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Logically structured and systematically sequenced variation is a cornerstone of an appropriately designed periodized strength and conditioning program (1).
The ability to introduce variation into a resistance training program can include manipulating training load, number of sets and repetitions, exercise order, exercise density (i.e., number of exercises in a session, cycle, or period of training), training focus or priority, and rest intervals between sets (5,6). A relatively new method for introducing training variation into a training program involves manipulating the training set structure. This can involve varying the intraset rest (or inter-repetition rest) interval and/or training load (5,6,7). This type of set modification has been defined as a cluster set and may offer a novel stimulus for maximizing power output and movement velocity (7,8).Types of SetsWhen examining the literature there are two main set structures, the traditional and cluster set, that can be utilized in a resistance training program (5). The most common set configuration used by strength and conditioning professionals is the traditional set. This involves a series of repetitions that are performed with the same load in a continuous fashion (5,6,7). While these traditional set structures are generally better suited for inducing hypertrophy, increasing strength, or improving strength endurance, they may not be the best configurations for improving movement velocity, power endurance, or power output (5,17).The second most common set structure is the cluster set, which was developed as a means for improving the overall quality of the training set by increasing the velocity and power profile of the training set (5,6,7). Typically, the cluster set structure utilizes a 5 - 45 s rest interval between each individual or series of repetitions, which results in increases in power output, barbell velocity, and barbell displacement when compared to traditional set configurations (5,6,7,8,9,13,14). A variety of cluster set structures can be created (Table 1), but there are generally four basic variants. These variants include the standard, undulating, wave, and ascending cluster set (5). The standard cluster set involves placing a 5 - 45 s rest interval between individual repetitions or clusters of repetitions, while resistance or training load is held constant.
For example, a total of 10 repetitions at 75% of 1-repetition maximum (1RM) can be programmed for a given set with 15 s separating every pair of repetitions. This example provides the following loading pattern for a 10/2 cluster set:
Figure 1. 10/2 Cluster Set Loading Pattern
Another strategy could be to utilize a 5-s rest after each individual repetition, yielding the following loading pattern for a 10/1 cluster set:
Figure 2. 10/1 Cluster Set Loading Pattern
Overall, the standard cluster set structure can be oriented in several different fashions where the number of repetitions contained in the cluster are varied or the rest interval is manipulated to change the physiological stress stimulated by the cluster set (Table 1) (5).
Table 1. Example Cluster Set Variants
For example, if the target goals of a phase of training are strength endurance or hypertrophy, the overall number of repetitions contained in the set should be higher and the rest interval between each individual repetition, or a series of repetitions, should be lower. Conversely, if strength and power are the target goals less repetitions should be contained in the set, but longer rest intervals should be placed between the series of repetitions. The standard cluster set configuration can be further adapted by introducing variations to the resistance used during each repetition or series of repetitions (5). Three basic loading modifications can be made to the cluster set that allow for the creation of the undulating, wave, and ascending cluster set variants. With the undulating cluster, the resistance used in the set is structured in a pyramid fashion where the load is progressively increased and then decreased. For example, an undulating cluster might look like the following:
Figure 3. Undulating Cluster Set
Generally, the undulating cluster is constructed with an even number of repetitions so that a pyramid loading scheme (i.e., ascending to a target load and descending back to the original load) can be formatted. The overall programmed load for the cluster set is the average of all the intensities performed for each of the repetitions completed in the set. For example, in the 6/1 cluster above the average intensity of the set is 71%. Conceptually, the descending arm of the undulating cluster series results in a potentiation effect, with higher power outputs and velocities of movement (7). Because of this potentiation effect, the undulating cluster set is recommended for use during strength, strength-power, or peaking phase of a periodized training plan. Another potential loading modification to the structure set is the wave loading structure (5). The wave loading structure utilizes a loading pattern where loading alternates between two or more targeted loads throughout a set. For example, a wave loaded cluster set could be structured in the following fashion:
Figure 4. Wave Loaded Cluster Set
As with the undulating cluster set model, the wave loaded cluster set attempts to create a potentiation effect by performing lighter loads after the completion of higher intensity efforts. Because of the basic structure of this type of cluster set, it is generally employed during strength or strength-power phase of a periodized training plan. The ascending cluster is slightly different than the undulating cluster in that the weight is increased with each repetition, or cluster of repetitions, and ends with the highest load as the last repetition or cluster. For example, an ascending cluster might look like the following:
Figure 5. Ascending Cluster
The central premise that underlies the ascending cluster is to work the athlete up to higher intensity efforts. Because of this targeted outcome, the use of the ascending cluster is typically restricted to the strength-power or peaking phase of an overall strength and conditioning plan. More complex cluster sets can also be created by altering the number of repetitions contained in the cluster, while also employing undulating, ascending, or wave loading patterns. For example, the following set structure may be created:
Figure 6. Complex Wave Cluster Set
An additional method for varying cluster sets involves modifying the rest interval between clusters to target specific training outcomes (5,6). For example, a shorter rest interval (<10 s) will target more of the strength or power endurance end of the spectrum, while longer rest intervals (30 - 45 s) will focus more on strength and power development (Table 2). Ultimately, the structure of the cluster set is dictated by the phase of the periodized training plan and the adaptive response targeted by the employed cluster set and periodization plan implemented by the strength and conditioning coach (5,6).
Table 2. Basic Cluster Set Rest Interval Patterns