Changes in inter-muscular coordination appear critical. Adaptations in agonist muscle activation, as assessed by electromyography, tetanic stimulation and the twitch interpolation technique, suggest small, but significant increases.

Enhanced firing frequency and spinal reflexes most likely explain this improvement, although there is contrary evidence suggesting no change in cortical or corticospinal excitability.

The gains in strength with HRST are undoubtedly due to a wide combination of neurological and morphological factors. Whilst the neurological factors may make their greatest contribution during the early stages of a training programme, hypertrophic processes also commence at the onset of training.

It is both a structural and functional adaptation of the organ systems. Thanks to these adaptations movement abilities are developed. Supercompensation refers to the raised level of the energetic potential and the subsequent raised functional ability of the organism, both resulting from the previous workload — Fig.

Effective sports training must adhere to basic principles. Here we refer to the interconnectedness of elementary features of workload such as volume, intensity and recovery duration Fig.

Training intensity- effort made in a particular movement activity intensity of workload on the muscles and transport system.

Highly intensive training with a smaller volume workload develops muscle speed and strength while lower intensity training of great volume workload develops aerobic capacity, i.

Figure 25 Contribution of individual training periods during the macrocycle. Biologically, muscle hypertrophy is the result of a positive balance between the synthesis and breakdown of proteins, which is manifested as microscopic fiber thickness and macroscopic muscle thickness surrogate variables.

It is known that remodeling processes are highly dynamic in skeletal muscle, and that single bouts of RT immediately upregulate intramuscular anabolic signaling, amino acid transport and protein synthesis Bickel et al.

Therefore, if the protein balance remains positive after RT sessions Reitelseder et al. In line with this idea, in a recent clinical trial in humans, increases in Type II fiber cross-sectional area CSA were detected after only 2 weeks of RT Holloway et al.

Although this histological evidence suggests a continuous process of adaptation, the macroscopic evidence of adaptation in the early phase of RT is less convincing.

Given this background, the time course of macroscopic muscle hypertrophy was revisited in a recent review of studies involving different RT protocols and at least three muscle size measurements over time Counts et al.

Even though most of the studies analyzed used a similar sample untrained young volunteers , a lack of agreement between the results was observed. For the lower limbs, the reported results range from no significant changes in muscle size after 5—12 weeks Abe et al.

To date, the study conducted by Seynnes et al. The differences in the training stimulus, frequency of the assessments and sensitivity of the measurement methods all contribute to the spread of results in the available literature Counts et al. It is hoped that new studies using an optimized RT stimulus, and more sensitive and frequent assessment will shed light on the topic.

In order to produce fast and significant increases in muscle size, the training stimulus has to meet certain characteristics. Mechanical tension is the primary driver of muscle hypertrophy Schoenfeld, Thus, RT interventions aiming to increase muscle volume should focus on the magnitude and the length of time producing tension.

Several training systems and techniques can be used to achieve an enhanced stimulus. For instance, inertial flywheels allow for accommodated maximal or near maximal actions from the very first repetition of a set in the concentric CON and eccentric ECC phases Tesch et al.

To the best of our knowledge, Lundberg et al. In a recent meta-analysis, flywheel training showed to be more effective than conventional weights in promoting increases in muscle volume, strength and power Maroto-Izquierdo et al. FIGURE 1. A,C Bottom position of the exercise and transition between the eccentric and the concentric phases.

B,D Top position of the exercise and transition between the concentric and the eccentric phases. The vest is attached to a strap wound on the flywheel axis. As resistance is the moment of inertia of the flywheel, the force applied during the concentric phase to unwind the strap determines the force needed during the eccentric phase as the strap rewinds to impede the spin of the axis.

A wide range of techniques can be employed to estimate changes in whole-muscle size Counts et al. Magnetic resonance imaging MRI is regarded as the gold standard for clinical and research imaging of skeletal muscle, and is commonly used to compute 1 middle , 2 proximal and distal , or 3 proximal, middle and distal CSAs of a given muscle Seynnes et al.

However, single CSAs may not be representative of whole-muscle changes, given that different patterns of hypertrophy ventral or distal have been reported in response to specific CON or ECC loading Franchi et al.

New MRI approaches for the assessment of total muscle volume, and not only CSAs of specific muscle sites, provide a better measurement of whole-muscle changes Nordez et al.

Other acute physiological processes need to be carefully taken into account when measuring muscle hypertrophy.

