Effect of different types of conditioning contraction on upper body postactivation potentiation

Muscle contractions preceding an activity can result in increased force generation (postactivation potentiation [PAP]). Although the type of muscular contractions could affect subsequent strength and power performance, little information exists on their effects. The purpose of this study was to examine PAP effects produced by isometric (ISO), concentric (CON), eccentric (ECC), or concentric-eccentric (DYN) conditioning contractions on upper body force and power performance. Ten male, competitive rugby players (mean ± SD: age 20.4 ± 0.8 years, height 177.0 ± 8.1 cm, body mass 90.2 ± 13.8 kg) performed a ballistic bench press throw (BBPT) followed by a 10-minute rest and one of the conditioning contractions. After a 12-minute rest, the subjects performed another BBPT (post-BBPT). The conditioning contractions, applied on separate days and in counterbalanced randomized order, were a 7-second isometric barbell bench press for ISO and 1 set of 3 bench press repetitions at 3 repetition maximum for CON, ECC, and DYN (each repetition lasting 2 seconds for CON and ECC, overall execution time <7 seconds for DYN). Peak power (Ppeak), peak force (Fpeak), maximum distance (Dmax) and rate of force development (RFD) were measured using a linear position transducer. Electromyography (EMG) of the pectoralis major and triceps brachii was also recorded. The ISO produced significantly higher Ppeak (587 ± 116 and 605 ± 126 W for pre- and post-BBPT, respectively; p < 0.05). No significant differences in Ppeak were revealed for CON, ECC, and DYN (p > 0.05), and no significant differences existed in Fpeak, Dmax, and RFD for ISO, CON, ECC, and DYN (p > 0.05). Finally, EMG was not significantly different between pre- and post-BBPT for any of the conditioning contractions (p > 0.05). Isometric contractions appear to be the only conditioning contractions increasing upper body power output after long resting periods.     

Mechanical load increases muscle IGF-I and androgen receptor mRNA concentrations in humans

The mechanism(s) of load-induced muscle hypertrophy is as yet unclear, but increasing evidence suggests a role for locally expressed insulin-like growth factor I (IGF-I). We investigated the effects of concentric (CON) vs. eccentric (ECC) loading on muscle IGF-I mRNA concentration. We hypothesized a greater IGF-I response after ECC compared with CON. Ten healthy subjects (24.4 +/- 0.7 yr, 174.5 +/- 2.6 cm, 70.9 +/- 4.3 kg) completed eight sets of eight CON or ECC squats separated by 6-10 days. IGF-I, IGF binding protein-4 (IGFBP-4), and androgen receptor (AR) mRNA concentrations were determined in vastus lateralis muscle by RT-PCR before and 48 h after ECC and CON. Serum total testosterone (TT) and IGF-I were measured serially across 48 h, and serum creatine kinase activity (CK), isometric maximum voluntary contraction (MVC), and soreness were determined at 48 h. IGF-I mRNA concentration increased 62% and IGFBP-4 mRNA concentration decreased 57% after ECC (P < 0.05). Changes after CON were similar but not significant (P = 0.06-0.12). AR mRNA concentration increased (P < 0.05) after ECC (63%) and CON (102%). Serum TT and IGF-I showed little change. MVC fell 10% and CK rose 183% after ECC (P < 0.05). Perceived soreness was higher (P < 0.01) after ECC compared with CON. Results indicate that a single bout of mechanical loading in humans alters activity of the muscle IGF-I system, and the enhanced response to ECC suggests that IGF-I may somehow modulate tissue regeneration after mechanical damage.     

Older adults are less steady during submaximal isometric contractions with the knee extensor muscles.

This study compared the steadiness of submaximal contractions with the knee extensor muscles in young and old adults. Twenty young and twenty old subjects underwent assessment of isometric maximum voluntary contraction (MVC), one-repetition maximum (1-RM) strength, and steadiness during isometric, concentric, and eccentric contractions with the knee extensor muscles. The old adults displayed 33% lower MVC force and a 41% lower 1-RM load. The coefficient of variation for force was significantly greater for the old adults during isometric contractions at 2, 5, and 10% of MVC but not at 50% MVC. The decline in steadiness at low forces experienced by the men was marginally greater than that experienced by the women. The steadiness of concentric and eccentric contractions was similar in young and old adults at 5, 10, and 50% of 1-RM load. Old subjects exhibited greater coactivation of an antagonist muscle compared with young subjects during the submaximal isometric and anisometric contractions. These results indicate that, whereas the ability to exert steady submaximal forces with the knee extensor muscles was reduced in old adults, fluctuations in knee joint angle during slow movements were similar for young and old adults.     

