Progression Models in Resistance Training for Healthy Adults

Nicholas A. Ratamess, Ph.D.; Brent A. Alvar, Ph.D.; Tammy K. Evetoch, Ph.D., FACSM; Terry J. Housh, Ph.D., FACSM (Chair); W. Ben Kibler, M.D., FACSM; William J. Kraemer, Ph.D., FACSM; N. Travis Triplett, Ph.D.

Disclosures

March 01, 2010

In This Article

Trainable Characteristics

Muscular Strength

The ability to generate force is necessary for all types of movement. Muscle fiber cross-sectional area (CSA) is positively related to maximal force production.[71] The arrangement of fibers according to their angle of pennation, muscle length, joint angle, and contraction velocity can alter the expression of muscular strength.[95,145] Force generation is further dependent upon motor unit activation, and motor units are recruited according to their recruitment threshold that typically involves the activation of the slower (lower force-producing) motor units before the faster (higher force-producing) units, that is, size principle.[114] Adaptations to RT enable greater force generation through numerous neuromuscular mechanisms. Muscle strength may increase significantly within the first week of training,[39] and long-term strength enhancement manifests itself through enhanced neural function (e.g., greater recruitment, rate of discharge),[234] increased muscle CSA,[5,176,250] changes in muscle architecture,[138] and possible adaptations to increased metabolites, for example, H+,[242] for increased strength. The magnitude of strength enhancement is dependent on the type of program used and the careful prescription of muscle actions, intensity, volume, exercise selection and order, rest periods between sets, and frequency.[157]

Muscle Action

Most RT programs primarily include dynamic repetitions with both concentric (CON; muscle shortening) and eccentric (ECC; muscle lengthening) muscle actions, whereas isometric (ISOM; no net change in muscle length) actions play a secondary role (e.g., during nonagonist muscle stabilization, core strength, grip strength, pauses between ECC and CON actions, or specific agonist ISOM exercises). Greater force per unit of muscle size is produced during ECC actions[147] than either CON or ISOM actions. Moreover, ECC actions require less motor unit activation per specific load,[147] are less metabolically demanding,[26] and are conducive to promoting hypertrophic adaptation[112] yet result in more pronounced delayed onset muscle soreness[58] as compared with CON actions. Dynamic CON muscular strength improvement is greatest when ECC actions are included with CON actions,[56] and independently, ECC isokinetic training has been shown to produce greater muscle action-specific strength gains than CON training.[64] The role of muscle action manipulation during RT is minimal with respect to overall progression because most programs include both CON and ECC actions in a given repetition. However, the inclusion of additional ISOM exercise may be beneficial. In some programs, the use of different forms of ISOM training, for example, functional ISOM[131] and supramaximal ECC actions,[143] has been reported to produce additional benefit. Specifically, certain ISOM actions have been recommended for promoting low back health and have been demonstrated effective for the selective recruitment of postural, spinal-stabilization musculature.[181]

Evidence Statement and Recommendation.Evidence Category A. For progression during RT for novice, intermediate, and advanced individuals, it is recommended that CON, ECC, and ISOM muscle actions be included.[56,64,112,131,143]

