Secondary Logo

Journal Logo

Original Research

Effects of Carbohydrate Supplementation on Force Output and Time to Exhaustion During Static Leg Contractions Superimposed with Electromyostimulation

Wax, Benjamin; Brown, Stanley P.; Webb, Heather E.; Kavazis, Andreas N.

Author Information
Journal of Strength and Conditioning Research: June 2012 - Volume 26 - Issue 6 - p 1717-1723
doi: 10.1519/JSC.0b013e318234ec0e
  • Free



Resistance exercise is important to the training regime of many athletes and the general population (13). In this regard, people who perform resistance exercise consume nutritional supplements to enhance force output and improve exercise performance. One of the most popular supplements consumed before, during, and after exercise comprise food items rich in carbohydrate. Specifically, carbohydrate intake has been proposed to sustain prolonged anaerobic and aerobic exercise performances (32).

It is well known that resistance exercise demands high levels of adenosine triphosphate (ATP) to meet the demands of the working muscle. Originally, it was believed that almost all energy used during resistance exercise was obtained from the ATP-PC system (22). However, studies have reported that glycolytic activity occurs during multiple sets of resistance exercise (9,12,31) that can lead to a decrease in muscle glycogen (27,29). The decrease in muscle glycogen has been shown to be detrimental to exercise performance by causing a reduction in force output (33). Theoretically, carbohydrate intake before and during resistance exercise may augment performance by providing a glycogen sparing effect. This glycogen sparing effect has been found to transpire during intermittent types of resistance exercise (10,12). In this regard, Conley and Stone (5) speculated that carbohydrate intake may enhance performance during resistance exercise by delaying fatigue.

However, research investigating the ergogenic effects of carbohydrate supplementation during resistance exercise has produced contradictory results. Lambert et al. (20) reported an increase in exercise performance during a carbohydrate supplementation trial. These authors reported that subjects supplemented with carbohydrate were able to perform a larger number of sets and repetitions during multiple sets of leg extension to voluntary failure (20). In addition, Haff et al. (15) reported that consumption of carbohydrates before, during, and after a resistance exercise protocol increased exercise performance during an exercise session later on the same day. However, these results are in conflict with the findings of Kulik et al. (19), who reported that carbohydrate intake did not enhance resistance exercise performance during squat exercise performed at intensities of 85% 1-repetition maximum (1RM) to volitional failure. These findings are in line with those of Conley et al. (6), who reported that carbohydrate consumption did not provide any augmentation to subjects' exercise performance variables during a squat protocol at 65% 1RM. These disparate results may be related to subject selection criteria, methodological differences, and central nervous system factors (e.g., central fatigue).

Central fatigue is considered to be any fatigue proximal to a motor nerve; therefore, the lack of consistency in performance output during resistance exercise may be the result of reduced motor unit recruitment (3). For example, Requena et al. (26) reported an increased maximal force output in quadriceps femoris after following a protocol consisting of superimposed electromyostimulation with static contractions. Furthermore, Bigland-Ritchie et al. (3) reported that subjects produced greater force output during maximum isometric contractions when superimposed stimulation was applied to the quadriceps muscle. The increase in force output may be attributed to dissimilar patterns of muscle fiber activation occurring between electromyostimulation and voluntary contractions (8).

To our knowledge, there are no studies examining the potential ergogenic effects of carbohydrate ingestion while performing static contractions superimposed with electromyostimulation. Therefore, the objective of this investigation was to examine the effects of carbohydrate ingestion during concomitant electromyostimulation and static voluntary leg contractions on muscular performance. It was hypothesized that carbohydrate ingestion would allow for greater muscular performance by increasing force output and extending time to exhaustion compared with placebo.