The effects of isokinetic contraction velocity on concentric and eccentric strength of the biceps brachii

The purpose of this investigation was to determine the influence of contraction velocity on the eccentric (ECC) and concentric (CON) torque production of the biceps brachii. After performing warm-up procedures, each male subject (n = 11) completed 3 sets of 5 maximal bilateral CON and ECC isokinetic contractions of the biceps at speeds of 90, 180, and 300 degrees x s(-1) on a Biodex System 3 dynamometer. The men received a 3-minute rest between sets and the order of exercises was randomized. Peak torque (Nm) values were obtained for CON and ECC contractions at each speed. Peak torque scores (ECC vs. CON) were compared using a t-test at each speed. A repeated measures analysis of variance was used to determine differences between speeds. ECC peak torque scores were greater than CON peak torque scores at each given speed: 90 degrees x s(-1), p = 0.0001; 180 degrees x s(-1), p = 0.0001; and 300 degrees x s(-1), p = 0.0001. No differences were found between the ECC peak torque scores (p = 0.62) at any of the speeds. Differences were found among the CON scores (p = 0.004). Post hoc analysis revealed differences between 90 degrees x s(-1) (114.61 +/- 23) and 300 degrees x s(-1) (94.17 +/- 18). These data suggest that ECC contractions of the biceps brachii were somewhat resistant to a force decrement as the result of an increase in velocity, whereas CON muscular actions of the biceps brachii were unable to maintain force as velocity increased.     

Strength training improves the steadiness of slow lengthening contractions performed by old adults.

When old adults participate in a strength-training program with heavy loads, they experience an increase in muscle strength and an improvement in the steadiness of submaximal isometric contractions. The purpose of this study was to determine the effect of light- and heavy-load strength training on the ability of old adults to perform steady submaximal isometric and anisometric contractions. Thirty-two old adults (60-91 yr) participated in a 4-wk training program of a hand muscle. Both the light- and heavy-load groups increased one-repetition maximum and maximal voluntary contraction (MVC) strength and experienced similar improvements in the steadiness of the isometric and shortening and lengthening contractions. The increase in MVC strength was greater for the heavy-load group and could not be explained by changes in muscle activation. Before training, the lengthening contractions were less steady than the shortening contractions with the lightest loads (10% MVC). After training, there was no difference in steadiness between the shortening and lengthening contractions, except with the lightest load. These improvements were associated with a reduced level of muscle activation, especially during the lengthening contractions.     

Acoustic myography for investigating human skeletal muscle fatigue.

Sounds produced during voluntary isometric contractions of the quadriceps muscle were studied by acoustic myography (AMG) in five healthy adults. With the subject seated, isometric force, surface electromyography (EMG), and AMG were recorded over rectus femoris, and the EMG and AMG signals were integrated (IEMG and IAMG). Contractions lasting 5 s each were performed at 10, 25, 50, 60, 75, and 100% of maximum voluntary contraction (MVC) force. Fatigue was then induced by repeated voluntary contractions (10 s on, 10 s off) at 75% MVC until only 40% MVC could be sustained. After 15 min of rest, the different force levels were again tested in relation to the fresh MVC. Both before and after fatiguing activity the relationships between force and IEMG [r = 0.99 +/- 0.01 (SD), n = 10] and force and IAMG (r = 0.98 +/- 0.02) were linear. After activity, however, the slopes of the regression lines for force and IEMG increased (P less than 0.01) but those for force and IAMG remained the same (P greater than 0.05). The present results clarify the relationship between AMG and isometric force in fatigued muscle without the problem of fatigue-induced tremor, which hampered previous studies of prolonged activity. This study contributes to the validation of AMG and shows that it is a potentially useful method for noninvasive assessment of force production and fatigue. Further studies to establish the origin of AMG activity are required before AMG can be accepted for use in neuromuscular physiology or rehabilitation.     