Loading

Altering the training load affects the acute metabolic,[221] hormonal,[151,152,153,154,158,159,165,219] neural,[96,235] and cardiovascular[72] responses to resistance exercise. Depending on an individual's training experience and current level of fitness, proper loading during RT encompasses one or more of the following loading schemes: 1) increasing load based on a percentage of 1 RM, 2) increasing absolute load based on a targeted repetition number, or 3) increasing loading within a prescribed zone (e.g., 8-12 RM). The load required to increase maximal strength in untrained individuals is fairly low. Loads of 45-50% of 1 RM (and less) have been shown to increase dynamic muscular strength in previously untrained individuals.[9,33,255,268] Light loads that can be lifted a maximum of 15-25 repetitions have been shown to increase strength in moderately trained individuals.[227] It appears greater loading is needed with progression. At least 80% of 1 RM is needed to produce further neural adaptations and strength during RT in experienced lifters.[96] Several pioneering studies indicated that training with loads corresponding to 1-6 RM (mostly 5-6 RM) was most conducive to increasing maximal dynamic strength.[22,201] Strength increases have been shown to be greater using heavy weights for 3-5 RM compared with 9-11 and 20-28 RM.[33] Although significant strength increases have been reported using loads corresponding to 8-12 RM and lighter,[33,149,250] this loading range may be inferior for maximizing strength in advanced lifters.[96] Research examining periodized RT has demonstrated a need for variable-intensity loading schemes.[74,223] Contrary to early suggestions of 6 RM loading, it appears that using a variety of training loads is most conducive to maximizing muscular strength.[74] Meta-analytical data have shown that 60% of 1 RM produced the largest effect sizes for strength increases in novice individuals whereas 80% of 1 RM produced the largest effect sizes for strength increases in trained individuals[225] and 85% of 1 RM was most effective in athletes.[206] For novice individuals, it has been suggested that moderate loading (50-60% of 1 RM or less) be used initially as learning proper form, and technique is paramount. These dose-response data refer to average training dosages, that is, mean loads used for all exercises. Further, using a variety of loads appears to be most effective for long-term progression in muscular strength.[157] Recent studies have shown that self-selected RT intensities are lower than what is recommended, for example, 38-58% of 1 RM.[76,87,222] Thus, intensity needs to be prescribed above one's threshold (based on targeted repetition number) for progression in experienced populations.

Evidence Statement and Recommendation.Evidence Category A. It is recommended that novice to intermediate individuals train with loads corresponding to 60-70% of 1 RM for 8-12 repetitions and advanced individuals cycle training loads of 80-100% of 1 RM to maximize muscular strength.[9,33,96,206,225,227,255,268]

Evidence category B. For progression in those individuals training at a specific RM load, it is recommended that a 2-10% (lower percent for small muscle mass exercises, higher percent increase for large muscle mass exercises) increase in load be applied when the individual can perform the current workload for one to two repetitions over the desired number on two consecutive training sessions.[68]

Volume

Training volume is a summation of the total number of repetitions performed during a training session multiplied by the resistance used (kg) and is reflective of the duration of which muscles are being stressed.[262] Volume has been shown to affect neural,[102] hypertrophic,[258] metabolic,[221] and hormonal[92,151,152,191,220] responses and subsequent adaptations to RT. Altering training volume can be accomplished by changing the number of exercises performed per session, the number of repetitions performed per set, or the number of sets per exercise. Low-volume programs, for example, high load, low repetitions, moderate to high number of sets, have been characteristic of RT. Studies using two,[55,170] three,[149,250] four to five,[56,122] and six or more[123,236] sets per exercise have all produced significant increases in muscular strength in both trained and untrained individuals. In direct comparison, studies have reported similar strength increases in novice individuals between two and three sets[35] and two and four sets,[202] whereas three sets have been reported superior to one and two.[23] Although little is known concerning the optimal number of sets performed per muscle group per session, a meta-analysis of 37 studies has shown that approximately eight sets per muscle group produced the largest effect size in athletes.[206,207]

Another aspect that has received considerable attention is the comparison of single- and multiple-set programs. In many of these studies, one set per exercise performed for 8-12 repetitions at a relatively slow velocity has been compared with both periodized and nonperiodized multiple-set programs. A common criticism of these investigations is that the number of sets per exercise was not controlled from other variables such as intensity, frequency, and repetition velocity. Notwithstanding this concern, most research investigations comparing single- versus multiple-set training for muscular fitness have examined the effects of a standard single-set training program relative to that of any number of possible multiple-set programs of varying intensity. This design has made the process of identifying a clear-cut prescription recommendation very difficult because these studies have yielded conflicting results. Several studies have reported similar strength increases between single- and multiple-set programs,[40,132,248] whereas others reported multiple-set programs superior[23,27,237,251,256] in previously untrained individuals. Since 2002, six studies have shown multiple-set superiority for 33-100% of the dynamic strength assessments used, whereas the remaining dynamic strength assessments showed similar increases.[81,126,175,192,203,231] These data have prompted the notion that untrained individuals respond favorably to both single- and multiple-set programs and formed the basis for the popularity of single-set training among general fitness enthusiasts.[68] In resistance-trained individuals, multiple-set programs have been shown to be superior for strength enhancement[142,149,155,160,228,238] in all but one study.[110] Among resistance-trained postmenopausal women, multiple-set training led to 3.5-5.5% strength increases, whereas single-set training led to −1% to 2% strength reductions.[142] No comparative study has shown single-set training superior to multiple-set training in trained or untrained individuals.

