A COMPARISON OF OPEN AND CLOSED CHAIN KNEE EXTENSION EXERCISES ON PATTERNS OF QUADRICEPS HYPERTROPHY

Resistance training causes hypertrophy, however, the magnitude of muscle growth varies along the length of the muscle (i.e. proximo-distally). For running based athletes and those dependent on movement about the hip, preferential proximal hypertrophy of the quadriceps femoris (the primary knee extensor) shifts the center of mass (CoM) of the thigh closer to the hip which provides a direct biomechanical advantage by decreasing the moment of inertia of the high about the hip (I). This in turn can increase movement velocity and economy and has been observed in studies using mathematical modeling and when comparing elite national level sprinters. Recent studies have reported that the pattern of quadriceps hypertrophy differs between different types of training (plyometrics vs traditional heavy resistance training) or when different types of contractions (eccentric vs concentric) are performed. However, no study to date has explored how exercise selection affects patterns of hypertrophy. Therefore, the purpose of the present study was to compare the effects of open kinetic chain (OKC) and closed kinetic chain (CKC) exercises on quadriceps patterns of hypertrophy and to determine if patterns of hypertrophy differ and if so does this result in a significant effect on CoM and I. Given pilot data from our lab, we hypothesized that CKC would result in similar proximal hypertrophy but less distal hypertrophy of the quadriceps compared to OKC, thus shifting CoM proximally and decreasing I about the hip. To test our hypothesis, 12 untrained participants (male =7; female = 5) aged 18-35 years participated in an 8 week resistance training intervention where each participant trained by performing both unilateral CKC (squat) and OKC (knee extension) exercises on separate legs. Before and after the training program MRI of the quadriceps femoris was performed in order to measure changes in muscle cross sectional area in the proximal-thigh (1/3 thigh length), mid-thigh (1/2 thigh length) and distal-thigh (2/3 thigh length). Regional cross sectional area of the quadriceps femoris was compared between exercises and over time using a 2 x 2 mixed model ANOVA with Bonferoni post-hoc corrections. Results revealed that both conditions resulted in an increase in muscle volume which was similar between conditions (CKC Δ 60.2 ± 110.5 cm3, OKC Δ 79.5 ± 87.9 cm3, p = 0.285). However, the pattern of hypertrophy differed along the length of the thigh and between conditions with CKC experiencing a significant increase in cross sectional area in only the distal-thigh region (p = 0.044) and OKC experiencing a significant increase in both the midand distal-thigh regions (p = 0.003-0.004). Additionally, a significant interaction effect of exercise and time was observed for CoM (p < 0.001) and I (p < 0.001), where CKC resulted in CoM shifting proximally and I reducing about the hip when compared to OKC. Given running and other athletes can benefit from a proximal shift in CoM of the thigh and reduced I of the thigh about the hip, our results suggest that running based athletes should preferentially select CKC exercises over OKC exercises during their resistance training program.