RT results in a rapid activity-dependent influx of fluid and accumulation of osmolytes phosphate, lactate, and sodium and a subsequent acute inflammatory response that can last for several hours after exercise Damas et al. These processes temporally alter muscle volume and may interfere with hypertrophy measurements DeFreitas et al.

For instance, acute fluid changes in muscles can be assessed indirectly by muscle functional MRI mfMRI Damon et al. The T2 shift is positively correlated with electromyographic activity, at least when muscle groups are exercised in isolation Adams et al.

In consequence, and although it can only be considered as a proxy marker, mfMRI is commonly used as a tool to determine total or regional muscle metabolic activation in different exercises Fernandez-Gonzalo et al. Given that swelling is caused by muscular activity, and that cell swelling is a known upregulator of anabolic signaling pathways in a wide variety of cells Lang, , it would be reasonable to expect that muscles with greater T2 shifts after acute RT will display greater hypertrophy if the same exercise is systematically repeated over time.

In fact, previous studies have found a correlation between these parameters when an isolated muscle or muscle group is exercised over time Wakahara et al. The purpose of this study was to assess early changes in thigh muscles function force and power , hypertrophy total volume and regional CSAs , and muscle activation T2 shift , during a 4 weeks inertial-squat RT program.

Additionally, we studied the relationship between muscle hypertrophy and activation in the first session, under the hypothesis that different degrees of hypertrophy of thigh muscles would be directly related to muscle task-specific activity assessed by mfMRI.

A quasi-experimental design of repeated measurements was used to determine the effects of an intervention on different variables. The independent variable in this study was a 4 weeks training program consisting of bilateral half-squats performed on an inertial flywheel device.

The variables assessed for thigh muscles were: muscular activation, muscle hypertrophy and muscle performance. Activation was assessed through the T2 shift by mfMRI. Hypertrophy was assessed through increases in total volume and regional CSAs using MRI. Muscle performance was assessed through maximal voluntary isometric contraction force MVIC of the knee extensors and knee flexors, as well as by squat dynamic force and power.

The assessments were performed before PRE , and after 2 IN and 4 POST weeks of training; and also during each of the 10 training sessions. Figure 2 is a schematic representation of the study protocol.

FIGURE 2. Schematic representation of the study protocol. S1—S10 training sessions, SQUAT squat performance kinetics , MVIC maximal voluntary isometric contraction force of knee extensors and knee flexors, MRI muscle volume assessed by magnetic resonance imaging, mfMRI muscle activation assessed by muscle functional magnetic resonance imaging.

All were recreationally active physical education students who had not been involved in any RT program during the 6 months preceding the study. The experiment was conducted in accordance with the code of ethics of the World Medical Association Declaration of Helsinki and was approved by the Ethics Committee of the Catalan Sports Council Generalitat de Catalunya.

A familiarization session was conducted 2 weeks before starting the study. All the volunteers were informed of the aims, experimental protocol, procedures, benefits, and risks of the study, and their written informed consent was obtained.

The volunteers were instructed to maintain their usual level of physical activity throughout the experimental period, and to refrain from moderate or heavy physical activities in the 96 h before the MRI.

The training period comprised of 10 training sessions distributed over 4 weeks two or three sessions per week.

Each training session consisted of a standardized warm-up two sets of 10 body-weight squats, with 1 min of rest between the sets , followed by five sets of flywheel exercise.

Each set of exercise included three sub-maximal repetitions to accelerate the disk, followed by 10 maximal voluntary repetitions. There was 3 min of rest between sets. The start and end position of each repetition was a 90° knee angle. The participants were verbally encouraged to perform the concentric phase at their fastest voluntary speed.

Exercise kinetics were monitored during each session using a Chronopic friction encoder Chronojump, Barcelona, Spain , accuracy ± 1 mm, sampling rate Hz. The sensor was tightly attached at a known diameter of the flywheel, sharing the same linear speed.

Visual and acoustic feedback on the performance was provided for each repetition using a computer. The variables calculated for the CON and ECC phase of each repetition were displacement and mean values of force and power.

Chronojump is open-code software, and a complete repository of the code and formulas used can be found online Chronojump, Maximal voluntary force production of the knee extensors and knee flexors was tested in isometric conditions. For data collection, a strain gauge and a custom-built bench were used.

The signal was recorded at a frequency of Hz, using a Muscle Lab Ergotest Technology AS, Porsgrunn, Norway and real-time results were provided on a computer monitor.

