The effects of two forms of isometric training on the mechanical properties of the triceps surae in man

Effects of Low-Intensity Torque-Matched Isometric Training at Long and Short Muscle Lengths of the Hamstrings on Muscle Strength and Hypertrophy: A Randomized Controlled Study

ABSTRACT

Nakao, S, Ikezoe, T, Taniguchi, M, Motomura, Y, Hirono, T, Nojiri, S, Hayashi, R, Tanaka, H, and Ichihashi, N. Effects of low-intensity torque-matched isometric training at long and short muscle lengths of the hamstrings on muscle strength and hypertrophy: A randomized controlled study. J Strength Cond Res 37(10): 1978-1984, 2023-This study investigated the effects of low-intensity torque-matched isometric training on muscle hypertrophy and strengthening at long (LL) and short muscle lengths (SL). Twenty-eight young subjects completed an 8-week hamstring isometric training program (30% of maximal voluntary contraction (MVC) × 5 s × 20 repetitions × 5 sets × 3 times/week) at 30° knee flexion (LL) or 90° knee flexion (SL). The cross-sectional area (CSA) of the hamstrings and MVC were measured before and after the intervention. The active torque because of muscle contraction was calculated by subtracting the passive torque at rest from the total torque (30% MVC). The active torque was significantly lower in the LL training group than in the SL training group (p < 0.01), whereas there was no between-group difference in total torque during training. For CSA and MVC at 30° knee flexion, the split-plot analysis of variance (ANOVA) showed no significant time × group interaction; however, it did show a significant main effect of time (p < 0.05), indicating a significant increase after training intervention. As for MVC at 90° knee flexion, there was a significant time × group interaction (p < 0.05) and a significant simple main effect of time in both the LL (p < 0.01; Cohen's d effect size [ES] = 0.36) and SL (p < 0.01; ES = 0.64) training groups. Therefore, low-intensity isometric training at LL can induce hypertrophy and strengthening, even in cases where the active torque production is lower than that at SL, whereas the training at SL may be more effective for muscle strengthening at SL.

Association between interindividual variability in training volume and strength gain

This study aimed to examine the association between interindividual variability in strength changes and in training volume. A total of 26 untrained men completed 4-weeks of isometric knee extension (KE group, n = 12) and hip flexion (HF group, n = 14) training. Each training session comprised four sets of ten isometric contractions, 3-s contractions every 20 s. Training volume, which was defined as impulse during contractions, and maximal voluntary contraction (MVC) torque during KE and HF were evaluated. Based on the magnitude of MVC torque changes, the participants were divided into the high and low responders (n = 13; KE = 6 and HF = 7 per responders). The MVC torque changes (KE, 20.8%; HF, 22.4%) and total training volume did not significantly differ between the two groups. A higher training volume was demonstrated in the low responders than the high responders. The total training volume was positively associated with the MVC torque changes in low responders (r = 0.869%, 95% confidence interval [0.610, 0.960], p < 0.001), but not in high responders [r = 0.229, 95% confidence interval (−0.368, 0.693), p = 0.451], KE or HF group. Results showed that training volume was an important factor in determining the magnitude of strength gains in low responders, and MVC torque could improve by approximately 20% with the use of the study protocol regardless of joint actions involved during training.

Comparing Functional Motor Control Exercises With Therapeutic Exercise in Wrestlers With Iliotibial Band Syndrome