The results of meta-analytical studies have shown multiple-set RT superior to single sets for strength enhancement in untrained[224,225] and trained populations[224,225,278] and superior for strength increases for programs lasting 17-40 wk.[278] These studies have shown that performing three to four sets per exercise produced the most substantial effect sizes.[224,225] Thus, it appears that both program types are effective for increasing strength in untrained to moderately trained individuals during relatively short-term training periods. Long-term studies support the contention that a moderate increase in training volume is needed for further improvement.[27,224,225,278] However, there is a point where further increase in volume may be counterproductive. In weightlifters, a moderate volume was shown to be more effective for increasing strength than low or high volumes of training with similar intensity.[90] The key factor may be variation of training volume (and its interaction with intensity) rather than absolute number of sets.

Evidence Statement and Recommendation.Evidence Category A. It is recommended that one to three sets per exercise be used by novice individuals initially.[23,35,40,55,132,170,202,206,207]

Evidence Category B. For progression into intermediate to advanced status, data from long-term studies indicate that multiple sets be used with systematic variation of volume and intensity over time.[142,149,155,160,228,238] To reduce the risk of overtraining, a dramatic increase in volume is not recommended. It is important to point out that not all exercises need to be performed with the same number of sets, and that emphasis of higher or lower volume is related to the program priorities of the individual as well as the muscle(s) trained in an exercise movement.

Exercise Selection

Both single- and multiple-joint exercises have been shown to be effective for increasing muscular strength in the targeted muscle groups using multiple modalities, for example, free weights, machines, cords, etc..[47,157] Multiple-joint exercises, such as bench press and squat, require complex neural responses[37] and have generally been regarded more effective for increasing overall muscular strength because they enable a greater magnitude of weight to be lifted.[253] Single-joint exercises, such as knee extensions and knee curls, have been used to target-specific muscle groups and pose a reduced level of skill and technical involvement. It is important to note that alterations in body posture, grip, and hand width/foot stance and position change muscle activation and alter the exercise. Thus, many variations or progressions of single- and multiple-joint exercises can be performed. Another way to vary exercise selection is to include unilateral as well as bilateral exercises. The level of muscle activation differs when an exercise is performed bilaterally versus unilaterally. Unilateral training may increase bilateral strength (in addition to unilateral strength), and bilateral training may increase unilateral strength.[179] Unilateral training has been shown to improve some aspects of sports performance, such as single-leg jumping ability to a greater extent than bilateral training.[179] Of interest has been the performance of single- and multiple-joint exercises in unstable environments, for example, with stability balls, wobble boards, and BOSU balls.[144] These exercises have been shown to increase the activity of lower torso musculature and other stabilizer muscles (compared with stable environments); however, the magnitude of agonist force production is considerably lower resulting in lighter weights lifted.[10,21] There are a multitude of exercises that can be performed in a variety of conditions that leaves many options for RT variation.

Evidence Statement and Recommendation. Evidence Category A. Unilateral and bilateral single- and multiple-joint exercises should be included in RT with emphasis on multiple-joint exercises for maximizing overall muscle strength in novice, intermediate, and advanced individuals.[33,96,97,98,99,100,101,102,103,104,105,106,107,113,118,120,149,150,151,152,153,154,155,156,157,169,172,176]

Free Weights and Machines

Weight machines have been regarded as safer to use, easy to learn, and allow performance of some exercises that may be difficult with free weights, for example, knee extension. Machines help stabilize the body and limit movement about specific joints involved in synergistic force production, and machine exercises have demonstrated less neural activation when matched for intensity for most comparisons to free-weight exercises.[178] Unlike machines, free weights may result in a pattern of intra- and intermuscular coordination that mimics the movement requirements of a specific task. Both free weights and machines are effective for increasing strength. Research shows that free-weight training leads to greater improvements in free-weight tests and machine training results in greater performance on machine tests.[30] When a neutral testing device is used, strength improvement from free weights and machines appears similar.[274] The choice to incorporate free weights or machines should be based on level of training status and familiarity with specific exercise movements as well as the primary training objective.

