1) Most of us are capable of producing a little less than twice the force when performing an exercise with both limbs (bilateral) than we can by adding together the weight lifted by each limb independently (unilateral).
For example, if you can perform a leg extension with 25kgs when using one leg at a time, you are able to lift approximately 45kgs when using both legs together.
2) The difference in strength, described above, is known as the “bilateral deficit” and this deficit is typically around 10% in new exercisers.
3) The bilateral deficit is made larger by training unilaterally (one limb at-a-time).
When working with personal training clients, or those using our Online Program, correcting discrepancy in strength between right and left side is a priority. Why? Because I believe you’re less likely to sustain injury if you’re “balanced.” In fact, I would rather you were equally weak than be very strong with your right arm and average with your left arm. There is little research on the subject of strength discrepancy, as it relates to injury. This theory is based on experience and advice.
Research based evidence does exist that being balanced leads to improved sports performance. Researchers in strength and conditioning have been studying the bilateral deficit, and how to augment it, since the term was first coined in 1961 (Henry & Smith). Let’s skim the surface.
Why do we have a strength deficit?
There are a small number of reasons why we have a bilateral deficit and the primary is; task familiarity.
Most of our day-to-day movements are reciprocal. One limb will move independently of the other when walking, running, climbing etc. It makes sense then that we learn new movements more efficiently when exposed to them (or practicing them) one limb at a time. It’s not that we are designed to workout with only one limb at a time, as studies have shown consistently that this bilateral strength deficit decreases over time.
Only one study to date has examined the effect of the bilateral deficit on sports performance, finding that sprinters with a smaller bilateral deficit are able to produce more force ‘off the blocks’ than those with a larger deficit. The more balanced we are (in theory) the more explosive/stronger we are.
Our strength discrepancy between limbs is normal and unavoidable as we develop as humans with a dominant side (for whatever reason). The bilateral strength deficit is always present but can be reduced/improved over time with effective exercise programming. However, one of the goals of your training program should be to reduce discrepancy between right and left side, and reduce your bilateral deficit.
Hard science on this subject is lacking and far from conclusive. However, a number of things are likely and important to consider if training for longevity:
1. Beginners to strength training will likely demonstrate a large strength deficit.
2. Learning new movements might best be done one limb at a time.
3. Balance is never a bad thing. Whilst we do not yet have proof, it’s likely that bringing strength in to balance reduces injury and improves general physical performance.
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