In both positions, the volunteers were fixed using adjustable straps and the strain gauge was attached at the mid-level of the malleolus tibiae forming a perpendicular angle with the leg. Three unilateral 5 s trials were recorded for the dominant limb, with 30 s of recovery between trials.

Subjects were instructed and verbally encouraged to perform and maintain maximum force output. Pre-contraction conditions were standardized and attempts with counter movements were rejected. Maximum force values were selected using mean force over a 1 s mobile window once a force plateau had been established Tesch et al.

The best attempt was selected at each time point for further analysis. MVIC was assessed before each training session, and at IN and POST, after the standard warm-up.

MRI was used to compute the volumes of the individual muscles of the thigh and mfMRI to assess the acute muscle activation after exercise. The volunteers were placed supine inside a 3-T MRI scanner Magnetom VERIO, Siemens, Erlangen, Germany , with their heads outside the MR-bore and thighs covered with one and two flexible 4-channel coils, respectively, in the proximal and distal segments.

A custom-made foot-restraint device was used to standardize and fix limb position, and to avoid any compression of thigh muscles. To ensure the same anatomical area was assessed each time, the range was centered at the mid-length of the femur, as measured on the coronal plane image.

For the assessment of PRE and POST muscle activation, one scan was performed in basal conditions and another 3—5 min after finishing an acute bout of the exercise Cagnie et al.

To minimize the effects of fluid shifts caused by walking, the volunteers remained recumbent for a minimum of 10 min before basal data acquisition LeBlanc et al. In each scan, 12 contiguous each A parametric image was generated from the T2 mapping sequence using Leonardo workstation Siemens.

The T2 of the muscles in both thighs was measured using OsiriX 8. Pixmeo, Geneva, Switzerland. The assessment was performed by the same researcher on all occasions. All the sequences from a given volunteer were processed in parallel, with the researcher blinded.

A circular region of interest ROI was selected for the muscles gluteus maximus GM , rectus femoris RF , vastus intermedius VI , vastus medialis VM , vastus lateralis VL , adductor magnus AM , gracilis GR , biceps femoris long head BFL , biceps femoris short head BFS , semitendinosus ST , and semimembranosus SM in each of the T2 mapping images where these muscles were visible.

ROIs of similar size and anatomical location were placed in the subsequent image sets to ensure positioning identical to that in the first analysis Mendez-Villanueva et al. The intra-class correlation coefficients and coefficient of variation for the intra-rater agreement of the T2 values were: 0.

The T2 values for each muscle were computed as the mean value of the different ROIs. The T2 shift was then calculated by subtracting T2 basal values from T2 values post-exercise, and expressed as a percentage of the basal value.

The results are shown as the average T2 shift of right and left thighs. To assess muscle volume, PRE, IN, and POST scans were performed.

To minimize the effects of fluid shifts caused by walking LeBlanc et al. In each scan, contiguous each 1. Pixmeo, Geneva, Switzerland Figure 3.

Because there may appear to be substantial fusion between VL and VI on some slices Pareja-Blanco et al. The same approach was adopted for the assessment of ADD volume, which includes pectineus, and adductor longus, brevis and magnus. The total volume of muscles was computed from all the CSAs of the images where they were visible, in the range within the last image where the ischial tuberosity was visible and the last image where the femoral condyles were visible Figure 3.

The intra-class correlation coefficients and coefficient of variation for the intra-rater agreement of the CSA segmentation were 1.

The intra-class correlation coefficients and coefficient of variation for the intra-rater agreement of the volumetric values were 1. QUAD, hamstrings HAMS and total thigh muscle volume were computed post hoc as follows:. FIGURE 3. Volume changes were calculated by subtracting the PRE muscle volume from that IN or POST, expressed as a percentage of the PRE value.

The results are shown as the average of right and left thighs. As depicted in Figure 2 , the scans were performed with at least 96 h of recovery after the previous training session, so as to account only for hypertrophic changes and avoid any influence of acute muscle swelling Damas et al. To verify the absence of muscle edema at IN and POST, basal T2 values of the assessed muscles were compared with those obtained at PRE Damas et al.

Complementary analysis was performed to evaluate regional activation and hypertrophy of the muscles assessed and the relationship between these variables. PRE SE MULTIECO, and both PRE and POST VIBE 3D sequences were spatially synchronized.

Thus, PRE regional T2 shifts were matched with PRE and POST regional CSAs. The exact anatomical placement of the cross-sectional images where T2 ROIs were measured was used to assess regional CSAs.