Evidence Statement and Recommendation. Evidence Category A. For novice to intermediate training, it is recommended that free-weight and machine exercises are included.[30,169,172,178,248,249,250,274]

Evidence Category C. For advanced RT, it is recommended that emphasis be placed on free-weight exercises with machine exercises used to compliment program needs.[100,101,102,103,251]

Exercise Order

The sequencing of exercises significantly affects the acute expression of muscular strength.[240] This also applies when exercises are sequenced based on agonist/antagonist muscle group relationships. Muscle force and power may be potentiated when opposing exercises (antagonist movements) are performed;[16] however, force and power may be reduced if the exercises are performed consecutively.[171] Studies show that multiple-joint exercise (bench press, squat, leg press, and shoulder press) performance declines significantly when these exercises are performed later (after several exercises stressing similar muscle groups) rather than early in a workout.[244,245] Considering that these multiple-joint exercises have been shown to be effective for increasing strength, maximizing performance of these exercises by performing them early in a workout may be necessary for optimal strength gains.[247]

Evidence Statement and Recommendation.Evidence Category C. Recommendations for sequencing exercises for novice, intermediate, and advanced strength training for total body (all muscle groups trained in the workout), upper/lower body split (upper-body musculature trained 1 d and lower-body musculature trained another day), and muscle group split (individual muscle groups trained during a workout) workouts include large muscle group exercises before small muscle group exercises, multiple-joint exercises before single-joint exercises, higher-intensity exercises before lower-intensity exercises, or rotation of upper and lower body or agonist-antagonist exercises, that is, exercise performed for a muscle group followed by an exercise for the opposing muscle group.[244,245]

Rest Periods

The amount of rest between sets and exercises significantly affects metabolic,[150,221] hormonal,[158] and cardiovascular[72] responses to an acute bout during resistance exercise as well as performance of subsequent sets[149,279] and training adaptations.[212,230] Acute resistance exercise performance may be compromised with one versus 3-min rest periods,[149] and strength recovery may not be complete within 3 min.[20] Several studies have shown that the number of repetitions performed may be compromised with short rest intervals, and 3- to 5-min rest intervals produce less performance decrements than 30 s to 2 min.[221,229,269,270,271] In untrained individuals, circuit RT programs (using minimal rest in between exercises) have been shown to produce modest increases in strength.[108] However, most longitudinal training studies have shown greater strength increases with long versus short rest periods (e.g., 2-5 min vs 30-40 s[3,213,230]), and one study has shown a lack of strength increase with 40-s rest periods.[213] It is important to note that rest period length will vary based on the complexity of a given exercise (e.g., Olympic lifts and variations require longer rest periods) and the primary objective for incorporating the exercise into the training program (i.e., not every exercise will use the same rest interval).

Evidence Statement and Recommendation. Evidence Category B. For novice, intermediate, and advanced training, it is recommended that rest periods of at least 2-3 min be used for core exercises using heavier loads (those exercises included specifically to improve maximal strength such as the squat and bench press).[3,149,213,214,221,229,230,269,270,271]

Evidence Category C. For assistance exercises (those exercises complimentary to core exercises), a shorter rest period length of 1-2 min may suffice.[149,213,229,230,269]

Velocity of Muscle Action

The velocity of muscular contraction used to perform dynamic muscle actions affects the neural,[97] the hypertrophic,[123,241] and the metabolic[17,173] responses to resistance exercise and is inversely related to the relative load during maximal muscle contractions.[48,234] Isokinetic training has been shown to increase strength specific to the training velocity with some carryover in performance at other velocities in the proximity to the training velocity.[39,44,63,123,137,145] However, it appears that training at moderate velocity (180-240°·s−1) produces the greatest strength increases across all testing velocities.[137]

Dynamic constant external resistance (also called isotonic) or isoinertial training poses a different stress. Significant reductions in force production are observed when the intent is to perform the repetition slowly with submaximal loading. In interpreting the effects of intent to perform slow repetitions, it is important to note that two types of slow-velocity contractions exist during dynamic RT, unintentional and intentional. Unintentional slow velocities are used during high-intensity repetitions in which either the loading or the fatigue is responsible for the repetition tempo and duration (velocity of movement).[187] Conversely, intentional slow-velocity contractions are used with submaximal loads and occur when an individual has greater control of the velocity and influences the time the muscles are under tension.