Experimental Approach to the Problem

The subjects reported to the laboratory for 3 sessions at the same time of the day. All the subjects arrived at the laboratory before 07:30 for each session. Each session was separated by 7 days. The first session was used to determine subjects' anthropometric data and served as a familiarization session for the exercise protocol. During session 1, the subjects were instructed to refrain from strenuous leg exercise 48 hours before sessions 2 and 3. Also, the subjects were told to avoid caffeine and alcohol consumption during the 48-hour period preceding sessions 2 and 3. For 3 days before the second and third sessions, the subjects were also encouraged to select food items from a predetermined list based on a normal dietary intake of approximately 55% carbohydrates, 20% protein, and 25% fat (11,14,15,19) designed to ensure maximal preexercise glycogen levels. To increase compliance, the subjects maintained a log of their dietary intake for the 3 days before session 2 and were instructed to duplicate their nutritional intake before session 3. A randomized, counterbalanced, double-blind design was used for this study. Specifically, in session 2, the subjects either consumed carbohydrate (CHO) or placebo (PL) beverage before and during a single leg static exercise protocol (details described below). Seven days later, the subjects consumed the other beverage and performed the same exercise protocol. The dependent measures obtained were force output, time to exhaustion, and venous blood lactate.


Six apparently healthy, resistance trained male subjects, who participated and won or placed in the top 3 positions in their perspective weight classes at state and national level bodybuilding and power lifting competitions volunteered to participate in this investigation. The subjects were free (self-reported) of any anabolic steroid use for a year before the experimental sessions. All the subjects gave their signed informed consent and completed a health history questionnaire in accordance with the Institutional Review Board approval before data collection. Descriptive characteristics for the subjects are presented in Table 1.

Table 1
Table 1:
Anthropometric characteristics of the subjects used in the study.*



During the CHO condition, the subjects consumed a loading dose of a 10% carbohydrate solution (Carboplex, Unipro Inc., Atlanta, GA, USA) at 1 g of carbohydrate per kilogram of body mass. The subjects also consumed the same solution at a dose of 0.17 g of carbohydrate per kilogram of body mass before the exercise protocol started and thereafter at 6-minute intervals until the completion of the exercise protocol. Prior data indicate that this concentration of carbohydrate gets cleared from the stomach at a rate comparable with that of water, while maintaining a high rate of carbohydrate availability to the muscle (23). In addition, the carbohydrate intake timing was based on prior research, which reported that this amount tends to enhance resistance exercise performance during lower body muscle contractions (20). During the PL condition, the subjects consumed a solution that contained saccharin and aspartame that was similar in taste, texture, color, and volume to the carbohydrate solution at the same time points.

Session 1

During session 1, the subjects' height, body mass, and body composition were recorded. Body composition was determined using the 3-site skinfold method described by Pollock and Wilmore (25). Also, during session 1, the subjects performed mock submaximal leg contractions to familiarize themselves with the experimental procedures and equipment.

Sessions 2 and 3

Seven days after session 1, and after an overnight (10-hour) fast, the subjects reported to the laboratory and were questioned about their compliance in regard to their activity level (no strenuous leg exercise). The subjects also submitted their 3-day nutritional journal for review by a trained evaluator. We used the 3-day nutritional journal method because prior studies have shown that this facilitates consistent muscle glycogen stores (11,15,19,21,30). Following the evaluator's review and approval of the compliance guidelines, the subjects consumed their assigned solution (CHO or PL; minute 0).

Electrodes (10.16 × 10.16 cm) were securely placed on the subjects transversely to the proximal (about 10 cm below the groin) and as close as possible to the motor point of the vastus medialis and vastus lateralis muscles on the subjects' perceived dominant leg. The subjects were then seated with the dominant knee secured to a 300-lb load cell strain gauge (Transducer Techniques, Temecula, CA, USA) at an angle between 85 and 90° just above the malleoli. This position mimics a typical seated leg extension model used in training facilities. A restraint was placed around the waist (pelvic region) to maintain quadriceps muscle length and prevent the substantial use of the hip flexors during the exercise protocol.

To determine maximal voluntary force output, the subjects performed a static maximal voluntary contraction without electromyostimulation by using their dominant leg (minute 10). Force output was amplified (Model SGA-6, Advanced Mechanical Technology, Inc., Watertown, MA, USA) by using the signal sent via the load cell and the signal was recorded at 1,000 Hz using DataPac 2000 (Run Technologies, Laguna Hills, CA, USA). Each subject performed a series of three, 3-second maximal voluntary contractions. The subjects rested 90 seconds between maximal voluntary contractions. The greatest force held for 1 second was deemed to be the maximal voluntary contraction.

