muscle strength and power have been considered strong exogenous stimuli that are able to improve sports performance and promote therapeutic effects in several pathological conditions characterized by muscle wasting , such as cancer , disuse , sepsis , and sarcopenia [ 1 , 2 ] . in this context ,
resistance exercise ( re ) has demonstrated significant effects regarding neural , metabolic , and functional adaptations   in skeletal muscle .
for this reason , recent studies in humans and rodents have focused on the mechanisms behind such responses promoted by re , especially molecular and phenotypic characterization [ 69 ] .
although human studies provide strong evidence for practical application ( i.e. , greater external validity ) , it is not always possible to control the variables that may influence a biological response . furthermore , collection of muscle tissue samples in order to provide a significant body of cellular evidence [ 10 , 11 ] may limit mechanistic human studies .
therefore , experimental research has been used to provide primary results that may support subsequent studies in humans .   since the study of klitgaard
, literature has described some experimental models of re for rodents that have presented interesting results concerning skeletal muscle adaptations [ 1316 ] .
these studies have demonstrated that rodents are able to perform long- and short - term re with different re protocols and that their hindlimb muscle adapts similar to human muscles in terms of size , fiber type profile , and functionality .
furthermore , molecular characterization of re effects have provided some insight in terms of cell signaling pathways ( transcription and translation ) involved in the control of skeletal muscle growth ( hypertrophy , atrophy , and inflammation ) and functionality ( strength capacity ) . however , these experimental models present limitations that may compromise the extrapolation of data to humans .
there are many factors that can limit experimental models of re in comparison to those performed in humans and , among these , we can cite as the main one , the stimulus that is needed to encourage the animal to execute the exercise .
indeed , environmental stress is required , but many models use fasting and electric shock as reward and punishment stimulus , respectively , which is beyond the physiological context observed in humans , and could be considered a limiting factor in experimental models characterized by appetite suppression and energy restriction .
free  in a given area and remain  tied  in the training apparatus during exercise session . regarding the exercise physiology context
, these experimental models do not provide appropriate control of re variables : volume ( series and repetitions ) , intensity , and rest interval between repetitions and sets .
this lead us to question whether the stimuli is quantitatively enough or not for muscle tissue in terms of volume and overload .  
thus , the aim of the present report is to propose an experimental model of re for rodents that could mimic ( considering the limitation of the species ) the training carried out in humans .
we developed an re model that does not use fasting nor electric shock as stimuli for exercise execution , allows operant conditioning so that the animal does not stay  tied  to the training apparatus , and promotes the control of re variables to perform specific muscle function tests . with this model
it is possible to minimize environmental stress , to approach the animal in a physiological context , to study skeletal muscle adaptations in response to re in several experimental conditions , and to compare the responsiveness of rodents ' and humans ' muscles to different re protocols .
the experiments were conducted in accordance with the national research council 's guidelines for the care and use of laboratory animals .
six wistar male rats ( 400450  g ) were housed under controlled environmental conditions ( temperature , 22c ; 12-h dark period starting at 18  :  00  h ) .
they were given free access to commercial laboratory chow and water before the experiments were performed . during periods of energy restriction ,
the whole equipment consists of an re apparatus and an interface of control ( hardware ) on the rear panel of the equipment and was based on squat - type studies of klitgaard   and wirth et al . .
the re apparatus has its external structure built in acrylic , preferably dark , which holds all devices of the equipment and keeps the main tool of the experiment : the weight lifting cylinder .
thus , its composition can be divided to the acrylic structure , rear panel , drawer , copper bars , cylinder lid , weight lifting cylinder , sensors , and actuators . 

acrylic structure ( figure 1 , item 1 ) : the acrylic structure is bicompartmentalized , that is , it has two divisions : the lower box , to house devices of communication , sensors , actuators , and electronic boards , and the higher box , to place the animal and the weight lifting cylinder .
each compartment has distinct lids which have a slot for passage of sensor wires . 

rear panel ( figure 1 , item 2 ) : in the rear panel the connectors responsible for power supply and the hardware are allocated . 

drawer ( figure 1 , item 8 ; figure 2 , item 9 ) : it is an aluminum tray that lines the bottom of the higher box during the experiments , facilitating the removal of animal wasting .
it can be washed with water and is usually covered with paper for easy cleaning . 

copper bars ( figure 1 , item 7 ; figure 2 , item 8) : responsible for supporting the weight lifting cylinder and can be exchanged for another metal material . 

