the properties and interrelationships of various force-time parameters including the inflection point for the rate of decline in force during a maximal repeated rhythmic grip. Fifteen healthy males (age M=21.5, SD=2.1 yr, height M=172.4, SD=5.7 cm, body mass M=68.2, SD=9.2 kg) participated in this study. Subjects performed a maximal repeated rhythmic grip with maximal effort with a target frequency of 30 grip.min(-1) for 6 min. The force value decreased linearly and markedly until about 70% of maximal strength for about 55 s after the onset of a maximal repeated rhythmic grip, and then decreased moderately. Because all parameters showed fair or good correlations between 3 min and 6 min, they are considered to be able to sufficiently evaluate muscle endurance for 3 min instead of 6 min. However, there were significant differences between 3 min and 6 min in the integrated area, the final force, the rate of the decrement constant (k) fitting the force decreasing data to y=ae(-kx)+b and the force of maximal difference between the force and a straight line from peak force to the final force. Their parameters may vary generally by the length of a steady state, namely, a measurement time. The final force value before finishing and the rate of the decrement constant (k) reflect the latter phase during a maximal repeated rhythmic grip. Although many parameters show relatively high mutual relationships, the rate constant (k) shows relatively low correlations with other parameters. We inferred that decreasing the time until 80% of maximal strength and the amount of the decrement force for the first 1 min reflect a linear decrease in the initial phase.

Concurrent Endurance and Explosive Type Strength Training Increases Activation and Fast Force Production of Leg Extensor Muscles in Endurance Athletes

The purpose of this experiment was to examine the effects of concurrent endurance and explosive strength training on electromyography (EMG) and force production of leg extensors, sport-specific rapid force production, aerobic capacity, and work economy in cross-country skiers. Nineteen male cross-country skiers were assigned to an experimental group (E, n = 8) or a control group (C, n = 11). The E group trained for 8 weeks with the same total training volume as C, but 27% of endurance training in E was replaced by explosive strength training. The skiers were measured at pre- and post training for concentric and isometric force-time parameters of leg extensors and EMG activity from the vastus lateralis (VL) and medialis (VM) muscles. Sport-specific rapid force production was measured by performing a 30-m double poling test with the maximal velocity (V(30DP)) and sport-specific endurance economy by constant velocity 2-km double poling test (CVDP) and performance (V(2K)) by 2-km maximal double poling test with roller skis on an indoor track. Maximal oxygen uptake (Vo(2)max) was determined during the maximal treadmill walking test with the poles. The early absolute forces (0-100 ms) in the force-time curve in isometric action increased in E by 18 +/- 22% (p < 0.05), with concomitant increases in the average integrated EMG (IEMG) (0-100 ms) of VL by 21 +/- 21% (p < 0.05). These individual changes in the average IEMG of VL correlated with the changes in early force (r = 0.86, p < 0.01) in E. V(30DP) increased in E (1.4 +/- 1.6%) (p < 0.05) but not in C. The V(2K) increased in C by 2.9 +/- 2.8% (p < 0.01) but not significantly in E (5.5 +/- 5.8%, p < 0.1). However, the steady-state oxygen consumption in CVDP decreased in E by 7 +/- 6% (p < 0.05). No significant changes occurred in Vo(2)max either in E or in C. The present concurrent explosive strength and endurance training in endurance athletes produced improvements in explosive force associated with increased rapid activation of trained leg muscles. The training also led to more economical sport-specific performance. The improvements in neuromuscular characteristics and economy were obtained without a decrease in maximal aerobic capacity, although endurance training was reduced by about 20%.

Relations between the inflection point on the force-time curve and force-time parameters during static explosive grip.

Individual differences in muscle contractile speed during static explosive muscle contraction are reflected in the developmental phase of the force-time curve. The purposes of this study were to clarify the properties and reliability of the inflection point of force-time, statistically dividing speed during static explosive grip into two phases and to assess the relations between that inflection point and others. Static explosive grip data were measured two times with a 5-min. rest (sampling frequency; 100 Hz). 32 healthy, young men (age: 15.5 +/- 0.8 yr., height: 173.9 +/- 7.3 cm, body mass: 71.5 +/- 11.2 kg) participated. 8 static explosive grip parameters were selected: time of reaching, integrated area, and quotient values of the integrated areas up to 0.25, 0.5, and 1.0 sec. divided by maximal grip force. The inflection point was calculated statistically from two regression lines fitted to a developmental phase and the almost steady-state phase of reaching maximal grip force by applying a two-phase regression model. The reliabilities of maximal grip force, time of reaching 90% of maximal grip force, and the integrated area until 0.5 sec. and 1.0 sec. after the onset of grip were good (ICC=.77 to .93). The time of reaching an inflection force value appeared at 0.3 sec. after the onset of grip, corresponding to 80% of maximal grip force, and the reliabilities of the parameters regarding inflection point were good (ICC=.77 to .95). The time determined by boundary data between the former and the latter regression data set and the regression coefficient during the developmental phase correlated significantly with the time of reaching 90% of maximal grip force, the integrated area, and the quotient values of the integrated areas up to 0.25, 0.5, and 1.0 sec. divided by maximal grip force (rs=-.78 to -.96 and -.75 to 0.88, respectively, p<.05). However, these parameters did not correlate with maximal grip force. A force during the developmental phase and maximal grip force can depend on different physiological factors. The time determined by boundary data between the former and the latter regression data set and the regression coefficient during the developmental phase are useful parameters for evaluating static explosive grip.