. This hypertrophy can be beneficial to athletic performance given that greater cross sectional area of muscle yields greater force production potential during many common movements in sport such as sprinting, jumping and changing direction (2,(4)(5)(6)(7)(8). However, hypertrophy also negatively affects such movements, as hypertrophy results in an increase in mass, which is the primary form of resistance that must be overcome in these movements in the form of inertia (linear motion) and moment of inertia (for angular motion: 9). Therefore, the positive benefits of the added force production are balanced between the negative effects of the added mass. In response to this, a growing area of research has focused on means by which athletes can increase force production potential of muscles while minimizing the negative effects of mass related to hypertrophy. Selective or targeted regional hypertrophy is one such solution.
When considering the quadriceps femoris, the largest muscle group in the body by mass, hypertrophy provides a direct benefit to force production at the knee but this added mass increases the resistance that needs to be overcome for hip motion to occur. This resistance is referred to as the moment of inertia of the thigh about the hip (or I). Movement around the hip joint is important for athletic performance given many sports rely on hip dominant movements such as running (8,10). As the moment of inertia (I) of the hip is the product of the mass of the hip multiplied by the location of the center of mass squared of the hip (I = m r 2 ) the location at which hypertrophy occurs is exponentially more important mathematically than simply how much mass is increased. Therefore, hypertrophy that is more proximal to the hip will minimize the resistance the athlete needs to overcome while running thereby increasing how fast the limb can be moved (angular acceleration = torque / moment of inertia) decreasing energy costs more so than when the same amount of hypertrophy occurs closer to the knee / distally (1, 2, 6, 10).
As distribution of hypertrophy along a muscle may have important implications for athletic performance, creating resistance training practices that provide this direct biomechanical benefit may be of great importance for running athletes.
Recent research has suggested that it is possible to manipulate changes in the location of mass along the length of a muscle via manipulation of various acute programming variables in exercise prescription (2,(11)(12)(13)(14). However, it is presently unknown how exercise selection affects patterns of hypertrophy. Specifically, it is of interest to determine if training with open chained exercises (in which the foot moves freely around the knee, e.g. a knee extension) compared to closed chain exercises (where the feet are fixed and the body is moved, e.g. a squat) results in meaningful changes in muscle mass distribution, center of mass (CoM), and moment of inertia (I) about the hip as these are variables within a training program that can be easily manipulated and controlled.
Given CoM is a function of morphological conditions within muscle (i.e. the shape of the muscle), the overall distribution of mass will alter the location of CoM (2,10,15,16). Changes in the location of CoM will subsequently alter I about the hip (I =mr 2 , where I = I about the origin, m = mass, and r 2 = radius of CoM squared) (See figure 1). This is important as a smaller I increases the speed at which the hip can be moved (α = T I -1 , where α = angular acceleration, T = the maximum torque that can be produced by the hip muscles and I = moment of inertia). Therefore by reducing I the hip can attain higher angular velocities during a variety of motions such as running, thus improving athletic performance (6)(7)(8). This is most evident when considering rotation about the hip joint given its role in common athletic movements such as general locomotion, sprinting, jumping, and kicking (8). For example,  A smaller I also reduces the amount of torque necessary to move at a given velocity (T = I α, where T is the torque necessary to complete the movement, I = moment of inertia, and α = the acceleration necessary to complete a task) (8,10,16). The equation for I the location of CoM is squared unlike the mass therefore I encountered is more greatly influenced by location of changes in mass than magnitude of the change in mass (15,16). Thereby movement efficiency can also be increased by proximally shifting CoM given less muscle force will be required to generate movement about the hip with a reduced thigh resistance moment, potentially limiting and mitigating accumulated muscular fatigue during performance.
Being able to run faster and more efficiently can improve performance in a meaningful way for diverse groups of athletes. Thus, training that results in a more proximal shift in CoM of the thigh would be most preferable for those populations. Given CoM of the thigh is primarily influenced by the mass and shape of the quadriceps femoris muscle, and that muscle can undergo significant and inhomogeneous hypertrophy, controlling that pattern can result in directly improving athletic performance. (2,10,11,15,16).
Until recently, skeletal muscle hypertrophy had been assumed to be homogenous, with hypertrophy distribution occurring proportional to the muscle thickness of a region in response to muscle-growth inducing stimuli (3,12,17). This assumption thus implied relative hypertrophy was consistent, though absolute hypertrophy differed by muscle region. Recent published works have contested this, suggesting that muscles may experience hypertrophy in an inhomogeneous fashion along the length of a muscle (2,(11)(12)(13)(14). For instance, it has been demonstrated that hypertrophy occurs in an inhomogeneous manner following 12 week of isokinetic knee extension training, with mass distribution varying between the proximal and distal regions of the quadriceps femoris (2). This notion is further supported by the work of Wakahara et al. (17), who reported that hypertrophy occurs in an inhomogeneous fashion along the length of the triceps brachii, which experiences greater proximal hypertrophy from closed chain exercise.
However, as the proximal portion of the triceps brachii (the long head) has a role at both the shoulder and elbow, the authors assumed this selective activation of this part of the muscle due to its role at the shoulder was the driving mechanism for their results and therefore their results may apply to the biarticular rectus femoris muscle of the quadriceps femoris.
Inhomogeneity may also be influenced by manipulation of acute programming variables for exercise prescription. A study by Earp et. al (11) figure 4). Though the involved muscle group differs from the quadriceps femoris, this supports that imposing different task specific demands in the form of OKC and CKC exercise may elicit different hypertrophy responses in skeletal muscle. Differences in regional muscle activation have been supported to shift hypertrophy localization during a training intervention to the region with the greatest activity (2,12,13). This is in line with the findings of Wakahara et al. (17), whose research supported closed chain exercise inducing greater proximal hypertrophy in the triceps brachii, but whose results were attributed to activation of the long head of that muscle, which is known to have separate activation pathway to the rest of the triceps brachii muscle. The current study is the first to directly explore the effects of OKC and CKC extension exercises on quadriceps femoris muscle morphology via a training intervention. Developing a successful training model for influencing inhomogeneous hypertrophy and muscle mass distribution within the quadriceps will allow for more targeted and effective exercise prescription in both the athletic and rehabilitative sectors. Development of an intervention training model allows for both an exploration of mechanisms and effects as well as a template for evidence-based professionals to utilize in their own 13 practices. Therefore, the purpose of the current study was to determine if regional hypertrophy of the quadriceps femoris and mechanical parameters differ between OKC and CKC knee extensor exercises. We hypothesized that CKC knee extensor training will result in similar proximal quadriceps femoris hypertrophy but less hypertrophy of the distal quadriceps femoris when compared to OKC knee extensor training, thus shifting CoM more proximally and decreasing I about the hip.

