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Effects of weight and inertia on muscle properties in bench press

by A. Manolova | 1 February 2018

bench press, resistance training, training, weight, inertia, elastic bands, force, strength, power, velocity, sport, performance, fitness

Ballistic movements (with load projection) require high power output (the product of force and speed). In movements like bench press with bar projection, the vertically displaced loads, as well as the body segments involved in the movement are subjected to different external forces, the weight (P, in N) and the inertia (Finertia, in N). Weight represents the gravitational force. It depends on the mass involved in the movement (the additional load + the mass of the body segments involved in the motion) and the gravitational acceleration (g, 9.8065 m/s2) :

Inertia can be defined as the tendency of a body to maintain its movement invariable. If the forces acting on a body are equal to zero, then that body will retain its original state. If a dumbbell is on the ground, it will not change its state until a new force is applied to it. And to move this dumbbell, the force needed will depend on its mass and the acceleration it will undergo.

In other words, when you move a loaded bar in bench press, not only do you fight the gravitational force, the weight of the bar, but also its inertia, that is, its state (eg, still or moving).

The handling of external loads like iron plates influences the kinematic and kinetic parameters of a movement, but this necessarily affects weight and inertia. The elastic bands, in turn, have a light weight and therefore a very low inertia. On the other hand, they can make it possible to simulate the weight of a load thanks to their mechanical properties. Finally, by combining iron and elastic bands, it is theoretically possible to work only with the inertia of the load. So what would bring selective training of weight and inertia on force, speed and power output ?

The Study

To answer this question, a team of Serbian researchers studied the effect of an 8-week selective training against weight and inertia, weight alone and inertia alone in bench press with projection of the load on strength, speed and power. For this, the researchers recruited 48 students, which they divided into 4 groups :

  • Weight + Inertia (Fig. 1) : Participants (n = 12) trained with iron plates (1RM = 79.6 ± 11.4 kg).
  • Weight (Fig. 2) : Participants (n = 12) exercised with elastic bands (1RM = 82.9 ± 11.5 kg).
  • Inertia (Fig. 3) : Participants (n = 12) trained with iron plates whose mass was compensated by elastic bands fixed upward (1RM = 82.9 ± 11.0 kg).
  • Control : Participants (n = 12) did nothing during the entire protocol period.
Weight + Inertia

Figure 1. Weight + Inertia (Click to enlarge)

Weight

Figure 2. Weight (Click to enlarge)

Inertia

Figure 3. Inertia (Click to enlarge)

With "Weight" and "Inertia" conditions, the elastic bands were stretched so that the resistance provided by them varied only slightly (about 6%).

Before and after the training cycle, all participants participated in two test sessions. The first day of pre-test consisted of measuring anthropometric data and 1RM in bench press on Smith Machine. The second day of pre-test consisted of performing a bench press with load projection on Smith machine against different loads in random order. The participants carried out 3 tests against 8 charges which corresponded to 30, 37, 44, 51, 58, 65, 72 and 79% of 1RM, only in concentric phase with the objective of projecting the load as high as possible. This allowed determination of force-speed profiles for each participant. Once the protocol was completed, the participants re-performed these two days of testing. Measurements were made with a linear transducer (a device that measures displacement over time) to determine acceleration, velocity, force, and power.

For 8 weeks, the participants of the 3 training groups trained 3 times per week in the bench press with projection of the load corresponding to 50% of 1RM (of which 25% of the load consisted of the mass of the arms and bar). They started with 6 sets of 7 repetitions the first two weeks, and increased one set every two weeks. Between each rep, there was a break of 5 seconds, and between each series, a rest of 5 minutes.

Results & Analyzes

The main results of this study show that the 3 training conditions have significantly improved muscle power output. And the group "Inertia" improved the power output significantly more in comparison to the group "Weight". However, during the evaluation tests at the beginning of the study, the power produced during the inertial condition was lower than that produced during the weight condition. Finally, it appears that depending on the type of training, the increase in power is not caused by the same variables. The group "Weight" increased the power mainly via force increase, the group "Inertia" mainly via speed increase and the group "Weight + Inertia" via both (Fig. 4).

Figure 4. Force-velocity profiles before (blue) / after (orange) of representative participants from each group.

Finally, these conditions simply reflect different effort in the concentric phase. During the condition "Weight + Inertia", at the beginning of the movement, to accelerate the load, it is necessary to fight against its immobility (inertia) and against its weight, but once the load accumulates energy, its inertia increases. And over the range of motion, it becomes easier to move the bar and accelerate it. On the other hand, in the condition "Weight", the elastic bands being stretched to limit the variation of resistance means that throughout the concentric phase, the resistance will be more or less the same (in fact it will even increase slightly, 6% variation) and thus the acceleration will be greatly reduced. Finally, during condition "Inertia", the elastic bands are placed in assistance to cancel the weight of the bar, and the latex assists the rise of the bar, the force to produce to move the bar is greatly reduced and the speed is much faster.

Practical Applications

This study shows that it is possible to significantly increase muscle power by improving either force, velocity, or both. However, as shown by the work of Jean-Benoit Morin and Pierre Samozino, (Read from their work, here, there and here) there is for everyone an optimal force-velocity profile, and testing allows to highlight the deficits of force or velocity. It is therefore important to focus the work specifically on the deficits of the athlete.

Finally, although theoretically the idea of working only against weight or against inertia is attractive, the protocol presented here cannot fully respect these conditions since the mass of the arms and the weight of the bar must be considered (in this protocol this corresponded to 25% of the load). It turns out that regardless of the training condition, the type of resistance or the equipment used, if you want to improve the force-velocity profile of an athlete, it is necessary to define what is his deficit, then work with very light or negative loads to improve velocity and/or work with heavy loads to improve force.

Les bandes élastiques - Le livre

Références

  1. Djuric S, Cuk I, Sreckovic S, Mirkov D, Nedeljkovic A and Jaric S. Selective Effects of Training Against Weight and Inertia on Muscle Mechanical Properties. Int J Sports Physiol Perform 11 (7) : 927-932, 2016.

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