Lower limb performance in older female patients with Parkinson's disease.

This study examined lower limb performance in older sedentary patients with Parkinson's disease (PD). Fourteen female patients with mild to moderate PD and 12 age-matched controls were included in this study. The force preparation and production of the knee extensor muscles during maximal isometric contraction were measured by dynamometric chair. Two force plates were used to measure the chair rising performance. PD patients had longer visual reaction time during performing maximal isometric contraction, and lower maximal isometric force (MF) and rate of force development of the knee extensor muscles compared with controls. However, MF relative to body weight (BW) did not differ significantly in PD patients and controls. A longer chair rising time and lower maximal rate of vertical ground reaction force (VGRF) development while rising from a chair were found in PD patients compared with controls. No significant differences in maximal VGRF, and the sum of maximal VGRF of the right and left legs relative to the BW while rising from a chair were observed between the groups. In PD patients, chair rising time correlated negatively with MF of the knee extensor muscles. PD subjects also showed a positive correlation between the BW-related maximal VGRF while rising from a chair and MF of the knee extensor muscles relative to BW. These data suggest that subjects with PD are more deficient in the regulation of force-time parameters, rather than simply in force production of the knee extensor muscles.

Reliability of Maximum Isometric Force-Time Parameters during a Leg Press Test in Pubertal Basketball Players

Fifteen young basketball players (aged 14.4 – 0.5 yrs) underwent two identical testing sessions spaced one week apart, to determine the reliability of maximum isometric force and force-time parameters during a maximal bilateral isometric leg press effort. The maximal isometric force (MIF), the ratio of maximal force to time (TMIF) to attain maximal force (ARMIF), starting strength (F50), and on a relative scale the time taken to increase the force from 10% to 30%, 60%, and 90% of maximal force were calculated. High intraclass correlation coefficients (ICC) were found for MIF (0.96), ARMIF (0.85), and F50 (0.90). On the relative scale, the ICCs for the times to produce 30%, 60%, and 90% of maximum force were 0.94, 0.95, 0.95, respectively. The present results indicate that maximum isometric force and the force-time parameters during a bilateral leg press can be measured reliably in pubertal basketball players.

Effects of Concurrent Endurance and Explosive Type Strength Training on Neuromuscular Characteristics in Endurance Athletes

Introduction: In previously untrained male and women subjects explosive type strength training results in great increases in the amount of neural input to the trained muscles contributing largely to the improvements in explosive force production (Häkkinen 1994). It has been suggested, that also in endurance athletes, during relatively short training periods of some weeks, the improvements in force and velocity characteristics might primarily come from neural adaptations, although no electromyographic (EMG) measurements on the muscles have been performed to support this suggestion (Paavolainen et al. 1999). Therefore, the aim of the present study was to investigate the effects of concurrent endurance and explosive strength training on EMG and force production characteristics of leg extensor muscles in well-trained endurance athletes. Methods: Six experimental (E) and 6 control (C) endurance athletes trained for 8 wk. The total training volume was kept the same in both groups, but 26% of training in E and 2% in C was replaced by explosive type strength training. It consisted of various concentric and stretch-shortening cycle exercises for leg extensor muscles performed with low loads but high velocities. A dynamometer was used to measure maximal bilateral isometric force and various force-time parameters of the leg extensor muscles. Three to five maximal isometric actions were performed at the knee and hip angles of 90°. EMG-activity was recorded from the vastus lateralis (VL), vastus medialis (VM) and rectus femoris (RF) muscles of the right leg. Results: Maximal isometric force did not change after 8-wk training but in E the average force-time curve changed in the absolute scale so that the times to reach lower force levels shortened (p < 0.01) and early forces (in 0–100 ms) increased (p < 0.01). Maximal average integrated EMG (AEMG) of the VL, VM and RF muscles increased in E during training and the changes were significant (p < 0.05–0.01) in the early portions of the isometric actions. No changes were observed in C. In E the relative increases in the AEMG of VL (0–100 ms) correlated with the relative increases in force (0–100 ms) (r = 0.82, p < 0.05). Discussion: The present 8-wk explosive type strength training resulted in considerable improvements in selected neuromuscular characteristics, although a large volume of endurance training was performed concomitantly. This supports previous findings (e.g. Paavolainen et al. 1999) that in well-trained endurance athletes training-induced improvements in neuromuscular characteristics may not be fully inhibited by concurrent explosive strength and endurance training. A specific training-induced shift in the “early” portions of the AEMG-time curve indicated that the improvements in explosive force production were largely due to neural adaptations. The importance of the neural component was further supported by the correlation between the changes in the early AEMG and the changes in explosive force production during the early part of the isometric force-time curve. No changes were observed in maximal isometric force during training which further supports the concept of specificity of training, since the training program did not include resistance exercises with high loads.