Context: Iliotibial band syndrome (ITBS) is a common overuse injury in runners with parallels to our findings of overuse in Greco Roman wrestlers. Despite research indicating coordination and movement-based factors about the hip, no studies were found using functional motor control (FMC) in runners or wrestlers with ITBS. Thus, we compared FMC exercises and therapeutic exercises (TEs) on pain, function, muscle strength, and range of motion (ROM) in national-level Greco Roman wrestlers with ITBS. Design: Controlled laboratory study. Methods: Sixty national-level Greco Roman wrestlers diagnosed with ITBS were randomly assigned to 8 weeks of FMC exercises, TE, and a control group (20 individuals for each group). Pain (visual analog scale), function (triple hop test for distance, single-leg vertical jump test, and agility T test), muscle strength (handheld dynamometer), and ROM (goniometer) were measured at baseline and 8 weeks after intervention as posttest. Results: Although both interventions significantly reduced pain (P < .001, η2 = .87), improved function (triple hop test P = .004, η2 = .94; single-leg vertical jump P = .002, η2 = .93; and T test P < .001, η2 = .93) and strength (hip abduction (P < .001, η2 = .52), hip external rotation (P = .02, η2 = .95), knee flexion (P ≤ .001, η2 = .94), and knee extension (P < .001, η2 = .91) compared with the control group, FMC showed more significant improvements in comparison with TE. Significant differences (P = .001) were observed between FMC and TE compared with the control group in ROM outcome. However, TE was more effective than FMC in improving ROM hip abduction (P < .001, η2 = .93), hip adduction (P = .000, η2 = .92), hip internal rotation (P < .001, η2 = .92), and hip external rotation (P < .001, η2 = .93). Conclusion: FMC exercises were superior to TE in terms of pain, function, and muscle strength, whereas TE was more effective for improving ROM. FMC exercise is suggested as an effective intervention for improvement of the outcomes related to ITBS in national-level Greco Roman wrestlers.

A Systematic Review and Meta-Analysis on the Longitudinal Effects of Unilateral Knee Extension Exercise on Muscle Strength

The aim of the study was to investigate the time-dependent increase in the knee extensors' isometric strength as a response to voluntary, unilateral, isometric knee extension exercise (UIKEE). To do so, a systematic review was carried out to obtain data for a Bayesian longitudinal model-based meta-analysis (BLMBMA). For the systematic review, PubMed, Web of Science, SCOPUS, Chochrane Library were used as databases. The systematic review included only studies that reported on healthy, young individuals performing UIKEE. Studies utilizing a bilateral training protocol were excluded as the focus of this review lied on unilateral training. Out of the 3,870 studies, which were reviewed, 20 studies fulfilled the selected inclusion criteria. These 20 studies were included in the BLMBMA to investigate the time-dependent effects of UIKEE. If compared to the baseline strength of the trained limb, these data reveal that UKIEE can increase the isometric strength by up to 46%. A meta-analysis based on the last time-point of each available study was employed to support further investigations into UIKEE-induced strength increase. A sensitivity analysis showed that intensity of training (%MVC), fraction of male subjects and the average age of the subject had no significant influence on the strength gain. Convergence of BLMBMA revealed that the peak strength increase is reached after ~4 weeks of UIKEE training.

Training load but not fatigue affects cross-education of maximal voluntary force

The purpose of this study was to determine the effects of training load (25% vs. 75% of one repetition maximum [1RM]) and fatigue (failure vs. non-failure) during four weeks of unilateral knee extension resistance training (RT) on maximal voluntary force in the trained and the untrained knee extensors. Healthy young adults (n = 42) were randomly assigned to control (CON, n = 9, 24 ± 4.3 years), low-load RT to failure (LLF, n = 11, 21 ± 1.3 years, three sets to failure at 25% of 1RM), high-load RT to failure (HLF, n = 11, 21 ± 1.4 years, three sets to failure at 75% of 1RM), and high-load RT without failure (HLNF, n = 11, 22 ± 1.5 years, six sets of five repetitions at 75% of 1RM) groups. Before and after the four weeks of training, 1RM, maximal voluntary isometric force, and corticospinal excitability (CSE) were measured. 1RM in the trained (20%, d = 0.70, 15%, d = 0.61) and the untrained knee extensors (5%, d = 0.27, 6%, d = 0.26) increased only in the HLF and HLNF groups, respectively. MVIC force increased only in the trained leg of the HLF (5%, d = 0.35) and HLNF groups (12%, d = 0.67). CSE decreased in the VL of both legs in the HLNF group (-19%, d = 0.44) and no changes occurred in the RF. In conclusion, high- but not low-load RT improves maximal voluntary force in the trained and the untrained knee extensors and fatigue did not further enhance these adaptations. Voluntary force improvements were unrelated to CSE changes in both legs.