After determination of maximal voluntary contraction, the subjects underwent transcutaneous muscle stimulation to obtain a level of stimulation they could comfortably tolerate without discomfort (minute 20). The quadriceps muscle was stimulated using a transcutaneous electronic muscle stimulator (Dynapulse 8, Health Group, Vista, CA, USA), and pulse waves of 60 Hz lasting 100 microseconds were used. Stimulation intensity was determined by the pain tolerance of each subject. The maximal tolerated intensity varied between 70 and 110 mA. The level of stimulation and electrode placement remained constant for each subject throughout the study, and this stimulation was used to superimpose electromyostimulation during static maximal voluntary contractions. This assured minimal variability in fiber recruitment during each session. No subject reported any serious discomfort during the experiment.

Exercise Protocol

The subjects began the exercise protocol (minute 30) by performing repeated static contractions, at a target force of 50% of maximal voluntary contractions. The target force output was marked on a storage oscilloscope in front of the subjects to provide visual feedback. The subjects generated a continuous pattern of 20-second contractions without electromyostimulation followed by a 40-second rest period (duty cycle 0.20). During the last 3 seconds of the first 20-second voluntary static contraction, and thereafter at 5-minute intervals, electromyostimulation was superimposed during maximal voluntary contraction in a concomitant manner. The superimposed electromyostimulation with maximal voluntary contractions was carried out to quantify force output. The stimulation was employed to assure the force response reflecting the muscle strength entailed maximum fiber activation in the quadriceps region. Importantly, this technique has been reported to effectively elicit maximal force during voluntary contractions (24).

The exercise protocol continued until exhaustion. Exhaustion in this study was defined as the inability of a subject to maintain 50% target force output for 5 consecutive seconds. After exhaustion, the subjects remained seated for 5 minutes, after which a final maximal voluntary contraction with superimposed electromyostimulation was performed. Force output was calculated as the sum of the force produced during maximal voluntary contractions superimposed with electromyostimulation (i.e., force output = force produced during the last 3 seconds of the first 20-second contraction + force at 5-minute intervals until exhaustion + force produced at 5 minutes postexhaustion). These procedures are summarized in Figure 1.

Figure 1
Figure 1:
Illustration summarizing the experimental procedures and exercise protocol used in the study. PL = placebo; CHO = carbohydrate.

Blood Analyses

Blood samples were obtained at minute 10 (pre), within 15 seconds of exhaustion (failure), and 5 minutes after exhaustion (recovery). All the blood samples were obtained using a 20-gauge Teflon cannula placed in a superficial antecubital vein by a registered nurse. The cannula was maintained patent using an isotonic saline solution (with 10% heparin solution) placed in a 3-way stopcock with a male Luer lock adapter. Blood lactate concentrations were determined using an YSI 1500 Sport Lactate Analyzer (YSI Inc., Yellow Springs, OH, USA). Whole blood was used to determine hemoglobin in duplicate using the cyanmethemoglobin technique (Sigma Diagnostics, St. Louis, MO, USA), and hematocrit was analyzed in duplicate via standard microcapillary techniques and microcentrifugation. Plasma volume shifts (7) were calculated, and blood lactate was corrected for plasma volume changes.

Statistical Analyses

Dietary recall data were assessed using Food Processor SQL (version 10; ESHA Research, Salem, OR, USA). All other statistical analyses were performed by using the GraphPad Prism (GraphPad Software, Inc., La Jolla, CA, USA). A sample size analysis was performed, which showed that 6 subjects were required to achieve a power of 0.80. The intraclass correlations were ≥0.80. Force output and time to exhaustion between PL and CHO were analyzed using paired t-tests. Blood lactate data were analyzed by using a 2 (condition; PL or CHO) × 3 (time; pre, failure, recovery) repeated measures analysis of variance. The F ratio was not found to be significant, and thus, no further analysis was performed for blood lactate. An a priori of α ≤ 0.05 was set as statistically significant. All data are reported as mean ± SD.


The 3-day dietary intake before testing is shown in Table 2. No differences were detected in carbohydrate, protein, fat, or total kilocalories consumed for CHO or PL.

Table 2
Table 2:
Carbohydrate, protein, fat, and total kilocalorie intake of the subjects at 72, 48, and 24 hours before performing the exercise protocol.*†

As Figure 2 shows, force output was significantly increased (p < 0.05) in CHO compared with PL. Specifically, the subjects in the CHO had a force output of 5,540.1 ± 726.1 N, and the subjects in the PL had a force output of 3,638.7 ± 524.5 N.