Inhomogeneous Hypertrophy
A study conducted by Miyamoto et al. (13)  A potential limitation of this study relates to the use of EMG given the measurement tool records signals from motor unit endplates and the distance between the receiver and the end plate may result in a lapse in time between the occurrence of muscle activity and its recording. This delay may have confounded findings given points of peak activation may have been assessed to occur later in the range of motion of a given exercise than they had actually occurred. The findings however, support the hypothesis of the present study. imaging were performed at 15%, 35%, 50%, 55%, and 70% the length of the thigh as a basis for analysis of regional hypertrophy of the quadriceps femoris and its patterns. Hypertrophy occurred in an inhomogeneous manner along the length of the quadriceps femoris with relative change in cross sectional area of the vastus lateralis and rectus femoris differing between proximal and distal regions after the resistance training intervention (p < 0.05). It was found that overall greater hypertrophy occurred within the rectus femoris than the vastus lateralis and medialis muscles. However, greater distal hypertrophy occurred than proximal in all regions of the quadriceps femoris via the seated knee extension protocol. This supports the present study's hypothesis.
Another study conducted by the lab of Wakahara & Ema et al. (14) explored the association between regional muscle activation as assessed by EMG and regional hypertrophy caused by a 12 week knee extension intervention as a follow up to their previous study on inhomogeneity of hypertrophy in the quadriceps femoris. MRI scans were used pre and post intervention to assess volume and muscle cross sectional area of the quadriceps femoris and each of its muscles at differing percentages of the total length of the thigh. EMG measurements were taken at each site during working sets of knee extension to assess muscle activation differences. The researchers found that muscle activation has a strong association with training induced hypertrophy localization and quadriceps femoris morphology (p < 0.05), suggesting that Vastus medialis, vastus lateralis, vastus intermedius, and rectus femoris regional cross sectional area of muscle increased significantly for both age groups (average regional cross sectional area increase of quadriceps femoris muscles of 8-40 cm 2 ; p <0.05), with no significant differences between age groups for cross sectional area changes in any of the regions of the muscle.
This suggests that meaningful hypertrophy was induced in each region of the quadriceps femoris, however inhomogeneity of hypertrophic responses to an exercise stimulus was not a function of age, according to this research study.
This is important when considering and evaluating participant selection criteria of a study.
A study by Blazevich et al. A study by Earp et al. (11) has explored differences in proximal and distal hypertrophy within the quadriceps femoris in response to different exercise prescriptions. The study included 3 training conditions and a control condition for participants (n = 36) who do not habitually resistance train over an 8 week period. All training conditions involved the CKC back squat movement pattern and participants were assigned to either a parallel depth heavy squat, parallel depth jump squat, volitional depth jump squat or no resistance training program condition. The researchers found that hypertrophy was inhomogeneous as a function of the specific exercise prescription used.
Specifically, high velocity parallel jump squatting was the only condition to experience a significant proximal increase in quadriceps femoris muscle cross sectional area (p < 0.05), and heavy squatting to parallel depth was the only condition that induced significant hypertrophy at the mid thigh (P < 0.05). The authors concluded that heavy squat intervention at a parallel depth increases proximal quadriceps femoris cross sectional area greater than jump squatting to parallel, which experienced greater distal increases in muscle cross sectional area. The current study supports the role of exercise type selection in influencing muscle architecture and morphology of the quadriceps femoris during a resistance training intervention. This is important to the present study given differences are hypothesized to be observed by manipulation of exercise type, an acute exercise programming variable.