Eccentric Versus Concentric Resistance Training to Enhance Neuromuscular Activation and Walking Speed Following Stroke

Background. Impaired voluntary neuromuscular activation of agonist muscles is a primary determinant of weakness and motor dysfunction following stroke. Objective. To determine whether eccentric resistance training (ECC) resistance training is superior to concentric resistance training (CON) resistance training to enhance neuromuscular activation, strength, and walking speed after stroke. Methods. A total of 34 adults poststroke participated in a staged intervention comprising (1) either CON-only or ECC-only resistance training of the paretic leg followed by (2) gait training. Changes in voluntary neuromuscular activation and power were assessed for both the trained paretic and untrained nonparetic legs. Self-selected and fast walking speeds were also assessed. Results. In response to resistance training, the ECC group experienced larger improvements in neuromuscular activation of paretic leg muscles, rectus femoris and vastus medialis ( P < .005), and the largest gains in paretic leg power (+74% for ECC contractions, P < .0001). ECC also had greater cross-education of increased power to the untrained nonparetic leg (12%-14%, P = .006). Over the course of gait training, much of the gain in paretic leg activation in the ECC group was lost, such that the net change in agonist activation was comparable between the CON and ECC groups when the full intervention was completed. Nevertheless, improvement in walking speed postintervention was more prevalent in the ECC than CON group. Conclusion. ECC resistance training was more effective for improving bilateral neuromuscular activation, strength, and walking speed following stroke. Future research should assess whether a longer duration ECC training program can provide further benefit.

The potential of toe flexor muscles to enhance performance

The metatarsal phalangeal joint (MPJ) and its crossing toe flexor muscles (TFM) represent the link between the large energy generating leg extensor muscles and the ground. The purpose of this study was to examine the functional adaptability of TFM to increased mechanical stimuli and the effects on walking, running and jumping performance. Fifteen men performed a heavy resistance TFM strength training with 90% of the maximal voluntary isometric contraction (MVIC) for 7 weeks (560 contractions) for the left and right foot. Maximal MPJ and ankle plantar flexion moments during MVICs were measured in dynamometers before and after the intervention. Motion analyses (inverse dynamics) were performed during barefoot walking, running, and vertical and horizontal jumping. Athletic performance was determined by measuring jump height and distance. Left (0.21 to 0.38 Nm · kg(-1); P < 0.001) and right (0.24 to 0.40 Nm · kg(-1); P < 0.001) MPJ plantar flexion moments in the dynamometer, external MPJ dorsiflexion moments (0.69 to 0.75 Nm · kg(-1); P = 0.012) and jump distance (2.25 to 2.31 m; P = 0.006) in horizontal jumping increased significantly. TFM responded highly to increased loading within a few weeks. The increased force potential made a contribution to an athlete's performance enhancement.

Superimposed vibration confers no additional benefit compared with resistance training alone

Eighteen participants (22-43 years) were randomly allocated to one of two groups: resistance training combined with vibration (VIB; five males, four females) or resistance training alone (CON; five males, four females). Each participant trained three sessions per week (three sets of 10 seated calf raises against a load, which was increased progressively from 75% of one repetition maximum (1RM) to 90% 1RM for 4 weeks. For the VIB group, a vibratory stimulus (30 Hz, 2.5 mm amplitude) was applied to the soles of the feet by a vibration platform. The two groups did not differ significantly with respect to the total amount of work performed during training. Both groups showed a significant increase in maximum voluntary contraction and 1RM (P<0.01) with training. There were no significant changes in measures that assessed the rate at which force was developed. Countermovement jump height increased for the CON (P<0.01) but not for the VIB group. Comparisons between the groups revealed that they did not differ significantly from one another with respect to any measure of performance, before or following training. It appears that vibration superimposed upon resistance training does not alter or augment the increase in strength induced by resistance training alone.

Effects of training on muscle and tendon in knee extensors and plantar flexors in vivo

The purpose of this study was to compare the effects of resistance training on muscle and tendon properties between knee extensors and plantar flexors in vivo. Twenty healthy young men voluntarily participated in this study. The subjects were randomly divided into two training groups: knee extension group (n=10) and plantar flexion group (n=10). They performed five sets of exercises with a 1-min rest between sets, which consisted of unilateral knee extension for the knee extension group and plantar flexion for the plantar flexion group at 80% of 1 repetition maximum with 10 repetitions per set (4 days/wk, 12 wk). Before and after training, muscle strength, neural activation level (by interpolated twitch), muscle volume (by magnetic resonance imaging), and tendon stiffness (by ultrasonography) were measured. There were no differences in the training-induced increases in muscle strength, activation level, muscle volume, and tendon stiffness between knee extensors and plantar flexors. These results suggested that if the used protocol of training (i.e., intensity, repetition, etc.) were the same, there were no differences in the training-induced changes in muscle and tendon properties between knee extensors and plantar flexors.

