A recent study (kudos to @CyclingScience1) concluded that there was no difference in EMG (muscle activation) patterns of the leg muscles between upright and recumbent cycling.
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This post is not reviewing that paper, but offering a short, general conceptual explanation for why that conclusion would be true.
Main Effect - Why They are Similar
Coordinated contraction of lower extremity muscles are required to overcome inertial resistance in a cyclical motion (crank arm about the crank axis), regardless of the bicycle type.
Take a look at this figure. On the left is an upright position, on the right is a recumbent position.
As you may have guessed, I have simply rotated the upright image to create the recumbent one. Thus, the relative orientation of the lower extremities (foot, shank, and thigh) are in the same positions relative to the crank arm in both case.
In both cases, the cyclist is trying to push down on the pedal to produce a propulsive force (green arrow) that should rotate the crank arm. In turn, the pedal is pushing back with an equal but opposite force (red arrow) on the leg.
This red force creates moments - which are related to muscle demands - on the ankle, knee, and hip joints. The magnitude of the moments for each joint are dependent on the red force magnitude, as well as each joint's moment arm (the perpendicular distance from the joint and the force direction).
When you rotate the entire lower extremity, the forces and moment arms rotate accordingly. Thus, there is no difference in joint moment demands between the two cases.
The most important parameter in changing biomechanical loading is the distance between the hip and crank axis (also pedal position on the foot). As you change this distance, knee, hip, and ankle angles all change, changing the moment arm distances in the figure above, and thus changing the absolute and relative demands of the lower extremity muscles crossing those joints.
2nd Order Effects - Why They Could Be Different
It would be silly to dismiss any differences between the two cycling styles, but important to note that they have smaller effects than the fundamental mechanics as shown above.
1) The rotation of the body does change the body positioning relative to gravity. Gravity is always acting of course, but segmental positions when going with gravity and working against (lifting up) change and could slightly change overall effort.
2) Torso position. Even though the lower extremity may be in the same position, the torso could be more flexed forward when upright cycling than recumbent cycling. This is an important aspect. In recumbent style, it will be easier for the cyclist to keep their pelvis in a neutral position vs the posterior tilt seen in many upright cyclists.
The more flexed torso in upright cycling also keeps the hip in more flexed positions overall. A more flexed hip can change where hip muscles are working on the Force - Length and Force-Velocity relationships, altering how hard or easy it is for each muscle to generate the required force. And chronic cycling will change the # of sarcomeres in series between these two styles!
Also, I believe that the more flexed hip position has some consequences on reducing femoral arterial blood flow (like in speedskating) which can make the upright style more painful and demanding.
Thoughts?