Data are presented as mean ± SD. All statistical analysis was performed using SPSS v. When significant effects were found, post hoc testing was performed by applying paired t -tests with a Bonferroni correction for multiple comparisons. For the assessment of within-subject squat displacement variability, coefficients of variation were calculated for all the squat repetitions performed by each volunteer.

Differences between CON and ECC force and power were calculated using t -tests for paired samples. Mean displacement during the training sessions was ± 24 mm, with low within-subject displacement variability 4.

Higher forces 2. FIGURE 4. Progression of squat mean values of force and power over the 10 training sessions S. FIGURE 5. Progression of MVIC force values throughout the training period. The number of images analyzed per subject for each muscle was GM 3.

The primary morphological adaptations involve an increase in the cross-sectional area of the whole muscle and individual muscle fibres, which is due to an increase in myofibrillar size and number. Satellite cells are activated in the very early stages of training; their proliferation and later fusion with existing fibres appears to be intimately involved in the hypertrophy response.

Other possible morphological adaptations include hyperplasia, changes in fibre type, muscle architecture, myofilament density and the structure of connective tissue and tendons. Indirect evidence for neurological adaptations, which encompasses learning and coordination, comes from the specificity of the training adaptation, transfer of unilateral training to the contralateral limb and imagined contractions.

The apparent rise in whole-muscle specific tension has been primarily used as evidence for neurological adaptations; however, morphological factors e. preferential hypertrophy of type 2 fibres, increased angle of fibre pennation, increase in radiological density are also likely to contribute to this phenomenon.

Changes in inter-muscular coordination appear critical. Adaptations in agonist muscle activation, as assessed by electromyography, tetanic stimulation and the twitch interpolation technique, suggest small, but significant increases.

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J Appl Physiol ; —9. Download references. No sources of funding were used to assist in the preparation of this review. The authors have no conflicts of interest that are directly relevant to the content of this review.

School of Sport and Exercise Sciences, Loughborough University, Ashby Road, Loughborough, LE11 3TU, UK. Institute for Biophysical and Clinical Research into Human Movement, Manchester Metropolitan University, Manchester, UK.

You can also search for this author in PubMed Google Scholar. Correspondence to Jonathan P. Reprints and permissions. Folland, J. Morphological and Neurological Contributions to Increased Strength. Sports Med 37 , — Download citation. Published : 09 January Issue Date : February Anyone you share the following link with will be able to read this content:.

Sorry, a shareable link is not currently available for this article. Provided by the Springer Nature SharedIt content-sharing initiative. Abstract High-resistance strength training HRST is one of the most widely practiced forms of physical activity, which is used to enhance athletic performance, augment musculo-skeletal health and alter body aesthetics.

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The muscle cross sectional area CSA increased pre-post training p 5 0. Maximal isometric strength increased pre-post training p, 0. Total work performed during an isokinetic endurance task increased pre-post training p, 0.

Perceptions of pain p 5 0. Furthermore, the additional metabolic stress that is proposed to occur with a continuous BFR protocol does not seem to translate into proportionally greater training adaptations.

The current findings promote the use of both intermittent BFR and low-load resistance training without BFR as suitable alternative training methods to continuous BFR. These approaches may be practically applicable for those less tolerable to pain and discomfort associated with ischemia during exercise.

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The count includes downloads for all files if a work has more than one. Export: EndNote Dublin Core BibTeX. Repository Staff Only: item control page. QUT Home Contact. Chronic exposure to this type of activity produces marked increases in muscular strength, which are attributed to a range of neurological and morphological adaptations.

This review assesses the evidence for these adaptations, their interplay and contribution to enhanced strength and the methodologies employed. The primary morphological adaptations involve an increase in the cross-sectional area of the whole muscle and individual muscle fibres, which is due to an increase in myofibrillar size and number.

Satellite cells are activated in the very early stages of training; their proliferation and later fusion with existing fibres appears to be intimately involved in the hypertrophy response. Other possible morphological adaptations include hyperplasia, changes in fibre type, muscle architecture, myofilament density and the structure of connective tissue and tendons.

Muscle quality: II. Effects of contribute to increased strength and the apparent rise strength training in to yr-old men and women. J Appl in whole-muscle specific tension, despite the fact Physiol ; that individual fibre-specific tension does not 6. Abe T, DeHoyos D, Pollock M, et al.

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