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These data support reducing volume during periods of CT. Two and three-week training days seem to have a greater frequency of success when compared to four, five, and six day studies. Though not all higher volume programs attenuated strength, many did, leaving exercise practitioners a warning to program volume in CT periods with caution. Also, scheduling RT work before END work has proven a successful method. These data are encouraging for exercise practitioners who need to augment simultaneous physical qualities to compete in their respective sports. This article reviewed 19 concurrent training (CT) studies that simultaneously sought to increase strength and endurance qualities.
Concurrent training (CT) is the simultaneous physical preparation of two or more exercise modalities. Common combinations include strength and power, hypertrophy and strength, and power and power endurance. The combination of strength and endurance (END) is common, but the efficacy of such a combination has conflicting findings. END can be defined as maximal oxygen consumption (VO2 Max) for END athletes and anaerobic power output (for field sport athletes). The purpose of this review is to discuss the abundance of research for and against the concurrent training of strength and END. A total of 19 CT studies were analyzed. The results are listed in Table 1. Based on the results, it can be inferred that CT training can be used successfully in training regimes to simultaneously improve multiple physiological qualities, namely strength, and END. However, research indicates that lower training volumes are needed. Also, completing resistance training (RT) work prior to END work is necessary to improve both END and strength simultaneously.
Hawley  concluded that both strength training and hypertrophy training are attenuated by endurance work because the physiological processes are diametrically opposed. According to Hawley, muscle protein anabolism occurs when the rate of protein synthesis is greater than the rate of muscle protein degradation. This process is slow because "protein synthesis must exceed protein breakdown for an extended period (weeks to months)". Hawley attributed the attenuation to an interruption of intracellular signaling pathways (E.g. Phosphatidylinositol 3-kinase, rapamycin) ultimately causing a retardation of protein synthesis.
A meta-analysis by Wilson 2012 examined the interference of aerobic and resistance exercise. Of the 21 studies and total of 422 effect sizes, the authors concluded that running had the strongest relationship to interference. "For moderator variables, resistance training concurrently with running, but not cycling, resulted in significant decrements in both hypertrophy and strength". Other studies support this finding [2-5]. Other studies do not support a strong correlation between interference of strength development and running based CT [6-9]. Based upon the scientific evidence, the effect of high speed sprinting and slower running on strength cannot be determined more favorable or negative.
Of the eight studies that used cycling as an endurance modality listed in Table 1, three of the studies indicated that END attenuated strength development [3,5,10]. No discernable inferences can be made based on speed or duration of these attenuations. However, in two cases the volume of the work was high, indicating that volume may be a more important determinant than mode of exercise.
Five of the nine studies that indicated an interference effect exists between END training and strength training presented situations where the weekly training volume was high [35,10,11]. Bell et al.  engaged in both END and RT training six days a week by alternating RT and END days. Likewise, both Hennessy & Watson  and Hickson  used an alternating RT and END schedule five days and six days of the week respectively. Also, using a daily alternating RT and END protocol, Karavirta et al.  had participants train at less frequency (4 days) but employed long duration exercise bouts (45-90 minutes). Jones et al.  cited the importance of accounting for total volume in CT
High volumes of endurance training resulted in the inhibition of lower body strength, whereas low volumes did not. Lower body power was attenuated by high and low frequencies of endurance. High frequencies of endurance resulted in increased cortical responses to training. These data suggest that if strength development is the primary focus on a training intervention, frequency of endurance must be low. Not all examples of low volume CT experiences had positive outcomes [2,12] and not all high-volume studies resulted in negative outcomes [13,14]. However, several studies give great insight to exercise professionals when planning concurrent training periods. Higher volumes of endurance work may interfere with strength development and should be programmed carefully.
a) Low Volumes: Whereas high exercise volumes have indicated a negative effect on CT, low to moderate volumes have shown to be beneficial in improving both strength qualities and aerobic qualities. Alves et al.  found success increasing strength and power measures in young children using same day training of RT and END protocols for two days a week. Davis et al.  indicated strength increases close to 20% by employing a three-day schedule of RT work and subsequent treadmill running. In a similar study that utilized a three-day workweek encompassing subsequent END work after RT, Gravelle and Blessing  indicated no strength interference effect. Petros, Toubekis and Platanou  combined high intensity maximal strength training (85-90%) and low volume END work (32 total minutes per week) to increase strength and swim speed in water polo players. Similarly, Sale, MacDougall and Jacobs  programmed only 45 minutes of weekly END work and found little interference. Lastly, Wong et al.  studied professional male soccer players engaged in two days of RT and two days of END work that encompassed 16 high-speed interval sprints. All lifts including bench press, squat, jump squat, and high pull increased.
b) Strength before END: In addition to lower volume, successful outcomes were seen in studies where RT training was performed before END training. Both Laird et al.  and McCarthy, Agre, Graf, Pozniak and Vailas  had participants train three days a week and scheduled END work after RT; one group four hours after and another group immediately after respectively. Both groups saw no interference effect and realized positive strength adaptations. Even under high volume training, cases where RT precedes END work resulted in positive strength adaptations . Varela Sanz, Tuimil, Abreu & Boullosa  combined relatively low volumes of RT with subsequent volumes of END work and results indicated strength gains across all lifts. However, groups that underwent high volumes of END work did jump less high than the lower volume END group.
A lack of compelling evidence exists in the reviewed literature to make credible inferences about study length, mean age of population, and mode of END exercise (cycling vs running). These data support reducing volume during periods of CT. Two and three-week training days seem to have a greater frequency of success when compared to four, five, and six day studies. Though not all higher volume programs attenuated strength, many did, leaving exercise practitioners a warning to program volume in CT periods with caution. Also, scheduling RT work before END work has proven a successful method. These data are encouraging for exercise practitioners who need to augment simultaneous physical qualities to compete in their respective sports.