Why are strength gains greater at some joint angles than others?
After including an exercise in our strength training program, we often find that it increases our ability to produce force with a muscle more at some joint angles, and less at others.
This happens most often when the exercise does not require us to produce a constant force at all points throughout the exercise range of motion. Surprisingly, this actually occurs very often, even when all we are doing is lifting a weight.
Force differs across the range of motion of an exercise because of changes in the length of the lever between the joint we are pivoting and the weight we are lifting. Along with the size of the weight, the length of this lever determines the amount of turning force the joint (and therefore the muscle) needs to produce. Longer levers between the joint and the weight mean we need to produce more force.
As a result of changes in the lever length over the exercise range of motion, a muscle experiences a greater mechanical load at some lengths, and a smaller load at other lengths. When we use a full range of motion, mechanical load is greatest at long muscle lengths. When we use partial ranges of motion, mechanical load is usually greatest at short muscle lengths.
So how does this lead to different strength gains at each joint angle?
Ways in which strength can increase at some joint angles, and not at others
There are three main ways in which strength can appear to increase more at some joint angles than at others, by imposing a high mechanical load on either a long or a short muscle length.
#1. Changes in the optimum muscle length for force production
Every muscle has a length at which it produces maximum force. We can call this the optimum muscle length for force production. This optimum length exists because muscle fibers are made up of long chains of linked sarcomeres (contractile units) that shorten to produce force.
When sarcomeres elongate to allow a muscle to lengthen, this alters the amount of overlap between the actin and myosin filaments, and this affects the amount of force that they can produce. Since more overlap means more force can be exerted, the optimum muscle length for force production is where sarcomere overlap is greatest, and this is where all the sarcomeres in a chain are neither too short nor too long.
When there are lots of sarcomeres in a chain, the length of each sarcomere at a given muscle length will be short. When there are fewer sarcomeres, the length of each individual sarcomere at a given muscle length must be longer, because the length of the whole muscle fiber is the same.
Applying a high level of mechanical loading to a muscle at a long muscle lengths stimulates sarcomeres to be added, which shifts the optimum muscle length for force production to a longer muscle length, and hence a different joint angle. Applying a high level of mechanical loading to a muscle at a short muscle length stimulates sarcomeres to be lost, and the optimum muscle length for force production shifts to a shorter muscle length.
When either of these things happen, we observe greater strength gains at one joint angle than at others, because the joint angle that corresponded to the previous optimum muscle length for force production will experience a drop in force, while a nearby joint angle will experience an increase.
#2. Changes in regional muscle size
Muscles can alter their ability to produce force at different lengths by increasing in size at certain specific regions and not others. This is called “regional hypertrophy” and appears to occur in tandem with joint angle-specific strength gains, although exactly why is still unclear.
Some researchers have suggested that regional hypertrophy may be observed because of increases or decreases in the number of sarcomeres in series. This is because the muscle growth that occurs after strength training at long muscle lengths is often observed at the very ends of a muscle, which is where sarcomeres would typically be added to their chains.
Alternatively, it may be that certain subdivisions of a muscle are more strongly activated at certain muscle lengths, while others are more strongly activated at others, and this provides a mechanism by which applying mechanical loads at either long or short muscle lengths affects subsequent changes in muscle size, and therefore joint angle-specific strength.
#3. Changes in joint angle-specific neural drive
Muscles can alter their ability to produce force at different lengths by increasing the signal from the central nervous system more at some joint angles, and less at others.
This is logical, as training with heavy loads tends to cause greater increases in neural drive than training with light or moderate loads, and neural drive is increased where mechanical load is largest.
Differences between full and partial range of motion exercises
Interestingly, the way in which the above mechanisms contribute to joint angle-specific strength gains differs between full range of motion exercises (which challenge the muscle at long muscle lengths) and partials (which challenge muscles at short muscle lengths).
Joint angle-specific strength gains after partial range of motion exercises seem to be caused predominantly by increases in the size of the signal from the central nervous system at the relevant joint angles. The mainly neural origin of the strength gains explains why the strength gains after partial range of motion training are so localized to one particular joint angle (and also why partials tend to produce less muscle growth).
In contrast, when we train using full range of motion exercises to long muscle lengths, strength gains tend to be greatest at long muscle lengths, and small-to-moderate at other lengths.
These joint angle-specific strength gains seem to be caused largely by regional hypertrophy, and also by a shift in the angle of peak torque to longer muscle lengths. The regional hypertrophy explains why the strength is less limited to one joint angle. The gains in muscle size are better able to transfer to strength gains across the whole exercise range of motion than a very joint angle-specific increase in neural drive.
What is the takeaway?
After including an exercise in our strength training program, we often find that it increases our ability to produce force with a muscle more at some joint angles, and less at others. This happens because the exercise produces its greatest mechanical load on the muscle at a specific muscle length, which is then the muscle length that adapts most markedly to training.
When the greatest mechanical load is applied to a long muscle length (as in full range of motion training), this tends to cause the largest gains in strength at long muscle lengths through regional hypertrophy and an increase in the number of sarcomeres in series inside the muscle fibers. When the greatest mechanical load is applied to a short muscle length (as in partial range of motion training), this tends to cause the largest gains in strength at short muscle lengths through joint angle-specific neural drive, and potentially also a decrease in the number of sarcomeres in series .