By Chris Beardsley, S&C Research columnist
Partial squats make you stronger at partial squats, but do not transfer to full squats. On the other hand, full squats make you stronger at full squats and also make you stronger at partial squats (although usually not quite as well as partial squats).
This is probably because the mechanisms that produce joint angle-specific strength gains are different after training at long muscle lengths, compared to training at short muscle lengths. Training at longer muscle lengths involves more regional hypertrophy, which seems to transfer better to strength across the whole range of motion.
Even so, many coaches have noted that the joint angles in partial squats are similar to the joint angles in the stance phase of running gait, or during jumping. Because of this similarity between joint angles, they suggest that partial squats should transfer better to sport than full squats, as they should produce the greatest gains in strength exactly where we need them.
And this makes a lot of sense.
On the other hand, most research shows that full squats are superior compared to partial squats for improving athletic performance in many respects, particularly jumping.
So what mechanism could be causing this disparity?
What is the background?
You should be able to follow this article without too many problems if you remember that we are normally stronger at one joint angle compared to all the rest, which we call the angle of peak torque.
This angle of peak torque can be changed in different ways, by different types of training.
Training programs using full ranges of motion, using long muscle lengths, or eccentrically all tend to move the angle of peak torque to a joint angle corresponding to a longer muscle-tendon length. In contrast, training programs using a partial range of motion, or short muscle lengths, tend to move the angle of peak torque to a joint angle corresponding to a shorter muscle-tendon length.
And most importantly, changing the angle of peak torque is very likely one of the main mechanisms that causes joint angle-specific gains in strength.
However, angles of peak torque are normally measured using isometric tests, and they might differ during dynamic contractions, particularly at higher speeds.
So does this happen?
Do angles of peak torque differ with angular velocity?
Full range of motion exercises might transfer better to sport than partial range of motion exercises if the angles of peak torque are different when we measure them at different speeds.
This will be particularly relevant if our exercises are traditional, heavy squats, as they involve much slower movement speeds than jumping or sprinting.
And this does happen!
The angle of peak torque is seen at joint angles corresponding to shorter muscle-tendon lengths as angular velocity increases (Moffroid et al. 1969; Knapik et al. 1983; Kannus & Jarvinen, 1991; Yoon et al. 1991; Khalaf et al. 1997; Khalaf et al. 2001; Khalaf & Parnianpour, 2001; Anderson et al. 2007; Ripamonti et al. 2008), although this effect is not always observed consistently in every study, and is much less marked above 180 degrees/s (Frey-Law et al. 2012).
The following charts derived from data reported by Yoon et al. (1991) show how the angle of peak torque alters with increasing angular velocity. Each line represents a different angular velocity moving through the same joint angle range of motion.
Here is knee flexion (contracting from left to right):
As you can see, as the movement speed increases, two things happen.
Firstly, the lines shift downwards, because force reduces as angular velocity increases (because of the force-velocity relationship).
Secondly, the angle of peak torque moves further to the right as angular velocity increases. This means that the angle of peak torque occurs at progressively shorter and shorter muscle-tendon lengths as angular velocity increases.
Here is knee extension (contracting from left to right):
Why do angles of peak torque differ with changing speed?
As you can see from the charts, the angle of peak torque moves to a joint angle that corresponds to shorter and shorter muscle-tendon lengths, with increasing speed.
This probably happens because even though the muscle-tendon lengths are the same at each joint angle, the muscle and tendon do not change length in the same way at different contraction speeds (don’t forget that tendons always lengthen to a greater or lesser extent when a muscle contracts, even when the contraction is purely a concentric contraction that involves a shortening of the muscle-tendon unit).
Fast contractions involve small muscle forces, which cause a smaller amount of tendon elongation at the start of the contraction.
The smaller amount of tendon elongation in fast contractions means that the muscle stays lengthened for longer in the concentric phase of the contraction. This allows the muscle to stay on the plateau of the length-tension curve for longer. Therefore, the angle of peak torque is shifted to much later in the overall joint angle range of motion (Murray et al. 1980).
Slow contractions involve high muscle forces, which cause much more tendon elongation at the start of the contraction.
This greater tendon elongation means that the muscle does not remain lengthened for very long during the concentric contraction. So it drops off the plateau of the length-tension curve quickly. Therefore, the angle of peak torque is seen earlier on in the overall joint angle range of motion (Murray et al. 1980). And isometric contractions are the slowest, strongest contractions of all.
Why is this important?
Why is contraction speed important for the angle of peak torque?
There are two key implications.
Firstly, it means that the angle of peak torque in dynamic movements is always at joint angles corresponding to shorter muscle-tendon lengths compared to the isometric angle of peak torque.
Secondly, it means that sporting movements at very high angular velocities have angles of peak torque at joint angles corresponding to very short muscle-tendon lengths. However, even when measured in the same person, these are not the same angles of peak torque as slower, barbell exercises or isometric tests. Those angles of peak torque occur at much longer muscle-tendon lengths.
This may be why full range of motion heavy resistance training exercises transfer better than similarly-loaded partial range of motion exercises to many high-velocity athletic movements.
What does this mean for jumping?
The quadriceps are key for jumping, and most jumping requires an angle of peak torque at moderate quadriceps lengths, as neither jumpers nor team sports athletes bend their knees down to the levels seen during a full squat before take-off.
This has led some coaches to assume that partial squats might be helpful, as they seem to involve a peak contraction around the same sort of joint angle.
But although this sounds logical, it ignores how the angle of peak torque changes with movement speed.
During a slow, heavy squat, the angle of peak torque will be observed at long muscle lengths. On the other hand, a jump is clearly a very fast movement and so the corresponding angle of peak torque will be at a much shorter muscle length.
If we train at long quadriceps muscle lengths, such as in the deep squat, we shift the angle of peak torque towards a longer muscle length. Because increasing movement speed moves angles of peak torques towards shorter muscle lengths, however, this will correspond to an angle of peak torque at moderate muscle lengths when we measure it at a fast velocity.
This is exactly where we need them for the jump.
If we train at short-to-moderate quadriceps muscle lengths, such as in the partial squat, we shift the angle of peak torque towards a shorter muscle length. Because increasing movement speed moves angles of peak torques towards shorter muscle lengths, however, this will correspond to an angle of peak torque at very short muscle lengths when we measure it at a fast velocity.
This is not where we want them for the jump.
And this is why deep squats transfer much better to jumping than partial squats (Weiss et al. 2000; Hartmann et al. 2012; Bloomquist et al. 2013).
Although there is less research available for sprinting, the same principles will apply.
Some people have proposed that partial squats should transfer better to sport than full squats because of the similar joint angles involved. However, full squats are definitely superior, and this is very clear in relation to jumping.
The reason for this discrepancy is that the angle of peak torque changes with movement speed. The angle of peak torque is found at shorter muscle-tendon lengths when measured at fast speeds, compared to when measured at slow speeds.
This is likely because even though the muscle-tendon lengths are the same at each joint angle, the muscle and tendon do not lengthen to the same extent at different speeds, and the amount of tendon elongation is less during fast contractions, which allows the muscle to remain on its length-tension plateau for longer.
Heavy, slow exercises such as full squats produce peak contractions at long muscle-tendon lengths. Because of differences in the amount that the tendon changes length, these angles of peak torque correspond very well to the peak contractions in athletic movements at joint angles corresponding to shorter muscle-tendon lengths, such as in jumping.
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