cylinder lid : this device is attached on the top of the weight lifting cylinder to provide better fixation of the cylinder to the acrylic structure , avoiding fluctuations during the experiment due to the rise and fall of weights . the weight lifting cylinder is located inside the re apparatus and is composed by the following . 
 acrylic cylinder ( figure 2 , item 1 ) : it is a holder that allows the animal to lift the weight as squat - type exercise . in this cylinder are coupled : weight plates , lifting ring , lifting bar , and the measurement axis . 

lifting ring ( figure 2 , item 6 ) : it is an acrylic piece which is located inside the acrylic cylinder , with one side attached to the measurement axis and the other to a vertical  rail  found in the body of the acrylic cylinder .
the height of this device can be adjusted according to the size and species of the animal . 

lifting bar ( figure 2 , item 10 ) : it is a steel piece responsible for supporting weight plates for the lifting movement .
weight plates must be placed at the predetermined distances relative to the  rail  found in the acrylic cylinder . 

weight plates : 50  g cylinders that should be placed in the lifting bar and fixed with the aid of elastics ( i.e. , orthodontic elastics ) .
more than one weight plate can be placed in the lifting bar at a time , but they should stay together side by side .
the point at which they are placed on the lifting bar must be considered as the midpoint of the set of weight plates on  rail  of the acrylic cylinder . 

axis measurement ( figure 2 , item 5 ) : it consists of a potentiometer and an acrylic base .
this axis should enable the establishment of the lifting ring trough a cross - bolt to the measurement axis .
it enables the measurement of angular displacement of the lifting bar through the angle sensor .
support base ( figure 2 , item 7 ) : it is responsible for acrylic cylinder fixation to the copper bars .
the support base consists of two symmetrical parts and must be put side by side below the acrylic cylinder .
u  shape on both sides of the support base allows its attachment to the copper bars .
angle sensor : it is the sensor responsible for measuring the angle of the lifting bar .
the angle sensor is attached to the weight lifting cylinder acting as the axis of the rotation of the lifting bar , allowing the software to calculate the angular displacement of the lifting bar when it is rotated . from this and other data
, the software can also calculate the concentric and eccentric strength exerted by the animal as well as the length of each movement . 

nose poke ( figure 1 , item 5 ; figure 2 , item 12 ) : it is through this device that the system receives each  nose poking  signal of the animal .
it has a circular hole in which the animal introduces the nose cutting an infrared light and triggering the device . at the bottom of the hole
there is as light - emitting diode ( led ) that emits yellow light , in order to stimulate the animal when it is operating .
the actuators , located inside the apparatus , are responsible for performing specific actions on the re apparatus .
automatic feeder ( figure 1 , item 6 ; figure 2 , item 2 ) : it is installed inside the apparatus . a pellet at a time is released , requiring replacement of a new pellet into the feeder after the previous release .
the release of pellets occurs through a door controlled by a motor servo which is controlled by the hardware in the rear panel of the equipment .
light device ( figure 2 , item 11 ) : it is placed inside the apparatus in order to illuminate most of the distribution of acrylic structure through a lamp , preferably red and with low power . during the re session
sound device ( figure 2 , item 4 ) : it is placed inside the apparatus and generates noise through a speaker that is also located inside the equipment and protected by a grid . during the exercise session ,
the sound is activated as a conditioning tool ( encourage / disturb the animal ) . 

fan ( figure 2 , item 3 ) : it is installed into the upper compartment of the equipment and remains ventilating while the apparatus is on trial .
it keeps the air circulation inside the equipment , produces a very low noise , and is not within the reach of the animal .
the present equipment has all the sensors and devices described highly integrated into its structure and is highly resistant , supporting all actions and movements of the animal .
moreover , it is not necessary to perform the assembly of each component individually prior to use and it is easy to operate and to clean .
the phases of operant conditioning adopted in the present equipment are the same described by wirth et al . .
however , we modified the logistics of such phases in order to optimize the exercise execution by the animal .
as previously mentioned , we did not use fasting as a reward stimulus to execute the exercise , but for conditioning the animal .
therefore , the environment where the experiment was conducted was extremely well - controlled : dark ( illuminated by a low - power red light ) , silent , and devoid of atypical odors .
briefly , the phases of operant conditioning were separated by 48  h and carried out as follows . 