Figure 2
Figure 2:
Force output (A) and time to exhaustion (B) after the consumption of carbohydrate (CHO) or placebo (PL). Force output = force during the last 3 seconds of the first 20-second voluntary contraction superimposed with electromyostimulation and thereafter at 5-minute intervals + force produced during the last 3 seconds of the 20-second voluntary contraction superimposed with electromyostimulation at 5 minutes postexhaustion. Data shown are mean ±SD. *p < 0.05.

The individual response of the subjects to force output after CHO or PL supplementation is shown in Figure 3. In this regard, all the 6 subjects had greater force output when they consumed CHO compared with that after consuming PL.

Figure 3
Figure 3:
Data showing each subject's force output (A) and time to exhaustion (B) the in carbohydrate (CHO) and placebo (PL) groups. Force output = force during last 3 seconds of the first 20-second voluntary contraction superimposed with electromyostimulation and thereafter at 5-minute intervals + force produced during the last 3 seconds of 20-second voluntary contraction superimposed with electromyostimulation at 5 minutes postexhaustion. *p < 0.05.

Analysis of performance variables (Figure 2) also revealed that CHO had a significantly higher (p < 0.05) time to exhaustion (CHO = 29.0 ± 13.1 minutes; PL = 16.0 ± 8.1 minutes).

Specifically, 5 subjects had a greater time to exhaustion when they received CHO compared with when they received PL. One subject had the same time to exhaustion after both CHO and PL (Figure 3).

Repeated measure analysis of variance did not show any significant interaction effect for blood lactate (p = 0.09) (Figure 4). Therefore, no further posttest analysis was conducted for blood lactate.

Figure 4
Figure 4:
Blood lactate after carbohydrate (CHO) or placebo (PL) supplementation. Blood was collected at PRE (before the exercise protocol begun), FAILURE (at exhaustion), and RECOVERY (5 minutes after exhaustion). TheF value of the repeated measures analysis of variance did not show any significant difference (p = 0.09).


Results of this study demonstrate that carbohydrate consumption during static leg contractions superimposed with electromyostimulation provides ergogenic benefits for resistance trained athletes. Prior research has yielded conflicting results regarding carbohydrate intake during different modes of resistance exercise. This may be related to the type of exercise modality used and selection criteria for the subjects involved in the study. Our findings are in line with those investigations proposing an ergogenic benefit from carbohydrate intake during moderate to high levels (>50% of 1RM) of resistance training exercise (14,15,20).

Prior investigations report that resistance exercise results in a decrease in muscle glycogen stores (12,15,27,29). Although muscle glycogen stores in the quadriceps muscles were not directly measured in this study, we speculate that significant muscle glycogenolysis did indeed occur in these muscles because the subjects reported for the exercise sessions in a fasted state. Further, with the repeated bouts of muscle contraction in the quadriceps, it may be expected that glycogen depletion has occurred in these muscles.

Investigators have postulated that intermittent carbohydrate ingestion during prolonged resistance exercise would mitigate the glycolytic effect occurring in the muscle by providing an exogenous source of glucose. Because most forms of resistance exercise have intermittent characteristics, carbohydrate ingestion may foster an internal environment within the exercising muscle that augments performance (14). This may be attributed to decreased reliance on muscle glycogen (1,2,12,33) increased rate of glycogen synthesis (15), increased hormonal activity (28), and continued delivery of exogenous glucose to the working muscle (5,14,20).

The ergogenic effects yielded in this resistance exercise protocol are similar to that of prior investigations using lower body exercise (14,20). In the present investigation, carbohydrate ingestion delayed time to exhaustion, which allowed for more overall work to be performed. As a result of delayed time to exhaustion, increased force output yielded a significantly greater performance difference in the CHO compared with PL. In a similar study, Lambert et al. (20) reported that carbohydrate ingestion approached significance for subjects performing 10 repetitions of leg extensions at 80% of 1RM to muscular exhaustion. Similarly, Haff et al. (15) reported a significant increase in performance during multiple sets of squats performed at 55% of 1RM to volitional failure. Furthermore, in another study carried out by Haff et al. (14), carbohydrate ingestion increased performance load during sessions in which time was held constant at 57 minutes. The duration of exercise in this study was not held constant, because exhaustion was defined as an inability to maintain 50% force output for 5 consecutive seconds.