Protein synthesis and the expression of growth-related genes are altered by running in human vastus lateralis and soleus muscles

Recent evidence suggests aerobic exercise may help preserve soleus muscle mass during unloading. The purpose of this investigation was to examine the muscle-specific metabolic response to running as it relates to muscle growth. Mixed-muscle protein synthesis [fractional synthetic rate (FSR)] and gene expression (GE) were examined in the vastus lateralis (VL) and soleus (SOL) muscles from eight men (26 +/- 2 yr; Vo(2max) 63 +/- 2 ml.kg(-1).min(-1)) before and after a 45-min level-grade treadmill run at 77 +/- 1% intensity. Muscle glycogen utilization was similar between muscles. Resting FSR was similar between the VL (0.080 +/- 0.007 %/h) and SOL (0.086 +/- 0.008 %/h) and was higher (P < 0.05) 24 h postexercise compared with rest for both muscles. The absolute change in FSR was not different between muscles (0.030 +/- 0.007 vs. 0.037 +/- 0.012 %/h for VL and SOL). At baseline, myostatin GE was approximately twofold higher (P < 0.05) in SOL compared with VL, while no other muscle-specific differences in GE were present. After running, myostatin GE was suppressed (P < 0.05) in both muscles at 4 h and was higher (P < 0.05) than baseline at 24 h for VL only. Muscle regulatory factor 4 mRNA was elevated (P < 0.05) at 4 h in both SOL and VL; MyoD and peroxisome-proliferator-activated receptor-gamma coactivator-1alpha (PGC-1alpha) were higher (P < 0.05) at 4 h, and forkhead box [FOXO]3A was higher at 24 h in SOL only, while muscle-RING-finger protein-1 (MuRF-1) was higher (P < 0.05) at 4 h in VL only. Myogenin and atrogin-1 GE were unaltered. The similar increases between muscles in FSR support running as part of the exercise countermeasure to preserve soleus mass during unloading. The subtle differences in GE suggest a potential mechanism for muscle-specific adaptations to chronic run training.

Combined functional task practice and dynamic high intensity resistance training promotes recovery of upper-extremity motor function in post-stroke hemiparesis: a case study

BACKGROUND AND PURPOSE

Weakness is a significant impairment in persons with post-stroke hemiparesis, yet traditional clinical perspectives caution against strengthening in neurological populations. Significant correlations between weakness and functional movement have been demonstrated, however, a clear relationship between increased strength and functional improvement has been elusive. This case study describes a combined program of dynamic, high-intensity resistance training and functional task practice for the upper-extremity in adult hemiparesis.

CASE DESCRIPTION

The patient was a 65-year-old, right hand dominant woman who presented to the Neural Control of Movement Laboratory at the Palo Alto VA Rehabilitation Research and Development Center 16 weeks following clipping of an unruptured aneurysm with consequent dense right hemiparesis. She received 7 weeks of acute rehabilitation according to CARF guidelines (ie, at least 3 hours of two or more disciplines, 6 days per week). Her baseline research evaluation revealed significant upperextremity deficits at the ICF body structure/function level including: weakness, shoulder pain, mild resistance to passive movement, and need for moderate to maximal assistance in many activities of daily living including bathing and dressing. The Stroke Impact Scale score reporting her perspective indicated she had recovered from her stroke only 50%. The hybrid resistance training-functional task practice intervention, detailed in this report, was delivered 3 times per week for 6 weeks with each session lasting 75:00.

OUTCOMES

The subject revealed marked improvements in isometric and dynamic force production in 5 key upper-extremity actions: elbow flexion, elbow extension, shoulder flexion, shoulder abduction, and shoulder external rotation. Strength gains were accompanied by increased EMG activation immediately postintervention and by a combination of increased activation and apparent hypertrophic effects at 6 month follow up. Marked improvements were noted in all clinical and functional measures and in an elbow trajectorytracking task which served as a surrogate measure of motor control.