magazine : in this phase we only placed the animal inside the apparatus to adapt to the environment .
after 12  h of energy restriction the animal remained inside the equipment for 1 hour . during this period , pellets of ~3040  mg were released ( a total of 30 pellets ) at regular intervals for the animal to identify the automatic feeder .
nose - poke 1 : as shown in figure 2 , nose - poke is installed besides the automatic feeder .
again , the animal was placed inside the equipment after 12  h of energy restriction .
the automatic feeder was triggered each time the animal introduced its nose into the nose - poke .
a total of 30 cycles was established for each experiment . based on our experience with the apparatus ,
some food smell in the nose - poke was necessary to encourage the animal to introduce its nose inside it .
nose - poke 2 : in the second phase of nose - poke , the sound device was integrated to the conditioning process .
a constant noise was emitted and when the animal introduced its nose into the nose - poke , the automatic feeder was triggered and the noise stopped .
the sound device was used to disturb the animal so that when the task (  poking  ) was performed correctly , the animal was food - rewarded and the noise stopped disturbing .
this phase introduced a new variable in order to replace the food reward as the only way to encourage the animal to perform the task proposed .
a total of 30 interleaved cycles of pellets ( 15 with and without food ) release were performed .
standing 1 : the nose - poke device was removed from the wall and placed inside the weight lifting cylinder , above the lifting ring , and a lid was placed in the nose - poke hole on the wall .
this phase was conducted to encourage the animal to enter into the weight lifting cylinder , but without lifting the ring , and was performed twice . 

standing 2 : in the second phase of standing , food reward was completely removed and the animal had the constant sound as the only stimulus to perform the task .
two sessions were required . 
 lifting : in the last phase , the nose - poke device remained within the weight lifting cylinder , however , at an unreachable height to the animal . using the software ,
a minimum lifting height was set depending on the species and size of the animal .
we observed that a height of 4  cm was necessary for an animal of ~400  g to complete the movement ( full concentric contraction ) . during the lifting task , animals performed concentric muscle contractions being plantaris and soleus the main muscles involved in contraction .
of note , the access of the lifting ring allowed the animal to pass only the head through the ring hole and its height was adjusted according to the animal 's characteristics .
the sound was turned on and once the animal raised the lifting bar to the pre - determined height , the noise stopped .
it was necessary to carry out one session without load and one session with light load .
it is important to emphasize that the animal did not stay  tied  in the training apparatus .
once conditioned , the animal underwent a muscle function test : maximal voluntary strength capacity ( mvsc ) as previously described by our group   using the same apparatus and procedures described by klitgaard .
the advantages of the present equipment are that the rest intervals between each repetition in the mvsc were optimized because of the sound stimulus , it was possible to calculate concentric and eccentric strengths of the animal during each muscular contraction and relativize the training intensity with better precision ( percent of mvsc , percent of body weight ) . additionally , the mvsc test can be performed during and at the end of the experimental protocol to assess the progression of muscle function . to configure the training protocol in the software it was necessary to enter the following data : training volume ( sets and repetitions ) , rest intervals between sets and repetitions , desired height of the lifting bar , and weight plate position in the lifting bar .
the experiment could be interrupted at any time and all data were stored in a report file by the software .
differences between nose - poke - phases and body weight values were tested by paired t - test . the level of significance was set at p < 0.05 .
statistical analysis was performed using sas 8.2 ( sas institute inc . , cary , nc , usa ) .
as previously described , to condition the animal to the experimental model it was necessary to carry out periods of energy restriction on the previous day of nose - poke 1 and 2 and standing 1 phases . however , as shown in figure 3 , energy restriction did not promote significant changes in body weight at the end of the conditioning process ( 414.1  4.8  g in pre versus 410.8  4.2  g in post period ; p > 0.05 ) . 