Force developmental phase and reliability in explosive and voluntary grip exertions.

The purposes of this study were to clarify the reliability of two types of grip exertions, explosive grip exertion and voluntary grip exertion up to the maximal grip strength and to examine their force patterns using force-time parameters. Subjects were 100 healthy young male volunteers (age: 17.8+/-2.50 yr.) who had no upper limb injury. Grip strength was measured two times with voluntary grip and then two times with explosive grip. 11 parameters derived from the force-time curve were selected to measure the developmental phase of muscle contraction. The reliability of maximal grip strength in explosive and voluntary grip exertions was very high (intraclass correlation = .95, .93). The difference between two trials in explosive grip tended to be smaller than that for voluntary grip, and reliability of the exertion pattern was higher for explosive grip than voluntary grip. The times to reach 90% of the maximal grip strength and the maximal grip strength in explosive grip were shorter than those in voluntary grip. The other 8 force-time parameters had higher values in explosive grip than voluntary grip and higher reliabilities. The force-time parameters reflect the properties of explosive exertion. The results suggest the possibility that static explosive strength could be evaluated using these force-time parameters.

FORCE DEVELOPMENTAL PHASE AND RELIABILITY IN EXPLOSIVE AND VOLUNTARY GRIP EXERTIONS

The purposes of this study were to clarify the reliability of two types of grip exertions, explosive grip exertion and voluntary grip exertion up to the maximal grip strength and to examine their force patterns using force-time parameters. Subjects were 100 healthy young male volunteers (age: 17.8 ±2.50 yr.) who had no upper limb injury. Grip strength was measured two times with voluntary grip and then two times with explosive grip. 11 parameters derived from the force-time curve were selected to measure the developmental phase of muscle contraction. The reliability of maximal grip strength in explosive and voluntary grip exertions was very high (intraclass correlation = .95, .93). The difference between two trials in explosive grip tended to be smaller than that for voluntary grip, and reliability of the exertion pattern was higher for explosive grip than voluntary grip. The times to reach 90% of the maximal grip strength and the maximal grip strength in explosive grip were shorter than those in voluntary grip. The other 8 force-time parameters had higher values in explosive grip than voluntary grip and higher reliabilities. The force-time parameters reflect the properties of explosive exertion. The results suggest the possibility that static explosive strength could be evaluated using these force-time parameters.

Examination of force-production properties during static explosive grip based on force-time curve parameters.

The present study attempts to clarify the properties of force-time curves in the force-development phase during static explosive grip exertion and to determine force-time parameters in relation to static explosive strength. 80 healthy young males (age 17.8 +/- 2.5 yr.) exerted maximal isometric force as fast and forcefully as possible. In total, 21 variables, e.g., time to fixed level, integrated area, average force, integrated area to fixed time, force at the maximal rate of force development, and maximal rate of force development, were selected as force-time parameters. Good reliability was obtained for average force in force levels above 90% of maximal strength (.64-.93), integrated area to a fixed time of 1.0-2.0 sec. (.86-.93), force at the maximal rate of force development (.67), and maximal rate of force development (.73). In addition, these values correlated closely with maximal grip strength (r= .65-.93). Further, significant differences in maximal grip strength, average force in force levels above 90% of maximal strength, integrated area to fixed time, and force at the maximal rate of force development were found among three groups categorized by average force in force levels of maximal strength. These results suggested that individual differences were present in the force-time curve patterns during the force-development phase of static explosive grip exertion and that the average force in force levels of maximal strength might be a valuable tool for evaluating static explosive strength.