DISCUSSION

Improvements in strength and positive outcome effects at the physiological, clinical, and functional levels were observed in this subject following the experimental hybrid upper-extremity rehabilitation intervention described. Importantly, no deleterious effects were observed including exacerbation of spasticity or musculoskeletal compromise. Observations of improved EMG activation in this case study suggest that improvements in motor activation underlie these strength gains and can likely be attributed to working at a high intensity level.

Strength training: Isometric training at a range of joint angles versus dynamic training

Strength training with isometric contractions produces large but highly angle-specific adaptations. To contrast the contractile mode of isometric versus dynamic training, but diminish the strong angle specificity effect, we compared the strength gains produced by isometric training at four joint angles with conventional dynamic training. Thirty-three recreationally active healthy males aged 18 - 30 years completed 9 weeks of strength training of the quadriceps muscle group three times per week. An intra-individual design was adopted: one leg performed purely isometric training at each of four joint angles (isometrically trained leg); the other leg performed conventional dynamic training, lifting and lowering (dynamically trained leg). Both legs trained at similar relative loads for the same duration. The quadriceps strength of each leg was measured isometrically (at four angles) and isokinetically (at three velocities) pre and post training. After 9 weeks of training, the increase in isokinetic strength was similar in both legs (pooled data from three velocities: dynamically trained leg, 10.7%; isometrically trained leg, 10.5%). Isometric strength increases were significantly greater for the isometrically trained leg (pooled data from four angles: dynamically trained leg, 13.1%; isometrically trained leg, 18.0%). This may have been due to the greater absolute torque involved with isometric training or a residual angle specificity effect despite the isometric training being divided over four angles.

Maximal motor unit firing rates during isometric resistance training in men

This study measured changes in maximal voluntary contraction (MVC) force, percentage maximal activation, maximal surface EMG, M-wave amplitude and average motor unit firing rates during the initial 3 weeks of isometric resistance training of the quadriceps muscle. Ten men participated in a resistance training programme three times a week for 3 weeks and 10 men participated as a control group. In the training group, MVC increased by 35% (from 761 +/- 77 to 1031 +/- 78 N) by the end of the 3 weeks. There were no changes in mean motor unit firing rates during submaximal or maximal voluntary contractions of 50 (15.51 +/- 1.48 Hz), 75 (20.23 +/- 1.85 Hz) or 100% MVC (42.25 +/- 2.72 Hz) with isometric resistance training. There was also no change in maximal surface EMG relative to the M-wave amplitude. However, there was a small increase in maximal activation (from 95.7 +/- 1.83 to 98.44 +/- 0.66%) as measured by the twitch interpolation technique. There were no changes in any of the parameters measured in the control group. It is suggested that mechanisms other than increases in average motor unit firing rates contributed to the increase in maximal force output with resistance training. Such mechanisms may include a combination of increased motor unit recruitment, enhanced protein synthesis, and changes in motor unit synchronization and muscle activation patterns across the quadriceps synergy.

Exercise for people with peripheral neuropathy

BACKGROUND

Peripheral neuropathies are a wide range of diseases affecting the peripheral nerves. Demyelination or axonal degeneration gives rise to a variety of symptoms including reduced or altered sensation, pain, muscle weakness and fatigue. Secondary disability arises and this may result in adjustments to psychological and social function. Exercise therapy, with a view to developing strength and stamina, forms part of the treatment for people with peripheral neuropathy, particularly in the later stages of recovery from acute neuropathy and in chronic neuropathies.

OBJECTIVES

The primary objective was to examine the effect of exercise therapy on functional ability in the treatment of people with peripheral neuropathy. In addition, secondary outcomes of muscle strength, endurance, broader measures of health and well being, as well as unfavourable outcomes were examined.

SEARCH STRATEGY

We searched the Cochrane Neuromuscular Disease Group register (July 2002 and updated February 2004) and MEDLINE (from January 1966 to June 2004), EMBASE (from January 1980 to June 2004), CINAHL (from January 1982 to July 2002) and LILACS (from January 1982 to July 2002) electronic databases. Bibliographies of all selected randomised controlled trials were checked and authors contacted to identify additional published or unpublished data.

SELECTION CRITERIA

Any randomised or quasi-randomised controlled trial comparing the effect of exercise therapy with no exercise therapy or drugs or an alternative non-drug treatment on functional ability (or disability) in people with peripheral neuropathy at least eight weeks after randomisation was included.