OKC and CKC Exercises and Movement Mechanics
An investigation by Wilk  A novel investigation by Azizi (29) into the occurrence of variable muscle gearing explored changes in muscle fascicle length and angle due to different training stimuli. Participants performed contractions of varying velocities and forces, resulting in changes in fascicle length, angle, and muscle thickness in each condition. It was found that greater fiber rotation occurs in low force high velocity movements, and lesser fiber rotation occurs in high force low velocity movements (P < 0.05). This suggests that the internal environment of a muscular compartment is dynamic and may be influenced by the type of stimuli encountered. If an OKC or CKC exercise results in increasing the amount of muscle force produced within muscles of the quadriceps femoris differentially, then they will experience different muscle fascicle orientations and architecture intraset. Thus, they will be differentially susceptible to muscle fascicle strain, a known driver of hypertrophy. This may result in driving inhomogeneous hypertrophy across the length of the quadriceps femoris.
An investigation by Browning et al. (15) had explored the biomechanical and energetic effects of increasing thigh and leg mass. The investigation aimed to understand the differences between net metabolic rate, movement kinematics, muscle activity and net muscle moments during gait with different magnitudes and locations of mass added to the legs using a within-subjects design.
Participants (n = 5 males) walked on a treadmill with a built in force plate at a in all quadriceps muscle regions and that these lengths which resulted in greater proximal muscle thickness were significantly related to record 100 meter sprint performance (r = 0.40 to 0.57). This suggests that those with longer fascicles and mass distributed proximally with the same amount of overall mass in their thigh will experience greater athletic performance. This may be due to an associated proximal shifting of CoM resulting from the proximal hypertrophy localization these sprinters experienced in past training.
This further supports the practical application of the present study in consideration of sports performance.

Experimental Design
The present study was a randomized control trial utilizing a within subjects repeated measures design (See Figure 5). Participants (n = 12) were recruited via email outreach campaign, in-class announcements, and local flier using IRB approved recruitment methods and designs. Initially, 15 participants were recruited, however only 12 were able to successfully complete the study and were thus the only participants included in all data and calculations. Of the three participants, one was removed for failure to adhere to study protocols and another two were unable to continue due to reasons outside of the study's control. Participants were allowed to participate upon completing a health history questionnaire and an informed consent document, which participants were required to convey their understanding using the teach-back method.
Participants of either gender between the ages of 18 and 35 were recruited. Research lab #120.

Figure 5. Study Flowchart
The above figure depicts the design of the present study. Originally 15 participants were recruited, but only 12 completed the entirety of the study.  Table 1). Resistance training occurred on 3 non-consecutive days per week and periodization was used to facilitate muscular hypertrophy dependent on guidelines set forth by the National Strength and Conditioning Association (31). Following the completion of the intervention, further muscular strength testing and MRI imaging took place, repeating the earlier protocols.  The calculated ratio of these two tissues was affected by hydration status when using the BIA device so body composition testing validity was supported by participant hydration status testing (34,35). Hydration status was assessed via refractometer upon collection of a urine sample inserted mid-stream (ATAGO USA, Inc.). Euhydration was defined as having a urine specific gravity ≤ 1.025. If urine specific gravity was greater than this value, participants were asked to consume an appropriate amount of water and urine specific gravity was assessed again every 90 minutes until euhydration status was achieved, in accordance with past validated research practices and the approximate time of the full absorption rate of fluid in the human body (34)(35)(36).
This had occurred one time and no participants were found to be hyperhydrated. Other controls for body composition analysis such as limiting pre-testing exercise and caffeine consumption were enacted.

