To Lift or to Lower? Negative reps in detail.
The use of concentric (shortening) & eccentric (lengthening) muscle actions are the most common functional muscle actions performed in resistance training exercise. It is important to understand that skeletal muscle can eccentrically lower a higher weight than it can concentrically raise up. Bodybuilders especially use training methods such as; eccentric reps or ‘negatives’ to take advantage of the perceived benefits of the higher loads ‘lifted’ eccentrically.
If you’re not sure what ‘eccentric’ or ‘negative’ reps are – they are a method of training where the lifter raises the bar concentrically, often using a ‘cheat’ technique, in order that they can then lower the heavier load eccentrically. The eccentric phase of the rep is completed slowly with control taking approximately 4 seconds to lower.
Figure 1: See Reference (1).
As shown in fig 1, concentric muscle action increases muscle torque (rotational force), whilst eccentric muscle action increases load torque. Decreasing the velocity of a movement also increases the force produced in the muscle (1,2). The precise mechanics of this function are still not fully understood (3), although some key properties of eccentric movements have been identified;
increased muscle fibre stretching (3),
brain electrical stimulation (4),
neural recruitment of fast twitch fibres (4)
and force in passive tension movements (5).
In both eccentric and concentric muscle contractions, the pre-requisite for muscle growth is that muscle damage must occur to then be repaired. This can be stimulated by 3 possible methods (fig 2):
Figure 2: See References (6)
Mechanical tension refers to the high load bearing of eccentric actions; this causes a higher stretch induced stimulus which has been associated with increases in protein synthesis stimulation at a higher level than concentric actions (6). So, in principle, higher loads could mean higher stimulation of repair mechanisms (4).
Metabolic stress refers to the build-up of metabolites energy producing reactions in the muscle cell that stimulate repair responses through molecular functions (6). This is in addition to causing lactic acid build-up which decreases muscle contractile function (7).
Both of these exercise stimuli cause muscle damage as the end result.
However, these factors seem to be interlinked, not separate (6). They can occur as a product of each other, but in some applications of training principles one can be stimulated more than the other. This is through the specificity of the movement, its intensity, load, and frequency, as well as the voluntary effort that is put into the exercise.
The Evidence of Change
Amongst literature there is a wide variety of training methods applied to research studies, all with different identified effects due to the 3 major area of stimulation previously mentioned (figure 2). For a full review see reference (3 & 10).
According to recent research findings, eccentric training has been suggested to stimulate changes in:
Mechanical overload induced protein synthesis (4)
Increase in muscle fibre length (4)
Increased muscle size (4,9,11)
Increased force production (11)
Increased electrical excitation of muscle (4)
Increased magnitude of cortical electrical signals (4)