DATA COLLECTION AND ANALYSIS

Two reviewers independently selected eligible studies, rated the methodological quality and extracted data.

MAIN RESULTS

Only one trial fully met the inclusion criteria. An additional two trials assessed outcomes less than eight weeks after randomisation and were also included. Methodological quality was poor for several criteria in each study. Data used in the three studies could not be pooled due to heterogeneity of diagnostic groups and outcome measures. The results of the included trials failed to show any effect of strengthening and endurance exercise programmes on functional ability in people with peripheral neuropathy. However, there is some evidence that strengthening exercise programmes were moderately effective in increasing the strength of tested muscles.

REVIEWERS' CONCLUSIONS

There is inadequate evidence to evaluate the effect of exercise on functional ability in people with peripheral neuropathy. The results suggest that progressive resisted exercise may improve muscle strength in affected muscles.

Human soleus muscle protein synthesis following resistance exercise

AIM

It is generally believed the calf muscles in humans are relatively unresponsive to resistance training when compared with other muscles of the body. The purpose of this investigation was to determine the muscle protein synthesis response of the soleus muscle following a standard high intensity bout of resistance exercise.

METHODS

Eight recreationally active males (27 +/- 4 years) completed three unilateral calf muscle exercises: standing calf press/heel raise, bent-knee calf press/heel raise, and seated calf press/heel raise. Each exercise consisted of four sets of 15 repetitions (approximately 15 repetition maximum, RM, or approximately 70% 1RM). Fractional rate of muscle protein synthesis (FSR) was determined with a primed constant infusion of [2H5]phenylalanine coupled with muscle biopsies immediately and 3 h following the exercise in both the exercise and non-exercise (resting control) leg.

RESULTS

FSR was elevated (P < 0.05) in the exercise (0.069 +/- 0.010) vs. the control (0.051 +/- 0.012) leg. Muscle glycogen concentration was lower (P < 0.05) in the exercise compared with the control leg (Decrease from control; immediate post-exercise: 54 +/- 5; 3 h post-exercise: 36 +/- 4 mmol kg(-1) wet wt.). This relatively high amount of glycogen use is comparable with previous studies of resistance exercise of the thigh (i.e. vastus lateralis; approximately 41-49 mmol kg(-1) wet wt.). However, the exercise-induced increase in FSR that has been consistently reported for the vastus lateralis (approximately 0.045-0.060% h(-1)) is on average approximately 200% higher than reported here for the soleus (0.019 +/- 0.003% h(-1)).

CONCLUSIONS

These results suggest the relatively poor response of soleus muscle protein synthesis to an acute bout of resistance exercise may be the basis for the relative inability of the calf muscles to respond to resistance training programs.

Randomized controlled trial of strength training in post-polio patients

Many post-polio patients develop new muscle weakness decades after the initial illness. However, its mechanism and treatment are controversial. The purpose of this study was to test the hypotheses that: (1) after strength training, post-polio patients show strength improvement comparable to that seen in the healthy elderly; (2) such training does not have a deleterious effect on motor unit (MU) survival; and (3) part of the strength improvement is due to an increase in voluntary motor drive. After baseline measures including maximum voluntary contraction force, voluntary activation index, motor unit number estimate, and the tetanic tension of the thumb muscles had been determined, 10 post-polio patients with hand involvement were randomized to either the training or control group. The progressive resistance training program consisted of three sets of eight isometric contractions, three times weekly for 12 weeks. Seven healthy elderly were also randomized and trained in a similar manner. Changes in the baseline parameters were monitored once every 4 weeks throughout the training period. The trained post-polio patients showed a significant improvement in their strength (P < 0.05). The magnitude of gain was greater than that seen in the healthy elderly (mean +/- SE, 41 +/- 16% vs. 29 +/- 8%). The training did not adversely affect MU survival and the improvement was largely attributable to an increase in voluntary motor drive. We therefore conclude that moderate intensity strength training is safe and effective in post-polio patients.