Carbohydrate intake during resistance exercise has yielded conflicting results in regards to its effect on blood lactate. Many studies have reported a significant increase in blood lactate (4,18) after carbohydrate supplementation, whereas other studies reported no differences (14,16,17). Hypothetically, an increased delivery of blood glucose via an exogenous source would attribute to an increased stimulation of the glycolytic pathway, thus resulting in an increase production of lactic acid. However, the results of this study indicate that there was no significant difference for blood lactate between subjects supplemented with carbohydrate or PL.

In summary, the data from this investigation support our hypothesis and suggest the consumption of a carbohydrate solution before and during static resistance exercise enhances static leg exercise by increasing force output and extending time to exhaustion.

Practical Applications

The results of this study demonstrate that carbohydrate intake extended time to exhaustion and increased force production during single leg static contractions intermittently superimposed with electromyostimulation. This increase in resistance exercise performance after carbohydrate supplementation lends support to the use of carbohydrate supplements during exhaustive bouts of resistance exercise. Therefore, our data suggest that carbohydrate supplementation before and during resistance exercise might help increase the training volume of athletes.


The authors thank Drs. Linda Chitwood, Wade Waters, and Jatuporn Wongsathikun for their valuable assistance with this project. No grant support was used to perform this study. The results of this study do not constitute endorsement of any products by the authors or National Strength and Conditioning Association.