It has been shown that CON force was significantly lower for an intentionally slow velocity (5:5; e.g., 5-s CON, 5-s ECC) compared with a traditional (moderate) velocity with a corresponding lower level of neural activation, for example, determined via electromyography.[143] The rate of energy expenditure is lower using an intentionally slow velocity.[173] Substantially, less peak force, power, and number of repetitions performed were observed with "super slow" repetition velocity (10:10) compared with a self-selected fast velocity when matched for intensity.[111] A 30% reduction in training load is necessary when using a "very slow" velocity (10:5) compared with a slow velocity (2:4).[141] Another study comparing "very slow" (10:5) to traditional velocity (1:1) showed that 37-40% reductions in training loads were needed to attain the same number of repetitions.[129] These data suggest that motor unit activity may be limited when intentionally slow velocities at lighter loads are incorporated and ultimately may not provide an optimal stimulus for strength enhancement in resistance-trained individuals.

Compared with slow velocities, moderate (1-2:1-2) and fast (< 1:1) velocities have been shown to be more effective for enhanced muscular performance capacities (e.g., number of repetitions performed, work and power output, and volume)[161,189] and for increasing the rate of strength gains.[113] The number of repetitions performed is based upon a continuum depending on the lifting velocity where the largest numbers of repetitions are performed with a fast velocity and decreases proportionately as velocity becomes slower.[234] The effect of lifting velocity on repetition performance appears largest with light to moderately heavy loading.[234] Most advanced RT studies examining fast velocities with moderately high intensities have shown these velocities to be more effective than traditionally slower velocities for strength increases.[133,190] It appears that the intent to maximally accelerate the weight during training is critical in maximizing strength gains.[19] Although loading may be moderate to heavy, the intent to lift the weight as fast as possible has been shown to be critical for maximizing strength increases.[19] Keeler et al.[141] showed that traditional velocity (2:4) RT produced significantly greater strength increases over 10 wk than "super slow" training in five of eight exercises trained (overall increase of 39% vs 15% in traditional and "super slow," respectively). Over 6 wk of RT in untrained individuals, it was shown that training at a faster velocity (1:1) led to ~11% greater strength increases than training at a slower velocity (3:3).[192] However, a study by Neils et al.[195] showed statistically similar increases in strength between "super slow" and slow-velocity (2:4) training.

Evidence Statement and Recommendation.Evidence Category A. For untrained individuals, it is recommended that slow and moderate velocities be used.[113,141,161,189,192,195]

Evidence Category B. For intermediate training, it is recommended that moderate velocity be used for RT.[113,141,161,189,192,195]

Evidence Category C. For advanced training, the inclusion of a continuum of velocities from unintentionally slow to fast velocities is recommended. The velocity selected should correspond to the intensity and the intent should be to maximize the velocity of the CON muscle action.[19,133]

Frequency

Optimal RT frequency (the number of workouts per week) depends upon several factors such as volume, intensity, exercise selection, level of conditioning, recovery ability, and number of muscle groups trained per workout session. Numerous studies have used frequencies of two to three alternating days per week in previously untrained individuals.[34,44,56,116] This frequency has been shown to be an effective initial frequency, whereas 1-2 d·wk−1 appears to be an effective maintenance frequency for those individuals already engaged in RT.[93] In several studies comparing strength gains, 1) 3 d of training per week was superior to 1[183] and 2 d,[94] 2) 3 d produced similar strength increases to 2 d·wk−1 when volume was equated,[34] 3) 4 d·wk−1 was superior to three,[127] 4) 2 d·wk−1 was superior to 1,[217] and 5) 3-5 d·wk−1 was superior to 1 and 2 d.[85] Meta-analytical data have shown that strength gains in untrained individuals were highest with a frequency of 3 d·wk−1.[225]