Spinal and supraspinal factors in human muscle fatigue

Muscle fatigue is an exercise-induced reduction in maximal voluntary muscle force. It may arise not only because of peripheral changes at the level of the muscle, but also because the central nervous system fails to drive the motoneurons adequately. Evidence for "central" fatigue and the neural mechanisms underlying it are reviewed, together with its terminology and the methods used to reveal it. Much data suggest that voluntary activation of human motoneurons and muscle fibers is suboptimal and thus maximal voluntary force is commonly less than true maximal force. Hence, maximal voluntary strength can often be below true maximal muscle force. The technique of twitch interpolation has helped to reveal the changes in drive to motoneurons during fatigue. Voluntary activation usually diminishes during maximal voluntary isometric tasks, that is central fatigue develops, and motor unit firing rates decline. Transcranial magnetic stimulation over the motor cortex during fatiguing exercise has revealed focal changes in cortical excitability and inhibitability based on electromyographic (EMG) recordings, and a decline in supraspinal "drive" based on force recordings. Some of the changes in motor cortical behavior can be dissociated from the development of this "supraspinal" fatigue. Central changes also occur at a spinal level due to the altered input from muscle spindle, tendon organ, and group III and IV muscle afferents innervating the fatiguing muscle. Some intrinsic adaptive properties of the motoneurons help to minimize fatigue. A number of other central changes occur during fatigue and affect, for example, proprioception, tremor, and postural control. Human muscle fatigue does not simply reside in the muscle.

A rat model of progressive isometric strength training

This study presents an isometric model of strength resistance training in rats. Seven rats were trained for five weeks with increasing load, once a day for six days per week while seven rats served as control group. Mechanical strength of the hindlimb muscle group was measured on anaesthetised rats with a force transducer linked to the Achilles tendon after electrical stimulation of the sciatic nerve. Training resulted in a 74 +/- 2% strength gain in experimental (E) vs control (C) rats and in a reduction of fatigability with no change in gastrocnemius, soleus and extensor digitorum longus weights. The fibres composition of the gastrocnemius showed a 50% increase of IIA fibres and a 17% fall of IIB fibres. Consequently, this new model of isometric training is suitable for physiological studies.

Effects of isokinetic training of the knee extensors on isometric strength and peak power output during cycling

Isokinetic training of right and left quadriceps femoris was undertaken three times per week for 16 weeks. One group of subjects (n = 13) trained at an angular velocity of 4.19 rad.s-1 and a second group (n = 10) at 1.05 rad.s-1. A control group (n = 10) performed no training. Maximal voluntary contraction (MVC) of the quadriceps, and peak pedal velocity nu p,peak) and peak power output (Wpeak) during all-out cycling (against loads equivalent to 9, 10, 11, 12, 13 and 14% MVC) were assessed before and after training. The two training groups did not differ significantly from each other in their training response to any of the performance variables (P > 0.05). No significant difference in MVC was observed for any group after the 16-week period (P = 0.167). The post-training increases in average Wpeak (7%) and nu p,peak (6%) during the cycle tests were each significantly different from the control group response (P = 0.018 and P = 0.008, respectively). It is concluded that 16 weeks of isokinetic strength training of the knee extensors is able to significantly improve nu p, peak and Wpeak during spring cycling, an activity which demands considerable involvement of the trained muscle group but with its own distinct pattern of coordination.

Adaptation of skeletal muscle to resistance training

This paper was prepared in partial fulfillment for Doctoral Degree in Physical Therapy at Texas Woman's University in Houston, TX. Resistance training is frequently used in rehabilitation to improve musculoskeletal function. The increased ability of skeletal muscle to generate force following resistance training results from two important changes: 1) the adaptation of the muscle fiber, and 2) the extent to which the motor unit can activate the muscle (neural adaption). The purpose of this article is to provide a review of research investigating the effects of resistance training on muscle fibers and on nervous system input. Muscle fiber adaptations caused by resistance training include increased cross-sectional area of the muscle (hypertrophy, hyperplasia, or both), selective hypertrophy of fast twitch fibers, decreased or maintained mitochondrial number and capillary density of muscle, and possible changes in energy sources. Changes in nervous system input resulting from resistance training include recruitment of an increased number and firing rate of motor units, increased reflex potentiation, and improved synchronization. An understanding of the adaptations occurring in muscle in response to resistance training provides a fundamental basis for which appropriate clinical exercise training programs can be developed for the rehabilitation of patients. J Orthop Sports Phys Ther 1990;12(6):248-255.