Exercise Familiarization and Technique:
Prior to the intervention and muscular strength testing, participants were familiarized with each of the exercises they would be performing during the    Table 2). The order in which exercises and legs were tested was dependent on the block randomization table participants were assigned to at the start of the study.  Whole quadriceps femoris and individual muscular compartmental volumes were calculated using cubic spline interpolation methods (11,43). These were manipulated using the 4x2 and 2x2 rules (See Table 3). Initial prescriptions of intensity was dependent on initial one repetition maximums for each exercise as determined by the muscular strength testing protocol. In accordance with the 2 for 2 rule, absolute training loads increased alongside increases in participant muscular strength to maintain desired training intensity (31 3  12  65  90  2  Hypertrophy  3  10  75  90  3  Hypertrophy  3  8  80  90  4  Strength  4  6  85  120  5  Strength  4  6  85  120  6  Hypertrophy  3  12  67  90  7  Hypertrophy  3  10  75  90  8  Hypertrophy  3  8  80  90  The above table depicts the exercise prescription used in the present study's 8 week intervention.

CoM & I:
The location of the CoM of the quadriceps femoris was similar between exercise conditions prior to the exercise intervention (p = 0.457  Figure 11) Accompanying changes in location of CoM, similar changes in I were observed as an interaction effect of exercise over time was observed (Δ 0.022 kgm 2 ± 0.003 kgm 2 , p < 0.001) and I was increased in the OKC condition (Δ 0.017 kgm 2 ± 0.014 kgm 2 , p < 0.001 ) but remained unchanged in the CKC condition (Δ -0.022 kgm 2 ± 0.020 kgm 2 , p > 0.05: Figure 11).  The present study's findings suggest that exercise selection can influence training outcomes in meaningful ways, which may mean that resistance training program design practices should account for exercise selection in ways that have not been previously recommended. Doing so will allow more precise control over the resulting adaptations from training and thereby improve resistance training efficacy. However, more research is needed to support specific programming recommendations within a comprehensive resistance training program.