The repeated-bout effect: influence on biceps brachii oxygenation and myoelectrical activity

This study investigated biceps brachii oxygenation and myoelectrical activity during and following maximal eccentric exercise to better understand the repeated-bout effect. Ten men performed two bouts of eccentric exercise (ECC1, ECC2), consisting of 10 sets of 6 maximal lengthening contractions of the elbow flexors separated by 4 wk. Tissue oxygenation index minimum amplitude (TOI(min)), mean and maximum total hemoglobin volume by near-infrared spectroscopy, torque, and surface electromyography root mean square (EMG(RMS)) during exercise were compared between ECC1 and ECC2. Changes in maximal voluntary isometric contraction (MVC) torque, range of motion, plasma creatine kinase activity, muscle soreness, TOI(min), and EMG(RMS) during sustained (10-s) and 30-repeated isometric contraction tasks at 30% (same absolute force) and 100% MVC (same relative force) for 4 days postexercise were compared between ECC1 and ECC2. No significant differences between ECC1 and ECC2 were evident for changes in torque, TOI(min), mean total hemoglobin volume, maximum total hemoglobin volume, and EMG(RMS) during exercise. Smaller (P < 0.05) changes and faster recovery of muscle damage markers were evident following ECC2 than ECC1. During 30% MVC tasks, TOI(min) did not change, but EMG(RMS) increased 1-4 days following ECC1 and ECC2. During 100% MVC tasks, EMG(RMS) did not change, but torque and TOI(min) decreased 1-4 days following ECC1 and ECC2. TOI(min) during 100% MVC tasks and EMG(RMS) during 30% MVC tasks recovered faster (P < 0.05) following ECC2 than ECC1. We conclude that the repeated-bout effect cannot be explained by altered muscle activation or metabolic/hemodynamic changes, and the faster recovery in muscle oxygenation and activation was mainly due to faster recovery of force.     

Maximal eccentric and concentric strength discrepancies between young men and women for dynamic resistance exercise

Although research has demonstrated that isokinetic eccentric (ECC) strength is 20-60% greater than isokinetic concentric (CON) strength, few data exist comparing these strength differences in standard dynamic resistance exercises. The purpose of the study was to determine the difference in maximal dynamic ECC and CON strength for 6 different resistance exercises in young men and women. Ten healthy young men (mean +/- SE, 25.30 +/- 1.34 years), and 10 healthy young women (mean +/- SE, 23.40 +/- 1.37 years) who were regular exercisers with resistance training experience participated in the study. Two sessions were performed to determine CON and ECC 1 repetitions maximum for latissimus pull-down (LTP), leg press (LP), bench press (BP), leg extension (LE), seated military press (MP), and leg curl (LC) exercises. Maximal ECC and maximal CON strength were determined on weight stack machines modified to isolate ECC and CON contractions using steel bars and pulleys such that only 1 type of contraction was performed. Within 2 weeks, participants returned and completed a retest trial in a counterbalanced fashioned. Test-retest reliability was excellent (r = 0.99) for all resistance exercise trials. Men demonstrated 20-60% greater ECC than CON strength (LTP = 32%, LP = 44%, BP = 40%, LE = 35%, MP = 49%, LC = 27%). Women's strength exceeded the proposed parameters for greater ECC strength in 4 exercises, p < 0.05 (LP = 66%, BP = 146%, MP = 161%, LC = 82%). The ECC/CON assessment could help coaches capitalize on muscle strength differences in young men and women during training to aid in program design and injury prevention and to enhance strength development.     

