Strengthen your knowledge of hypertrophy and its definition, significance and training variables.
Zachary Mang, PhD, Jeremy Ducharme, M.S., and Len Kravitz, PhD

Introduction: What is Skeletal Muscle Hypertrophy?
Fitness professionals design resistance training (RT) programs to help clients achieve several fitness goals, including increased power, strength, endurance, size and metabolic health. An increase in skeletal muscle size, as measured by an increase in the cross-sectional area of the muscle fiber (i.e., its thickness), is referred to as muscular hypertrophy (Haun et al., 2020). Skeletal muscle hypertrophy can be observed as a sarcoplasmic expansion (i.e., intracellular fluid, sarcoplasmic proteins, and glycogen) and/or by an increase in the diameter size of the contractile proteins of the muscle (Haun et al., 2020). Hypertrophy can be measured as whole-body changes in lean mass via body composition techniques such as hydrostatic weighing, dual x-ray absorptiometry, bioelectrical impedance, skinfolds and circumferences. For more direct measurements of hypertrophy, researchers may use magnetic resonance imaging, muscle biopsies, and ultrasound imaging.

How Does Hypertrophy Physiologically Occur?
Fundamentally, hypertrophy occurs when the rate of muscle protein synthesis (MPS) outpaces the rate of muscle protein breakdown inside of muscle cells for a prolonged period of time (Bamman et al., 2018). Resistance training is effective for increasing MPS because it imposes metabolic stress, mechanical stimuli, and exercise-induced muscle damage which effectively contribute to an increase in muscle cross-sectional size (Wackerhage et al., 2019). Wackerhage et al. explain that the stress of muscular contractions stimulate a protein known as the mammalian target of rapamycin complex 1 (mTORc1). When mTORC1 is stimulated, it activates several proteins inside of the muscle cell that initiate an increase in MPS. Also, RT has been shown to increase the efficiency of ribosomal proteins which positively influence MPS (Bamman et al., 2018). Ribosomes are macromolecule complexes that activate biological protein synthesis.

Why Does Hypertrophy Matter?
From a functional perspective, hypertrophy is important because it contributes to muscular strength, which is associated with increased mobility, gait speed, and independence, while also providing a robust intervention combatting chronic diseases in older clients (McLeod et al., 2019) (See Figure 1). From a cardiometabolic health standpoint, the research shows that an increase in muscle mass and strength is associated with improvements in several health-related variables and longevity (Wackerhage et al., 2019). The following section will discuss the essential training variables that help achieve optimal strength and hypertrophy outcomes.