Patterns of Hypertrophy:
Following completion of the 8 week training intervention, patterns of hypertrophy differed between conditions. Patterns of hypertrophy for both exercises were inhomogeneous with proximal, middle, and distal aspects of the quadriceps femoris undergoing differing degrees of 54 hypertrophy. The CKC training only resulted in significant distal hypertrophy while the OKC condition resulted in significant hypertrophy at both the mid and distal quadriceps femoris. Differences in patterns of hypertrophy may be attributed to task specific motor unit recruitment favoring mechanically favorable muscle fiber activation during one condition over the other, though limited research has been conducted on task specific motor unit recruitment to date (24).
Past work by Blazevich et al. (2006) supports that muscular compartments of the quadriceps femoris differ architecturally along their length and thus have mechanically different regional properties regarding their force transmission potential (3). This has been supported to be partly attributable to differences in muscle thickness, which positively relate to differences in muscle fascicle angle (3). Past research by Mitchell et al. (1997) had provided additional support for the existence of the regional differences noted by Blazevich et al.
(2006) across the length of muscular compartments of the quadriceps femoris by specifically investigating the vastus medialis (3,27). Mitchell et al. (1997) identified mechanical differences within the vastus medialis muscular compartment's proximal and distal aspects which suggests that withincompartment muscle fiber recruitment may have been task specific in the vastus medialis (24,27). Given motor unit recruitment has been demonstrated to be task specific and that architectural differences along the length of the individual muscular compartments of the quadriceps femoris have been demonstrated to be mechanically dissimilar, it is reasonable to conclude that recruitment within the quadriceps femoris specifically may be task specific.
CKC and OKC exercises are discrete tasks which place different demands on the body and thus may have resulted in differences in motor unit recruitment.
Differences in muscular compartment involvement, quadriceps morphology at the point of peak resistance torque, and differences in motion at the hip joint may have driven this (3,10,18). The greatest muscular hypertrophy has been reported to occur in recruited motor units and active tissue, with magnitude of hypertrophy varying by specific motor unit recruited, thus explaining the differential hypertrophy localization between CKC and OKC conditions reported in the present study (47). Due to co-contraction of other agonists to drive hip extension during the CKC movement that was not present in the OKC movement, the absolute load used for the CKC movement was greater than the OKC movement. However, the relative loading of the quadriceps femoris itself should not have differed between conditions given its force production capacity would not have been altered with the involvement of other muscles, and by extension relative hypertrophy should not have differed dependent on loading (48) . The effects on patterns of hypertrophy of these CKC and OKC knee extension movements should reasonably translate to other variations of CKC and OKC knee extension movements.
The quadriceps femoris may have experienced differences in task specific demands, and thus hypertrophy localization, between performing CKC and 56 OKC exercises due to inherent differences between exercise type. CKC exercises result in peak torque on the knee joint being reached when the knee is maximally flexed and the quadriceps are maximally lengthened (19).
However, OKC exercises result in peak torque on the knee joint being reached when the knee is maximally extended and the quadriceps are maximally shortened (20). Given fascicle angle changes with muscle length, peak torque: a meaningful driver of hypertrophy, was applied to the quadriceps femoris under mechanically different conditions between exercises (47). Fiber angle has been reported to alter muscle fiber force and shortening velocity, thus imposing different mechanical demands on the tissue (29). Furthermore, active muscle tissue has been reported to variably gear, altering muscle fiber angle dependent on task-specific demands to best meet those demands (29).
Differences in fascicle length and angle have been demonstrated to have implications for sports performance and facilitate performance differentially in sprinters and runners (6,22). It is reasonable that differences in torque-related demands on the knee joint resulted in differences in muscle gearing and thus different localized hypertrophy responses dependent on the regions that were more suited to the gearing required to meet task demands. However, more research is needed to investigate this.
Differences in patterns of hypertrophy may have also been driven by differences in rectus femoris involvement and subsequent hypertrophy.
Though rectus femoris hypertrophy did not significantly differ between the CKC and OKC condition (p = 0.376), it may have had a greater role in extending the knee during the CKC exercise, which is in line with past research due to its biarticular nature (13). This may have partially shifted emphasis away from the other compartments of the quadriceps femoris during training and in having done so reduced the overall amount of hypertrophy in those compartments due to differences in muscle activation (17,49). Those compartments may have been disproportionately responsible for mid-thigh hypertrophy, explaining the lack of significant hypertrophy in that region following the CKC intervention, but more research is needed to conclude this.
Thus, the middle of the thigh may have not received as much of a hypertrophy stimulus in the CKC condition. performance, given greater absolute muscular force production is associated with greater performance in many common sports activities such as running and jumping (52,53). Differences in muscular strength change between conditions were not assessed given the inherent differences in absolute load between the CKC and OKC conditions. During the CKC condition coactivation of the gluteus maximus contributes to muscular force production and increases the absolute load used at a given relative intensity contrasted to the OKC condition where the quadriceps femoris is the only large agonist involved in muscular force production. RPE during muscular strength testing was similar from pre to post intervention for the CKC and OKC conditions, suggesting that the strength tests were reliable measures for participants and that the technical difficulty of either movement did not limit pre intervention strength testing performance. Similar reported RPE from pre to post intervention also suggests that perceptions of exercise intensity and ability to exert force were not altered by changes in menstruation status, which have been previously linked to RPE (54). Given the duration of the 8 week intervention period, female participants were likely to have undergone strength testing in similar menstrual cycle phases if their cycles were of a normal length (55). However, given that significant increases in muscular strength and volume from pre to post intervention were observed in both conditions it is reasonably supported that both training conditions result in favorable adaptation in that CKC and OKC training both result in the development of more muscle mass and increased muscular strength. Though increased muscle mass makes a limb harder to move around a joint due to effectively increasing I, increasing its size can facilitate greater muscle force production and increase muscular strength, facilitating high performance (52,53,56).
Coupling this increase in mass with a favorable shift in CoM, the increase in I about a joint caused by increased mass can be negated by a decrease in I and performance predicated on movement around that joint may have meaningfully improved.
Performance and Therapeutic Applications: Given both conditions resulted in favorable adaptation in terms of muscular strength and size, difference in training efficacy was defined by shift in CoM and the resulting change in I.
Thus it is supported that CKC training is more beneficial for performance than OKC training during movements where the thigh moves about the hip, such as the swing phase of running, which has been demonstrated to be an important movement in sports performance (45,57). In movements requiring the thigh to move about the knee, OKC training would be more beneficial for performance.
It is favorable for athletes to require less muscle force to achieve any given angular acceleration around the hip joint as well as increase peak angular acceleration around the hip joint during a maximal power contraction of the quadriceps femoris (11,22,58,59). Reducing the muscle force required to achieve a given angular acceleration (T = Iꭤ, where T = effort torque, I = I about the origin, and ꭤ = angular acceleration) of the thigh about the hip is beneficial given it will reduce muscular fatigue and prolong the duration of high performance (59). Increasing peak angular acceleration is beneficial in many sports given greater angular acceleration is associated with greater propulsion which increases linear velocity of the whole body and greater linear velocity of the whole body is associated with greater success in many sports (7,(60)(61)(62).
The current study supports the use of CKC knee extension movements for athletes, which has been supported by the literature for reasons other than those presented in this study, such as strengthening multiple muscles involved in running action as opposed to just one (57,(64)(65)(66). Due to this, the current work can be used to inform evidence-based practice and strengthen support for and use of CKC knee extension movements over OKC as it supports current best practices. However, optimal exercise prescription within the context of a full periodized program with appropriate volume for a highly trained athlete is currently unknown. Due to this, skewing training towards prescription of CKC knee extension exercises can be recommended for evidence-based training, however exact prescription parameters are currently unknown.
Therapeutic applications of this research are promising but will vary and require further investigation. Given exercise selection has been suggested to influence hypertrophy localization, targeting hypertrophy induction in injured tissues in specific regions of the quadriceps may be possible and allow for case-specific and condition-specific rehabilitation practices (25,67). Those with general atrophy of the quadriceps femoris may benefit from CKC training as it would facilitate ease of activities of daily living such as standing from sitting and walking by making it easier for movement of the thigh about the hip (46,68). However, this would require further investigation to confirm.
These results are also informative and useful for the non-athletic general population, as improving I results in an improved running economy, and thus can increase duration of aerobic activity, which can contribute to helping regular individuals meet physical activity guidelines for aerobic exercise more readily (9,63). by other researchers to model the quadriceps femoris, having every slice analyzed may have allowed for a more comprehensive understanding of differences in hypertrophy localization between conditions (11,43).