Endurance capacity of untrained males and females in isometric and dynamic muscular contractions

The capacity to perform isometric and dynamic muscle contractions at different forces has been measured in two separate groups of subjects: 25 men and 25 women performed sustained isometric contractions of the knee-extensor muscles of their stronger leg to fatigue, at forces corresponding to 80%, 50% and 20% of the maximum voluntary force of contraction (MVC). The second experimental model involved a bilateral elbow-flexion weight lifting exercise. Eleven women and 12 men performed repetitions at loads corresponding to 90%, 80%, 70%, 60% and 50% of maximum load (1RM), at a rate of 10 X min-1 to the point of fatigue. Males were stronger (p less than 0.001) than females in both the static (675 +/- 120 N vs 458 +/- 80 N; mean +/- SD) and dynamic (409 +/- 90 N vs 190 +/- 33 N) contractions. Isometric endurance time of the males at a force corresponding to 20% of MVC was less than that of the females (180 +/- 51 s vs 252 +/- 56 s; p less than 0.001) but there was no difference between the sexes at 50% or 80% of MVC. Similarly, when the sexes were compared using dynamic elbow-flexion exercise, the female subjects were able to perform a greater number of repetitions than males at loads of 50% (p less than 0.005), 60% (p less than 0.001) and 70% (p less than 0.025) of 1RM, but there was no difference between the sexes at loads of 80% or 90% of 1RM.(ABSTRACT TRUNCATED AT 250 WORDS)     

Repeated contractions alter the geometry of human skeletal muscle.

The aim of this study was to investigate the effect of repeated contractions on the geometry of human skeletal muscle. Six men performed two sets (sets A and B) of 10 repeated isometric plantarflexion contractions at 80% of the moment generated during plantarflexion maximal voluntary contraction (MVC), with a rest interval of 15 min between sets. By use of ultrasound, the geometry of the medial gastrocnemius (MG) muscle was measured in the contractions of set A and the displacement of the MG tendon origin in the myotendinous junction was measured in the contractions of set B. In the transition from the 1st to the 10th contractions, the fascicular length at 80% of MVC decreased from 34 +/- 4 (means +/- SD) to 30 +/- 3 mm (P < 0.001), the pennation angle increased from 35 +/- 3 to 42 +/- 3 degrees (P < 0.001), the myotendinous junction displacement increased from 5 +/- 3 to 10 +/- 3 mm (P < 0.001), and the average fascicular curvature remained constant (P > 0.05) at approximately 4.3 m(-1). No changes (P > 0.05) were found in fascicular length, pennation angle, and myotendinous junction displacement after the fifth contraction. Electrogoniometry showed that the ankle rotated by approximately 6.5 degrees during contraction, but no differences (P > 0.05) were obtained between contractions. The present results show that repeated contractions induce tendon creep, which substantially affects the geometry of the in-series contracting muscles, thus altering their potential for force and joint moment generation.     

Efficacy of prior eccentric exercise in attenuating impaired exercise performance after muscle injury in resistance trained men

Previous research has demonstrated that prior exercise may reduce the magnitude of muscle soreness and impaired function (i.e., repeated bout effect [RBE]) observed during subsequent eccentric exercise. Previous investigations have predominantly used research designs that include single-joint exercise performed by untrained individuals. It is unknown how resistance trained individuals respond to novel multi-joint eccentric actions of the upper body and whether prior exercise offers protection. Thirty-one resistance trained men (23.4 +/- 3.5 y, 177.2 +/- 5.1 cm, 86.4 +/- 16.5 kg, mean +/- SD) were randomly assigned to repeated bout ([RB] N = 15) or single bout ([CON] N = 16) conditions. Both groups performed 100 eccentric actions of the bench press ([ECC] at 70% concentric 1 repetition maximum) to induce muscle injury. Bilateral maximal isometric force, dynamic exercise performance (e.g., bench press throws), and muscle soreness were measured before, immediately after, and at 24 and 48 hours post-ECC. Total work, percent fatigue, and rating of perceived exertion (ECC) data were collected during ECC. Those assigned to RB condition exhibited less fatigue (9.5 vs. 22.6%) and lower RPE (14.8 vs. 17.1) during ECC. A significant interaction (p < 0.05) was found such that RB individuals experienced less soreness at 24 (6.5 vs. 4.9) and 48 (6.6 vs. 3.9) hours postexercise than the CON condition. No significant group differences (p < 0.05) were found for any measured performance variable. Although soreness, fatigue, and RPE suggest a RBE, this was not found in regards to exercise performance. It appears that in trained men, performing a strenuous high-volume eccentric exercise bout 2 weeks prior to an identical future bout offers no additional amelioration of impaired exercise performance.     