1. Bagby GJ, Green HJ, Katsuta S, Gollnick PD. Glycogen depletion in exercising rats infused with glucose, lactate, or pyruvate. J Appl Physiol 45: 425–429, 1978.
2. Bergstrom J, Hultman E. A study of the glycogen metabolism during exercise in man. Scand J Clin Lab Invest 19: 218–228, 1967.
3. Bigland-Ritchie B, Jones DA, Hosking GP, Edwards RH. Central and peripheral fatigue in sustained maximum voluntary contractions of human quadriceps muscle. Clin Sci Mol Med 54: 609–614, 1978.
4. Coggan AR, Coyle EF. Metabolism and performance following carbohydrate ingestion late in exercise. Med Sci Sports Exerc 21: 59–65, 1989.
5. Conley MS, Stone MH. Carbohydrate ingestion/supplementation or resistance exercise and training. Sports Med 21: 7–17, 1996.
6. Conley MS, Stone MH, Marsit JL, O'Bryant HS, Nieman DC, Johnson RL, Butterworth D, Keith R. Effects of carbohydrate ingestion on resistance exercise. J Strength Cond Res 9: 201, 1995.
7. Dill DB, Costill DL. Calculation of percentage changes in volumes of blood, plasma, and red cells in dehydration. J Appl Physiol 37: 247–248, 1974.
8. Enoka RM. Activation order of motor axons in electrically evoked contractions. Muscle Nerve 25: 763–764, 2002.
9. Essen-Gustavsson B, Tesch PA. Glycogen and triglyceride utilization in relation to muscle metabolic characteristics in men performing heavy-resistance exercise. Eur J Appl Physiol Occup Physiol 61: 5–10, 1990.
10. Gollnick PD, Armstrong RB, Sembrowich WL, Shepherd RE, Saltin B. Glycogen depletion pattern in human skeletal muscle fibers after heavy exercise. J Appl Physiol 34: 615–618, 1973.
11. Haff GG, Jackson JR, Kawamori N, Carlock JM, Hartman MJ, Kilgore JL, Morris RT, Ramsey MW, Sands WA, Stone MH. Force-time curve characteristics and hormonal alterations during an eleven-week training period in elite women weightlifters. J Strength Cond Res 22: 433–446, 2008.
12. Haff GG, Koch AJ, Potteiger JA, Kuphal KE, Magee LM, Green SB, Jakicic JJ. Carbohydrate supplementation attenuates muscle glycogen loss during acute bouts of resistance exercise. Int J Sport Nutr Exerc Metab 10: 326–339, 2000.
13. Haff GG, Lehmkuhl MJ, McCoy LB, Stone MH. Carbohydrate supplementation and resistance training. J Strength Cond Res 17: 187–196, 2003.
14. Haff GG, Schroeder CA, Koch AJ, Kuphal KE, Comeau MJ, Potteiger JA. The effects of supplemental carbohydrate ingestion on intermittent isokinetic leg exercise. J Sports Med Phys Fitness 41: 216–222, 2001.
15. Haff GG, Stone MH, Warren BJ, Keith R, Johnson RL, Nieman DC, Williams FJ, Kirksey KB. The effect of carbohydrate supplementation on multiple sessions and bouts of resistance exercise. J Strength Cond Res 13: 111–117, 1999.
16. Haub MD, Haff GG, Potteiger JA. The effect of liquid carbohydrate ingestion on repeated maximal effort exercise in competitive cyclists. J Strength Cond Res 17: 20–25, 2003.
17. Hoffman JR, Ratamess NA, Ross R, Shanklin M, Kang J, Faigenbaum AD. Effect of a pre-exercise energy supplement on the acute hormonal response to resistance exercise. J Strength Cond Res 22: 874–882, 2008.
18. Kirkendall DT. Effects of nutrition on performance in soccer. Med Sci Sports Exerc 25: 1370–1374, 1993.
19. Kulik JR, Touchberry CD, Kawamori N, Blumert PA, Crum AJ, Haff GG. Supplemental carbohydrate ingestion does not improve performance of high-intensity resistance exercise. J Strength Cond Res 22: 1101–1117, 2008.
20. Lambert CP, Flynn MG, Boone JBJ, Michaud TJ, Rodriguez-Zayas J. Effects of carbohydrate feeding on multiple-bout resistance exercise. J Strength Cond Res 5: 192–197, 1991.
21. Luebbers PE, Potteiger JA, Warren BJ, Clore JN, Jennings C, Bond DS. Glucose uptake after resistance training of different intensities but of equal work volume. J Strength Cond Res 22: 1094–1100, 2008.
22. MacDougall JD, Ward GR, Sale DG, Sutton JR. Biochemical adaptation of human skeletal muscle to heavy resistance training and immobilization. J Appl Physiol 43: 700–703, 1977.
23. Owen MD, Kregel KC, Wall PT, Gisolfi CV. Effects of ingesting carbohydrate beverages during exercise in the heat. Med Sci Sports Exerc 18: 568–575, 1986.
24. Paillard T, Noe F, Passelergue P, Dupui P. Electrical stimulation superimposed onto voluntary muscular contraction. Sports Med 35: 951–966, 2005.
25. Pollock ML, Wilmore JH. Exercise in Health and Disease: Evaluation and Prescription for Prevention and Rehabilitation (2nd ed.). Philadelphia, PA: W.B. Saunders, 1990. pp. xiii, 741.
26. Requena B, Zabala M, Padial P, Feriche B. Sodium bicarbonate and sodium citrate: Ergogenic aids? J Strength Cond Res 19: 213–224, 2005.
27. Robergs RA, Pearson DR, Costill DL, Fink WJ, Pascoe DD, Benedict MA, Lambert CP, Zachweija JJ. Muscle glycogenolysis during differing intensities of weight-resistance exercise. J Appl Physiol 70: 1700–1706, 1991.
28. Tarpenning KM, Wiswell RA, Hawkins SA, Marcell TJ. Influence of weight training exercise and modification of hormonal response on skeletal muscle growth. J Sci Med Sport 4: 431–446, 2001.
29. Tesch PA, Ploutz-Snyder LL, Yström L, Castro MJ, Dudley GA. Skeletal muscle glycogen loss evoked by resistance exercise. J Strength Cond Res 12: 67–73, 1998.
30. Thyfault JP, Carper MJ, Richmond SR, Hulver MW, Potteiger JA. Effects of liquid carbohydrate ingestion on markers of anabolism following high-intensity resistance exercise. J Strength Cond Res 18: 174–179, 2004.
31. Vollestad NK, Tabata I, Medbo JI. Glycogen breakdown in different human muscle fibre types during exhaustive exercise of short duration. Acta Physiol Scand 144: 135–141, 1992.
32. Wildman R, Kerksick C, Campbell B. Carbohydrates, physical training, and sport performance. Strength Cond J 32: 21–29, 2010.
33. Yaspelkis BB, Patterson JG, Anderla PA, Ding Z, Ivy JL. Carbohydrate supplementation spares muscle glycogen during variable-intensity exercise. J Appl Physiol 75: 1477–1485, 1993.

lactate; isometric contraction; maximal voluntary contractions

Copyright © 2012 by the National Strength & Conditioning Association.