magazine phase is not described because it was carried out only to adapt the animal to the exercise environment . to complete the 30 cycles proposed in the nose - poke 1 phase
interestingly , when the sound device was integrated to the conditioning ( nose - poke 2 phase ) , animals took 58.9% less time to complete the cycles ( 768.4  109.1 seconds ; p = 0.03 ) .
it is important to emphasize that , although the animals were more conditioned to the  poking  task in nose - poke 2 phase , this factor did not contribute to reduce the time spent to complete the cycles .
we repeated the nose - poke 1 phase ( without sound ) and observed reduction of time ( data not shown ) .
standing phases time were not statistically different ( 1274  92.4 seconds in standing 1 versus  1234  113.3  seconds in standing 2 phases ; p > 0.05 ) .
animals took 960.8  254.2 seconds to complete the lifting phases and lifted the bar to 4.1  0.1  cm .  
height displacements during repetitions were quite similar to lifting phase and the result of the mvsc during the concentric phase of muscle contraction was 2.3  0.1  n ( figure 5 ) .
we did not describe the eccentric strength performed because this experimental model is totally based on concentric muscle actions , since during the eccentric phase the animal does not counteract the external load .
the rest interval between each repetition to perform a repetition ( contraction ) was 7.2  0.8 seconds ( figure 5 ) .
thus , each time the sound stimulus was released , animals took approximately 7 seconds to perform the next repetition .
the aim of the present report was to propose an experimental model of re for rodents with better conditioning ( no fasting and shocking ) and control of exercise variables ( volume , intensity and rest intervals ) .
as previously described , animals were able to perform the conditioning phases within the logistics proposed .
second , integrating the sound device into the conditioning process optimized the performance of the exercise learning .
regarding exercise physiology , it was possible to optimize the encouragement of the animal to perform muscle contractions controlling repetitions and rest interval between repetitions .
therefore , it was possible to control and optimize the variable of re  rest interval .    importantly ,
although this experimental model uses  energy restriction  as the first stimulus to condition the animal to the exercise , it did not promote significant changes in body weight and , consequently , in muscle mass .
this can be explained by the fact that the energy restriction was imposed only at the beginning and gradually abolished during the conditioning process .
the applicability of such factor is quite important since models of skeletal muscle atrophy ( i.e. , glucocorticoids , fasting , cancer , and sepsis ) are usually characterized by reduction in food intake .
furthermore , this experimental model demonstrates that it is not necessary to punish the animal ( shocking ) to encourage exercising . in this context , sound device proved to be a good  disturbing 
recently , we conducted an experiment to evaluate the effectiveness of this experimental model in counteracting dexamethasone - induced skeletal muscle atrophy , an experimental model characterized by severe muscle wasting and appetite suppression .
even with loss of appetite , animals performed , during 7 days , 3 sessions of re composed by 3 sets of 10 repetitions separated by 48 hours .
we demonstrated that , although re did not attenuate the atrophic response , it promoted significant phenotypic adaptations in plantaris muscles that in the long term may reflect structural remodeling .
furthermore , re - trained animals presented higher concentric mvsc when compared with nontrained animals at the end of the experimental protocol .
therefore , the experimental mode is effective in conditioning the animal to re and , according to the exercise protocol , to promote phenotypic , structural , and functional responses in skeletal muscle at least in dexamethasone - treated animals .  
although the present equipment promotes good control of re variables , the main limitation concerns the strength data .
concentric and eccentric measurements are calculated by estimative , that is , not directly measured by a strength platform as proposed by wirth et al . .
furthermore , it proposes a more specific control of other re variables , especially rest interval between sets and repetitions .
furthermore , it encourages the animal to exercise through short - term energy restriction and  disturbing  stimuli that do not promote alterations in body weight .
therefore , we believe that this re apparatus is closer to the physiological context observed in humans .  
future studies should evaluate the molecular characterization of distinct re protocols in order to understand the biological response of skeletal muscle and optimize the development of training methods .
experimental models characterized by skeletal muscle wasting should also be investigated in order to propose re as a nonpharmacological tool for humans .
additionally , this experimental model allows investigations regarding other stimulus associated with re , that is , nutritional , pharmacological , and so forth .
 this study aimed to develop an equipment and system of resistance exercise ( re ) , based on squat - type exercise for rodents , with control of training variables . 
 we developed an operant conditioning system composed of sound , light and feeding devices that allowed optimized re performance by the animal . with this system , it is not necessary to impose fasting or electric shock for the animal to perform the task proposed ( muscle contraction ) . 
 furthermore , it is possible to perform muscle function tests in vivo within the context of the exercise proposed and control variables such as intensity , volume ( sets and repetitions ) , and exercise session length , rest interval between sets and repetitions , and concentric strength . 
 based on the experiments conducted , we demonstrated that the model proposed is able to perform more specific control of other re variables , especially rest interval between sets and repetitions , and encourages the animal to exercise through short - term energy restriction and  disturbing  stimulus that do not promote alterations in body weight . 
 therefore , despite experimental limitations , we believe that this re apparatus is closer to the physiological context observed in humans . 