Mechanisms contributing to knee extensor strength loss after prolonged running exercise.

The aim of this study was to identify the mechanisms that contribute to the decline in knee extensor (KE) muscles strength after a prolonged running exercise. During the 2 days preceding a 30-km running race [duration 188.7 +/- 27.0 (SD) min] and immediately after the race, maximal percutaneous electrical stimulations (single twitch, 0.5-s tetanus at 20 and 80 Hz) were applied to the femoral nerve of 12 trained runners. Superimposed twitches were also delivered during isometric maximal voluntary contraction (MVC) to determine the level of voluntary activation (%VA). The vastus lateralis electromyogram was recorded. KE MVC decreased from pre- to postexercise (from 188.1 +/- 25.2 to 142.7 +/- 29.7 N x m; P < 0.001) as did %VA (from 98.8 +/- 1.8 to 91.3 +/- 10.7%; P < 0.05). The changes from pre- to postexercise in these two variables were highly correlated (R = 0.88; P < 0.001). The modifications in the mechanical response after the 80-Hz stimulation and M-wave peak-to-peak amplitude were also significant (P < 0.001 and P < 0.05, respectively). It can be concluded that 1) central fatigue, neuromuscular propagation, and muscular factors are involved in the 23.5 +/- 14.9% reduction in MVC after a prolonged running bout at racing pace and 2) runners with the greatest KE strength loss experience large activation deficit.     

Different neuromuscular recruitment patterns during eccentric, concentric and isometric contractions

Aim. The purpose of this study was to determine the neuromuscular fatigue profiles during 100 s isometric (ISO), concentric (CON), and eccentric (ECC) activity.

Methods. Twelve subjects (age 25.1±3.7 years, mass 70.1±8.2 kg, mean±SD) performed ISO, CON and ECC maximal voluntary contractions and 100 s endurance trials on an isokinetic dynamometer. Raw EMG data were recorded throughout each trial from the rectus femoris of the right limb. Corresponding data for integrated electromyography (IEMG), percentile frequency shifts (MPFS) and peak torque output were divided into five 5 s epochs and subsequently normalised with the first epoch being the reference point, in order to assess changes over time.

Results. There were no significant differences between ECC, CON and ISO peak torque output (211±63 vs 169±41 vs 177±61 Nm; ECC, CON, ISO) and IEMG activity (280±143 vs 305±146 vs 287±143 mV; ECC, CON, ISO) during maximal contractions. Serial reductions in torque output were greatest in ISO in which torque output during the final epoch was 31±13% of initial values, similar to the final torque values in CON (58±15%), but significantly less than ECC (108.6±38.6%; P<0.001) values. In CON and ECC, IEMG was maintained (95±27% and 93±21%; CON and ECC), whereas IEMG for ISO decreased to 38±13% of initial values. The greatest reduction in MPFS occurred in CON (69±10%) compared to ISO (78±9%; P<0.05) and ECC (93±6%; P<0.001).

Conclusion. These data demonstrate distinct neuromuscular fatigue profiles for the different types of muscle contraction. Whereas eccentric activity was largely fatigue resistant, isometric and concentric contractions displayed different neuromuscular fatigue profiles.     

Age-related enhancement of fatigue resistance is evident in men during both isometric and dynamic tasks.

