Muscle hypertrophy, or muscle growth, is the process of increasing the size of skeletal muscle fibers through resistance training. Time Under Tension (TUT) is a frequently discussed variable that dictates the speed at which a single repetition is performed. TUT measures the total duration the muscle is actively under stress during an exercise set. This measure is often manipulated in training programs to increase the stimulus for muscle building.
Deconstructing Time Under Tension (TUT)
Time Under Tension is calculated by summing the duration of the three distinct phases of a single repetition. The first phase is the eccentric action, where the muscle lengthens while under load (e.g., lowering a weight during a squat). The second phase is an isometric pause, which is any static hold at the turnaround point of the movement. The third phase is the concentric action, where the muscle shortens and overcomes the resistance (e.g., standing back up in a squat).
The duration of these three phases determines the total TUT for one repetition. For example, a 3-second eccentric, a 1-second pause, and a 1-second concentric lift results in a total TUT of 5 seconds. This calculation shows that movement speed is a programmable variable, allowing precise control over the duration the muscle works against resistance.
Research Consensus on Repetition Speed
Research indicates that a wide range of repetition durations can effectively stimulate muscle growth. Studies show that moderate repetition speeds, where a single rep lasts between 2 to 8 seconds, are highly effective for hypertrophy. This range allows for controlled movement that maintains tension while permitting the use of a sufficient load.
The hypertrophic response is similar whether repetitions last 2 seconds or 8 seconds, provided the lifter reaches a similar level of effort. This suggests that total repetition duration is less important than ensuring the muscle is adequately challenged. Many training programs recommend a total repetition duration of approximately 4 to 6 seconds, balancing control and load.
Very slow speeds, such as those exceeding 10 seconds per repetition, have not been shown to produce superior muscle growth. These prolonged tempos are often less effective because the extended TUT forces the lifter to use a significantly lighter weight. This reduction in load compromises the primary stimulus for muscle growth, overriding potential benefits from extended TUT.
Mechanical Tension as the Primary Stimulus
Moderate repetition speeds are effective because of the concept of mechanical tension. Mechanical tension is the primary signal that triggers the cellular processes leading to muscle growth. It is the force generated within the muscle fibers when they contract against resistance. This force stimulates mechanosensors, initiating anabolic pathways that promote protein synthesis and muscle building.
There is an inverse relationship between TUT and the load that can be used. When a lifter dramatically increases TUT for a repetition, they must reduce the weight lifted to complete the set. This reduction in load consequently reduces the overall magnitude of mechanical tension applied to the muscle fibers. Since load magnitude is a significant factor in creating high mechanical tension, repetition duration becomes less important than the intensity of the force overcome.
The objective is to maximize the product of load and time, ensuring sufficient force is applied to the muscle fibers. Training protocols that maintain high mechanical tension through challenging weights, even with moderate repetition speeds, are typically more effective for stimulating muscle growth.
Practical Application Implementing Rep Tempo
To translate the scientific consensus on repetition speed into an actionable training variable, trainers use a four-digit tempo notation. This notation precisely dictates the duration in seconds for each phase of a repetition. The sequence is always written as: Eccentric phase, Isometric Pause at the stretched position, Concentric phase, and Isometric Pause at the contracted position.
A common tempo notation of 4-0-1-0 means the lifter takes 4 seconds to lower the weight, has no pause at the bottom, takes 1 second to lift the weight, and has no pause at the top. The “X” notation is sometimes used for the concentric phase to indicate an explosive, maximal-effort lift performed with control.
A lifter applying a moderate tempo to a barbell squat might use a 3-1-1-0 tempo. This involves a 3-second descent, a 1-second pause in the deepest position, and a 1-second ascent back to the start. For a bench press, a 3-0-2-0 tempo translates to a 3-second lowering of the bar, no pause on the chest, and a 2-second press back up to the lockout. This systematic approach ensures the lifter maintains a controlled, moderate repetition speed aligned with hypertrophy recommendations.