Fundamental Training Variables for Hypertrophy
Intensity
The external load lifted during RT is referred to as intensity, which is typically quantified as a percentage of one-repetition maximum (%1-RM) or a repetition range. For example, if a client's 1-RM is 100 pounds, a fitness professional could design a set of 20 reps with 50 pounds, which could be denoted as a 20RM or 50% 1-RM. Pertaining to the effect of intensity on skeletal muscle hypertrophy, a recent review concluded that hypertrophic adaptations occur at &Mac179; 30% 1RM, thus proposing a new paradigm shift whereby muscular adaptations can be obtained across a wide spectrum of loading zones (Schoenfeld et al., 2021). In other words, hypertrophy will occur after training with low-intensity, moderate-intensity, and high-intensity external loads, as long as lifters perform their sets close to failure (Schoenfeld et al., 2021). Schoenfeld et al. (2021) continue that from a practical standpoint, a case can be made that moderate loads provide the most ef&Mac222;cient means to achieve muscle development. The researchers note that that light load training tends to produce more discomfort, displeasure, and a higher rating of perceived exertion than training with moderate-to-high loads. Therefore, the moderate loads are likely to be more enjoyable, leading to better exercise adherence. Uniquely, it is so interesting to highlight that fitness pros can successfully design resistance training programs with a variety of external loads and perhaps choose to rotate intensities in a daily, weekly, or monthly manner.
Proximity to failure
Whether sets of RT should be performed to failure or not has been a long-standing debate among fitness professionals and researchers. A recent meta-analysis concluded that failure and non-failure training are equally effective for hypertrophy and strength (Grgic et al., 2021), meaning that fitness professionals can use phases of both in their training programs. To strike a balance between failure and non-failure training, fitness pros are encouraged to teach their clients how to terminate their sets when they are 1-3 repetitions shy of true muscular failure.
Volume
Volume represents the amount of work performed during an RT session, which is typically quantified as the number of sets devoted to a specific muscle group. For example, performing 3 sets of 10 reps on a knee-extension machine would be 3X more volume than performing 1 set of 10 reps for the same exercise. As it pertains to set volume, Krieger (2010) reported in a meta-analytical study that multiple sets per exercise were associated with significantly greater increases in muscle size than a single set per exercise. This finding was true regardless of training status of the individuals or training program duration. When quantified as number of sets performed for a specific muscle group per week, Schoenfeld et al. (2017) reported that hypertrophy occurs in a dose-response manner, as low (<5 weekly sets), moderate (5-9 weekly sets), and high-volume (>10 weekly sets) training resulted in 5.4%, 6.6%, and 9.8% increases in muscle size, respectively. For best practice, fitness professionals should use set volume as a means to provide overload and/or tapering. In other words, if a lifter has been performing 6 weekly sets for their quadriceps, they may experience significant hypertrophic gains if their set volume is increased to 10 weekly sets. In contrast, a lifter who has been performing 20 weekly sets for their quadriceps may benefit from a tapering block where set volume is reduced to 10 weekly sets.
Frequency
Resistance training frequency is best described as the number of weekly sessions dedicated to a specific muscle group. When it comes to muscle hypertrophy, Grgic et al. (2019), in a meta-analysis study, summarized that RT frequency plays a secondary role in regards to muscle hypertrophy. The scientists state that the evidence suggests a potential, but slight, advantage with frequencies greater than one-day-per-week. Ultimately, fitness professionals must consider their client's schedule when determining how many training sessions to book per week.
Rest Intervals
A rest interval (RI) is defined as the time dedicated to recovering between sets and between exercises during RT sessions. A recent study by Schoenfeld et al. (2016) reported that 3-minute RIs were more effective than 1-minute RIs for hypertrophy and strength. Thus, by lengthening RIs, fitness professionals allow their client to feel more replenished before beginning subsequent sets, which may result in greater mental focus, total repetitions and muscular fitness output. If a client is pressed for time, fitness pros can balance the benefits of prolonged RIs with training efficiency by prescribing upper/lower-body- or push/pull-super sets, or by executing corrective/mobility exercises during their RIs.
Tempo
Repetition tempo, which is also referred to as repetition duration, denotes the amount of time dedicated to complete one repetition within a set of RT. In other words, if a lifter lowers the weight for 3 seconds (i.e., eccentric phase), and raises the weight for 1 second (i.e., concentric phase), the repetition tempo would be 4 seconds. Researchers are typically interested in repetition tempo because it has a direct effect on the time-under-tension during a set of exercise. Using the previous example, if a lifter completed 5 reps or 10 reps with the 4-second tempo, their TUT would be 20 or 40 seconds, respectively. Recently, a review by Lyons et al. (2020) concluded that hypertrophy and strength are stimulated by slow (&Mac179;14 sec), moderate (4-13 sec), or fast (< 4 sec) tempos, and all should be included in a RT programs. For practical application, we recommend that fitness pros incorporate a variety of repetition tempos when designing their RT programs for clients.

Major Message Going Forward
The benefits of resistance training are incredibly wide-ranging, from reducing chronic disease risk, to improvement of healthy lifestyle function particularly as we age, to enhancing athletic sport performance. As we begin our new journey forward from the pandemic, it is time for the fitness industry to really excel in our efforts to get every person 'lifting for life.' We have the evidence-based 'tools' to make this happen.

Bios:
Zachary Mang, PhD is a post-doc research associate for the wellness program at the Los Alamos National Lab where he specializes in strength and conditioning for structural firefighters. His research interests include resistance training for hypertrophy, oxidative adaptations to resistance training, and using resistance training as a frontline defense to prevent chronic disease.

Jeremy Ducharme, B.S. is a doctoral student in the Exercise Science program at the University of New Mexico in Albuquerque, where he works as a teaching and research assistant. His research interests include exercise amongst older adults, specifically assessments of maximal oxygen consumption and adaptations from resistance training.

Len Kravitz, PhD, CSCS is a professor and program coordinator of exercise science at the University of New Mexico where he received the Presidential Award of Distinction and the Outstanding Teacher of the Year award. Just recently he was awarded the canfitpro international presenter of the year.