It has been suggested that the effects of old age on the ability to resist fatigue may be task dependent. To test one aspect of this hypothesis, we compared the neuromuscular responses of nine young (26 +/- 4 yr, mean +/- SD) and nine older (72 +/- 4 yr) healthy, relatively sedentary men to intermittent isometric (3 min, 5 s contract/5 s rest) and dynamic (90 at 90 degrees /s) maximum voluntary contractions (MVC) of the ankle dorsiflexor muscles. To assess the mechanisms of fatigue (defined as the ratio of postexercise MVC to preexercise MVC), we also measured isometric central activation ratios (CAR), tetanic torque, contractile properties, and compound muscle action potentials before and immediately after exercise. Because dynamic contractions are more neurally complex and metabolically demanding than isometric contractions, we expected an age-related fatigue resistance observed during isometric exercise to be absent during dynamic exercise. In contrast, older men (O) fatigued less than young (Y) during both isometric (O = 0.77 +/- 0.07, Y = 0.66 +/- 0.02, mean +/- SE; P < 0.01) and dynamic (O = 0.45 +/- 0.07, Y = 0.27 +/- 0.02; P = 0.04) contractions (ratio of postexercise to preexercise MVC), with no evidence of peripheral activation failure in either group. We observed no obvious limitations in central activation in either group, as assessed using isometric CAR methods, after both isometric and dynamic contractions. Preexercise half-time of tetanic torque relaxation, which was longer in O compared with Y, was linearly associated with fatigue resistance during both protocols (r = 0.62 and 0.66, P < or = 0.004, n = 18). These results suggest that relative fatigue resistance is enhanced in older adults during both isometric and isokinetic contractions and that age-related changes in fatigue may be due largely to differences within the muscle itself.     

Different effects of concentric and eccentric muscle actions on plasma volume

The effects of concentric (CON) and eccentric (ECC) contractions on Delta plasma volume (PV), heart rate (HR), and lactate in responses to protocols in different body positions were investigated. CON or ECC contractions were performed in either a single-exercise (6 sets of 12 repetitions of leg extensions completed at 80% of 12 repetition maximum [12RM] with 3-minute rest periods) or multiexercise (4 sets of 10 repetitions for both CON and ECC trials of bench press, leg extension, military press, and leg curl at 80% of 10RM with 90-second rest periods) protocols. HR and lactate increased significantly for both protocols from pre- to postexercise for CON but not ECC trials. DeltaPV was greater following both CON single-exercise (-11.48 +/- 1.38%) and multiexercise (-4.64 +/- 0.33%) trials vs. ECC single-exercise (-1.62 +/- 1.69%) and multiexercise (-1.26 +/- 1.20) trials. Data demonstrate ECC exercise in response to single and multiexercise protocols at the same absolute workload as CON exercise produces less cardiovascular stress.     

Adaptations in the activation of human skeletal muscle induced by short-term isometric resistance training.

This study employed longitudinal measures of evoked spinal reflex responses (Hoffman reflex, V wave) to investigate changes in the activation of muscle and to determine if there are "linked" neural adaptations in the motor pathway following isometric resistance training. Twenty healthy, sedentary males were randomly assigned to either the trained (n = 10) or control group (n = 10). The training protocol consisted of 12 sessions of isometric resistance training of the plantar flexor muscles over a 4-wk period. All subjects were tested prior to and after the 4-wk period. To estimate changes in spinal excitability, soleus Hoffman (H) reflex and M wave recruitment curves were produced at rest and during submaximal contractions. Recruitment curves were analyzed using the slope method (Hslp/Mslp). Modulation of efferent neural drive was assessed through evoked V wave responses (V/Mmax) at 50, 75, and 100% maximal voluntary contraction (MVC). After 4 weeks, MVC torque increased 20.0 +/- 13.9% (mean +/- SD) in the trained group. The increase in MVC was accompanied by significant increases in the rate of torque development (42.5 +/- 13.3%), the soleus surface electromyogram (60.7 +/- 30.8%), voluntary activation (2.8 +/- 0.1%), and the rate of activation (48.7 +/- 24.3%). Hslp/Mslp was not altered by training; however, V/Mmax increased 57.3 +/- 34.2% during MVC. These results suggest that increases in MVC observed in the first few days of isometric resistance training can be accounted for by an increase in the rate of activation at the onset of muscle contraction. Augmentation of muscle activation may be due to increased volitional drive from supraspinal centers.     