Evidence Statement and Recommendation.Evidence Category A. It is recommended that novice individuals train the entire body 2-3 d·wk−1.[34,44,56,94,116,183,225]

It appears that progression from untrained to intermediate training does not necessitate a change in frequency for training each muscle group but may be more dependent upon alterations in other acute variables such as exercise selection, volume, and intensity. Increasing frequency enables greater specialization (e.g., greater exercise selection and volume per muscle group in accordance with more specific goals). Upper/lower body split or muscle group split routines are common at this level in addition to total-body workouts.[157] Similar increases in strength have been observed between upper/lower- and total-body workouts.[32]

Evidence Category B. It is recommended that for progression to intermediate training, a frequency of 3-4 d·wk−1 be used (3 d if using a total-body workout, 4 d if using a split routine thereby training each major muscle group twice).[34,85,94,183,225]

Optimal progression of frequency during advanced training varies considerably. It has been shown that football players training 4-5 d·wk−1 achieved better results than those who trained either 3 or 6 d·wk−1.[118] Advanced and elite weightlifters and bodybuilders use high-frequency training, for example, four to six sessions per week or more. Double-split routines (two training sessions per day with emphasis on different muscle groups) are common during training,[102] which may result in 8-12 training sessions per week. Frequencies as high as 18 sessions per week have been reported in elite Olympic weightlifters.[280] The rationale for high-frequency training is that frequent short sessions followed by periods of recovery, nutrition supplementation, and food intake allow for high-intensity training and performance (reduced fatigue). Häkkinen and Kallinen[103] reported greater increases in muscle cross-sectional area (CSA) and strength when training volume was divided into two sessions per day as opposed to one. Elite power lifters train 4-6 d·wk−1.[75] It is important to note that not all muscle groups are trained per workout during a high-frequency model of training. Meta-analytical data have shown that training a muscle group two times per week in advanced individuals yielded the highest effect size[225] and two to three times per week yielded similar effect sizes in athletes.[206]

Evidence Category C. It is recommended that advanced lifters train 4-6 d·wk−1. Elite weightlifters and bodybuilders may benefit from using very high frequency, for example, two workouts in 1 d for 4-5 d·wk−1.[102,118,206,225]

Muscular Hypertrophy

It is well known that RT induces muscular hypertrophy[156,176,249,250] through mechanical, metabolic, and hormonal processes. The process of hypertrophy involves a proportionate increase in the net accretion of the contractile proteins actin and myosin as well as other structural proteins. Mechanical loading leads to a series of intracellular events that ultimately regulates gene expression and protein synthesis. RT may alter the activity of nearly 70 genes,[232] up-regulate factors involved with myogenesis (e.g., myogenin, MyoD), and down-regulate inhibitory growth factors (e.g., myostatin).[148,233] Protein synthesis in human skeletal muscle increases after only one bout of vigorous RT[210] and peaks approximately 24 h postexercise. This anabolic environment remains elevated from 2 to 3 h postexercise up through 36-48 h postexercise.[83,166] Other factors such as fiber type,[176] muscle action,[84] metabolite formation,[242] amino acid intake,[80] and endocrine responses (testosterone, growth hormone [GH], cortisol, insulin, and insulin-like growth factor I) contribute to the magnitude of hypertrophy.[158] Optimal hypertrophy may comprise maximizing the combination of mechanical (use of heavy weights, ECC actions, and low to moderate volume) and metabolic (accumulation of metabolic waste products) stimuli.

The time course of hypertrophy has been examined in previously untrained individuals. Neural adaptations predominate during the early stages of training.[188] Muscle hypertrophy becomes evident within the first 6 wk,[211] although changes in the quality of proteins[250] and protein synthesis rates[211] take place much earlier. From this point onward, there appears to be interplay between neural adaptations and hypertrophy in the expression of strength. Less muscle mass is recruited during training with a given workload once adaptation has taken place.[215] These findings indicate that progressive overloading is necessary for maximal muscle fiber recruitment and, consequently, muscle fiber hypertrophy. This also indicates that alterations in program design targeting both neural and hypertrophic factors may be most beneficial for maximizing strength and hypertrophy.

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