What is the background?
The researchers begin by noting that several previous studies have found that humans cannot jump twice as high with two legs as they can with one leg, including Challis (1998 ), van Soest (1985 ) and Vint (1996 ). This is the bilateral deficit in jumping, or just the “bilateral deficit” for short. They suggest that the bilateral deficit in jumping might be related to the phenomenon known as the “bilateral force deficit,” which is where individuals are unable to express twice the force with both legs as with one leg. They note that while this phenomenon was first reported in the early 1960s, it has not been consistently reported since and some researchers have found no evidence of it.
The researchers explain that the theoretical mechanism by which a bilateral force deficit would occur is a reduced neural drive to the muscles when performing a bilateral task compared to when performing a unilateral task. However, force is not the same as work done, which is indicative of jump height. Work done is the area under the power/time curve so it is therefore dependent on both the force and the velocity of the movement, as power = force x velocity. Moreover, the researchers explain that in a two-legged jump, because of the greater impulse, the muscles must necessarily be contracting more quickly and therefore the muscles will reach faster shortening velocities, which moves them up the force-velocity curve.
The force-velocity curve is a simple relationship in which heavier objects are lifted at slower speeds than lighter objects. It is broadly asymptotic to the velocity axis but crosses the force axis at 1RM before becoming a yielding eccentric. The following chart shows this more clearly.
It’s quite important to understand this point before reading the rest of the review, as the conclusion is dependent upon it. If nothing else, just remember that if you try and contract your muscles faster, you will necessarily produce less force.
The researchers explain that van Soest (1985 ) discounted the role of the force-velocity relationship in the bilateral deficit because they thought that the muscle shortening velocities were similar in both the two-leg and one-leg jumps and because they found that the EMG activity of some of the leg muscles in the two-leg jump were lower than in the one-leg jump, which they took to imply a reduction in neural drive. The researchers also note that Challis (1998 ) created a simulation of a jumper and reported that if the maximal neural drive to each leg in the model was fixed, the maximal height reached in a two-leg jump was twice the height reached in a one-leg jump. This study also therefore suggested that the bilateral deficit was caused by reduced neural drive.
What did the researchers do?
To explore the reasons for the bilateral deficit, the researchers recruited 8 physically active male subjects who had considerable experience in jumping because of their training background in either volleyball or gymnastics. The researchers asked the subjects to perform maximum-height squat jumps using either two legs or only the right leg for push-off. The jumps were performed with no countermovement and 1-minute of rest was allowed between attempts.
While the subjects jumped, the researchers measured ground reaction forces with two force plates and the locations of various key anatomical landmarks was recorded using a motion capture system. They also took EMG measurements using surface electrodes on the soleus, gastrocnemius medialis, vastus lateralis, rectus femoris, gluteus maximus, and biceps femoris (long head).
The various data were incorporated into a computer model so that the researchers could attempt to simulate both one- and two-leg jumps and create equations to describe them. Once they had these equations, they then calculated energy changes and work done in each case and began to determine the factors that affected jump height in each case.
The researchers reported that while on first inspection, the initial posture of the body was quite similar in the two types of jumps, closer investigation revealed that the height of the center of mass was c. 3cm less at the start of the one-leg jump than of the two-leg jump. Additionally, they noted that at take-off, the center of mass was c. 2cm higher in the one-leg jump than in the two-leg jump.
The researchers found that the total change in mechanical energy of the center of mass during the push off was 4.9J/kg in the two-leg jump and 4.3J/kg in the one-leg jump. They noted therefore that the change in energy during the two-leg jump was not twice as great as the change in energy during the one-leg jump.
Ground reaction forces during jumping
The researchers reported that the peak vertical ground force of the right leg attained in the simulated two-leg jump was 15.6 ± 1. N/kg while the left leg was 14.2 ± 1.2N/kg), because of right-leg dominance in this group
of subjects. However, in the simulated one-leg jump, the force was 19.6 ± 2.2N/kg. So the right leg during the two-legged jump only produced 79% of the force that was produced by the same leg during the one-legged jump.
The researchers found that the change in mechanical energy during the two-leg jump was explained by the work done, which was 2.6J/kg for the right leg and 2.5J/kg for the left leg, for a total work of 5.1J/kg, of which 0.2J/kg was lost. However, the work done by the right leg during the one-leg jump was much higher, at 3.3J/kg. So the work done by the right leg during the two-legged jump was only 80% of the work done by the right leg during the one-legged jump.
Having said that, don’t lose sight of the fact that the overall work done during the two-legged jump was much greater than the overall work done during the one-legged jump. It’s only when we compare one leg at a time that we see the superiority of the one-leg jump. This greater overall work done means that power must be greater during the two-legged jump and therefore velocity.
The researchers found that in general the EMG activity of the leg muscles during the two-legged jump was very similar to that in the one-legged jump, as can be seen in the chart below.
The rectus femoris was the only muscle that had significantly lower EMG during the two-legged jump than during the one-legged jump. All the others were non-significantly different.
Reason for the bilateral deficit in jumping
Using their computer simulation, the researchers calculated the extent to which the higher muscle shortening velocities in the two-leg jump contributed to the reduction in work done.
They altered the shortening velocities for each muscle in the two-leg jump with the same numbers as were reported in the one-leg jump. They reported that this caused the work done of the right leg in the two-leg jump to increase by 1J/kg, which was c. 75% of the difference in work done of the right leg between the two-leg and the one-leg jump.
The researchers suggest that the remainder of the difference was likely due to the greater active state of the muscles during the squat jump in the one-legged variation because of the requirement to support the greater relative weight of the body.
What did the researchers conclude?
The researchers concluded that the ground reactions force produced by each leg during the two-legged jump were less than that produced during the one-leg jump. This was caused by lower joint moments.
The researchers concluded that while it is possible that the lower joint moments in the two-leg jump were smaller than those in the one-leg jump because of reduced neural drive, peak EMG levels in the two-leg jump were only slightly lower than those in the one-legged jump. They therefore suggest that a reduction in neural drive is unlikely to be the cause of the reduced moments.
Rather, the researchers suggest that because some of the muscles must have shortened at higher velocities in the two-legged jump, this caused them to produce lower forces because of the force-velocity relationship. They note that this was the result produced by their computer simulation, as enforcing the same muscle-shortening velocities in both jump variations eliminated 75% of the difference in jump height.
What were the limitations?
The study into the bilateral deficit was limited in that it did not record the EMG activity of each of the muscles involved in the jumping variations and only compared them to one another rather than to a maximum voluntary contraction.
Moreover, the researchers used a model to be able to manipulate the shortening velocity of the muscles, which made use of various assumptions based on anatomical data. Such anatomical data is often based on a limited number of older subjects and may not reflect the proportions of the athletic subjects used in this study.
What are the practical implications?
Humans jump higher using two legs than with one leg. However, they cannot jump twice as high with two legs as with one leg. This concept is called the “bilateral deficit”.
Ground reaction forces and work done by one leg during a two-legged jump are both c. 80% of the ground reaction forces and work done during a one-legged jump.
There is a small reduction in neural drive when jumping with two legs compared to when jumping with one leg. However, the main reason for the relative reduction in work done is likely the change in muscle shortening velocity, as a result of the force-velocity relationship.
Two-legged jumps involve much greater overall work done and therefore greater velocity, which suggests higher muscle-shortening velocity.
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