Limitations
During the duration of the intervention period, protein consumption habits were not tracked for any participants. However, participant protein consumption habits may have influenced results. Given underconsumption of protein has been linked with lesser hypertrophy following resistance training and high consumption has been linked with increases in muscular hypertrophy and strength, it is plausible that the magnitude of change in muscle volume and strength may have differed based on participant consumption patterns (69,70). This may have resulted in either a blunted observed effect of the intervention. Of additional note, the cohort of participants involved in the study did not have any unusual characteristics that would limit generalizability of the present study's findings.

Conclusion:
The present study was the first to investigate differences in regional hypertrophy of a muscle following CKC and OKC training. An

Consent Form for Research
We hope that you consider taking part in our study examining how exercise affects quadriceps muscle growth, shape, blood flow, and functional performance. We believe that this study (detailed below) has the potentially to Email: jacob_earp@uri.edu

KEY INFORMATION
Important information to know about this research study: • The purpose of the study is to determine if the exercises a person uses to train causes their muscle to hypertrophy (grow in thickness) at different locations.
• If you choose to participate, you will be asked to take part in 8 weeks of resistance training in which you will train using the squat exercise on one leg and the leg extension exercise on the other leg for. You'll be asked to train 3 days per week and each training session should last ~30 min.
• In addition to the training you will also be asked to take part in 2 days of testing before and after the training. As part of this testing you will have an MRI scan (imaging) of your legs taken at South County Hospital and be asked to perform a strength test.
• The total time commitment to take part in this study is approximately 13.5 hours.
• Risks or discomforts from this research include mild muscle soreness from performing the leg extension exercises.
• The study will be used to determine what aspects of a resistance training or physical therapy program should be emphasized to promote growth in different regions of the quadriceps (a muscle group in your upper leg). This can help people to target their training for specific parts of the muscle which are 1) injured, 2) at risk of injury or 3) important for sport performance.
• You will be provided a copy of this consent form.
participate and you can stop it any time.