Changes in dynamic exercise performance following a sequence of preconditioning isometric muscle actions

Complex training is the method of coupling heavy and light loads into an organized sequence with the aim of facilitating postactivation potentiation. Anecdotal evidence has supported the use of complex training sequences, but scientific studies investigating the effects of sequencing isometric loads with dynamic muscle actions have been limited. The purpose of this study was to examine the effects of a preconditioning sequence of maximal isometric knee extensions on performance standards in selected dynamic whole-body exercise. Fourteen track and field athletes (23 +/- 5.7 years; 71.53 +/- 6.93 kg; 172.6 +/- 5.8 cm) were randomly assessed in selected whole-body exercises (drop and countermovement jumps, 5-second cycle sprint, knee extension) following a sequence of maximal voluntary isometric contractions (MVC; 3 repetitions of 3 seconds or 3 repetitions of 5 seconds) or in the absence of prior loading (control). Electromyographic (EMG) assessments of muscle activity were also made during the knee extension assessment. Significant (p < or = 0.05) increases in jump height (5.03%), maximal force (4.94%), and acceleration impulse (9.49%) were observed in the drop jump following 3 repetitions of 3-second MVC only. Knee extension maximal torque was also significantly increased (6.12%) following the 3-second MVC. No significant changes in countermovement jump or cycle sprint measures were observed for any of the experimental conditions. Though adaptations were found, changes in EMG activity were not significantly different for any of the experimental conditions. These data indicate that performing a sequence of repeated maximal isometric knee extensions (3 repetitions of 3 seconds) prior to selected dynamic exercise (< or =0.25 seconds) may have favorable effects on performance beyond standards achieved without prior heavy loading.     

Age and contraction type influence motor output variability in rapid discrete tasks.

The purpose of this study was to examine the ability to control knee-extension force during discrete isometric (IC), concentric (CC), and eccentric contractions (EC) in 24 young (mean age +/- SD = 25.3 +/- 2.8 yr) and 24 old (mean age +/- SD = 73.3 +/- 5.5 yr) healthy and active individuals. Subjects were to match a parabola with a time to peak force of 200 ms during IC, CC, and EC at six target levels of force [20, 35, 50, 65, 80, and 90% of the maximum voluntary contraction (MVC)]. ICs were performed at 90 degrees of knee flexion, whereas CCs and ECs ranged from 90 to 80 degrees of knee flexion (0 degrees is full extension) at a slow velocity (25 degrees /s). Results showed that subjects produced similar MVC forces for the three types of contractions. Young subjects produced greater MVC forces than old subjects, and within each age group, men produced greater force than women. The variability (standard deviation) of peak force and impulse in absolute values was greater for young compared with old subjects. When variability was normalized to the force produced [coefficient of variation (CV)], however, old subjects exhibited greater CV than young subjects for peak force and impulse. Both the standard deviation and CV of time to peak force and impulse duration were greater for the old adults. In general, ECs were more variable than ICs and CCs, and old adults exhibited greater CV compared with young adults during rapid, discrete ICs, CCs, and particularly ECs of the quadriceps.     

Muscle weakness following sustained and rhythmic isometric contractions in man

The effects of sustained and rhythmically performed isometric contractions on electrically evoked twitch and tetanic force generation of the triceps surae have been investigated in 4 healthy male subjects. The isometric contractions were performed separately and on different occasions at 30%, 60% and 100% of the force of maximal voluntary contraction (MVC). The area under the maximal voluntary contraction (MVC) force/time curve during the rhythmic and sustained contractions was the same for each experiment. The results showed that following rhythmic isometric exercise there was a small decrease in low (10 and 20 Hz) and high (40 Hz) frequency tetanic tension which was associated with % MVC. However, there was no change in the 20/40 ratio of tetanic forces, MVC or the contraction times and force of the maximal twitch. In contrast, following sustained isometric exercise tetanic forces were markedly reduced, particularly at low frequencies of stimulation. The 20/40 ratio decreased and the induced muscle weakness was greater at 30% than 60% or 100% MVC. The performance of sustained isometric contractions also effected a decrease in contraction time of the twitch and MVC. The results are in accord with previous findings for dynamic work (Davies and White 1982), and show that if isometric exercise is performed rhythmically the effect on tetanic tensions is small and there is no evidence of a preferential loss of electrically evoked force at either high or low frequencies of stimulation following the contractions. For sustained contractions, however, the opposite is true, the ratio of 20/40 Hz forces is markedly reduced and following 30% sustained MVC there is a significant (p less than 0.05) change in the time to peak tension (TPT) of the maximal twitch.