INVITATION
You are invited to take part in this research study. The information in this form is meant to help you decide whether or not to participate. If you have any questions, please ask.

Why are you being asked to be in this research study?
You are being asked to be in this study because you may be interested in participating in research related to kinesiology, physical therapy or sports medicine. To take part you must be between the ages of 18-35 and currently free from any current injury or illness or any other lower leg injury which might prevent you from being able to safely perform leg extension or squat exercises. Additionally you must not have engaged in regular resistance training exercise (using weights for your lower body 2 or more days per week) for your lower body within the last 6 months.

What is the reason for doing this research study?
The way in which certain types of exercises (open and closed kinetic chain) affect the way the quadriceps muscle grows and its mechanics are not yet fully known. There is reason to believe that regions of the quadriceps will grow differently depending on whether open and closed chain exercises are performed. If the way that these exercises influence quadriceps growth becomes known then practitioners (in both physical therapy and strength and conditioning) will be better able to design programs for their respective patients and clients. Specific parts of the quadriceps that need to be strengthened and rehabilitated by therapists can be more efficiently targeted, and coaches can train their athlete's quadriceps to better optimize performance.

What will be done during this research study?
After signing this informed consent document we will ask you to complete a health history questionnaire and physical activity survey to ensure that you are free from any lower body injury, which might interfere with your ability to take part in testing and should your testing session and provide descriptive information (this should take about 10 min).
During the pre-training testing, you'll be met at the South County Hospital to undergo a lower body MRI scan. Before undergoing MRI testing you'll complete a food recall log where you'll have to recall what you'd eaten over the past day to the best of your ability from memory. You will additionally be asked to provide a urine sample in a specimen collection cup to be analyzed and then immediately discarded before undergoing body composition testing of your entire body via BIA, which is a noninvasive measurement tool that simply requires you stand on a scale. Afterwards you'll perform a standardized warm-up consisting of 5 min of low intensity aerobic exercise followed by a series of low intensity lower body exercises & stretches. Afterwards, a small probe will be placed on the skin over your quadriceps muscle that will record muscle activity. Once this set-up is completed you'll be asked to perform a series of two different types of leg exercises. One will be a single leg squat and the other a single leg leg extension. Afterwards, you will be asked to participate in 8 weeks of resistance training (3 days per week), in which you'll perform, these two different exercises on different legs. Once the intervention concludes you will undergo one final MRI scan. The entire study should take approximately 9 weeks and approximately 10.5 hours.

How will my data be used?
Your data will coded so that you cannot be identified and results from analysis of your data will presented at scientific conferences and published in scientific journal without any individual identifiers.
What are the possible risks of being in this research study?
There are minimal risks to you from being in this research study such as delayed muscle soreness from exercise or mild skin agitation from adhesives used to secure equipment to your skin. There are risks associated with MRI use, however the risks of MRI testing will be minimized via screening, however MRI use is contraindicated if pregnant or with certain other implantations or conditions. To ensure you are eligible to undergo MRI testing a pre-screening form will be administered to you.

What are the possible benefits to you?
You may experience increased muscle size and strength of your quadriceps muscles on both legs, as would be expected during an 8 week training intervention.

What are the possible benefits to other people?
The results from this study will provide information that can potentially be used to improve the effectiveness of exercise programs that are designed to help people to prevent or recover from tendon injury.
What are the alternatives to being in this research study?
Instead of being in this research study you can decide not to take part in this study without any repercussions.

What will being in this research study cost you?
There is no cost to you to be in this research study.

Will you be compensated for being in this research study?
You will receive $250 for the time commitment associated with the study.
Several payments will be made during the duration of the intervention to equal this amount for your time investment.

What should you do if you have a problem during this research study?
Your welfare is the major concern of every member of the research team. If you have a problem as a direct result of being in this study, you should immediately contact one of the people listed at the beginning of this consent form.
How will information about you be protected?
Reasonable steps will be taken to protect your privacy and the confidentiality of your study data. You will not lose any benefits to which you are entitled.