Speaker #0Hi and welcome to this new episode of BioPilates Deep Dive. Today I would like to take you into an in-depth exploration of the unilateral bicycle and of everything this movement reveals about pelvic stability. We will talk about the fundamental role of the deep stabilizer muscles, about the functioning of the major and minor psoas, about the characteristics of voluntary skeletal muscles and about the way in which each variation of the exercise subtly transforms the constraints imposed on the body. When one observes the unilateral bicycle on the reformer, one might think of a simple leg exercise, but behind this apparently simple gesture hides a very fine mechanics where every detail counts. This movement stresses the body on two planes at the same time. In the sagittal plane, it brings into play the flexions and extensions of the hip and the knee, but also the tendencies to anterior and posterior pelvic tilt. In the frontal plane, it tests the capacity of the pelvis to remain perfectly horizontal without inclination or rotation in spite of the asymmetry of support. Each time a leg acts, it generates an articular moment that the pelvis must absorb and neutralize. The requirement is therefore double, to produce movement with fluidity, while at the same time maintaining the stability of a solid base. To understand how this is possible, one must return to what defines a skeletal muscle. A skeletal muscle is voluntary, it is connected to the skeleton by tendons, and it responds to conscious nervous commands. It possesses three great properties that make it unique. Excitability, first. which is its capacity to react to a nervous signal. Contractility then, which is its ability to generate tension and to produce movement. And finally, extensibility and elasticity, which allow it to lengthen and to return to its initial length after a contraction. The composition in fibers also plays a determining role. Some fibers are slow, type 1, specialized in endurance and tonic contractions, while others are fast, type 2, capable of producing powerful and explosive movements. It is this combination that gives skeletal muscles their versatility and their capacity for adaptation. Among the muscles implicated in the unilateral bicycle, the major psoas occupies a particular place. It is a deep, powerful muscle which links directly the lumbar spine to the femur. It originates on the vertebral bodies, the intervertebral discs, and the transverse processes from T12 to L5. It descends crossing the pelvic region and joins with the iliacus to insert on the lesser trochanter of the femur. Its innervation comes from the ventral branches of the lumbar roots from L1 to L3, sometimes L4. This path and these attachments explain why it influences at the same time the mobility of the hip. and the statics of the pelvis. Its contraction flexes the hip, but it also exerts a traction on the pelvis capable of provoking an anterior tilt if it is not balanced by a coordinated activation of the stabilizer muscles of the trunk. The minor psoas, when it exists, is a more discrete muscle. Present only in a part of the population, it is born often from T12 and L1 and inserts on the pectineal line. and on the iliopectineal eminence. Its motor role is limited. It is above all a postural modulator. It contributes to tighten the anterior region of the pelvis. It acts on the lumbar lordosis, but it does not have the power of the major psoas. One could define it as a secondary stabilizer, a discrete assistant that refines the position without really directing the movement. What makes these muscles so important in the exercise of the unilateral bicycle is their double function. They are at the same time motors and stabilizers. When the major psoas contracts concentrically to flex the hip, it participates in the movement of the leg, but at the same time it applies a force on the pelvis and on the spine, which must be contained. In the sagittal plane, this can translate into an increase of the lumbar lordosis. in the frontal... plane, an asymmetrical contraction can pull the pelvis upwards on one side, provoking an obliquity or a contralateral drop, what is sometimes called a pelvic drop. The key consists in anticipating and balancing these effects through an intelligent co-contraction of the other deep muscles. Let us now see how this translates into the three variations of the unilateral bicycle. In the bent leg variation, the supporting foot presses the bar while the other leg remains in tabletop position. The psoas of the lifted leg works isometrically to maintain the thigh at right angle. while the quadriceps and the hip extensors of the supporting leg guide the carriage. The pelvis must remain neutral, horizontal, and if the psoas on the supporting side activates in an excessive manner, the anterior tilt appears. We understand here the importance of a prior activation of the transverse abdominis and of the multifidi to lock the trunk before the movement. In the reciprocal bicycle variation, variation the legs alternate. One pushes while the other bends, then they invert their roles. This adds a requirement of coordination. The psoas muscles activate alternately, and the slightest asymmetry becomes visible in the form of a rotation or a tilt of the pelvis. Here, it is no longer only strength that counts, but timing. The stabilizers must anticipate. The transverse must engage. even before the leg moves, and the obliques must contain the undesired rotations. It is a situation where precision matters more than power. Finally, the single heel variation. The supporting foot is in dorsiflexion, heel on the bar, the other leg stretched above. This simple change of support modifies deeply the levers. Dorsiflexion reduces the anterior lever arm and decreases the phasic solicitation of the psoas. on the supporting side. The psoas of the lifted leg then works more in isometry to maintain the thigh stretched. This position favors tonic endurance, but it requires a great coordination of the abductors and of the lateral muscles of the trunk to prevent the pelvis from inclining. These three variations illustrate the functional plasticity of the psoas. The same muscle can be, in turn concentric eccentric or isometric motor or stabilizer according to the position of the segments and the nature of the support. It is why it is essential not to isolate the psoas in a reductive vision of hip flexor, but to consider it as a global actor of the lumbopelvic dynamics. The question of pelvic stability faced with the action of the psoas is therefore a question of neuromuscular coordination. The anticipatory activation of the transverse and of the pelvic floor creates a solid base, a sort of natural belt that prevents the pelvis from tilting. The multifidi, small deep muscles of the spine, add a segmental stabilization that limits vertebral translations. The obliques intervene to contain the rotations. and to balance the lateral forces. The gluteus medius and the abductors control the frontal plane, avoiding the contralateral drop. Without them, the action of the psoas becomes too dominant, and the global balance is lost. What is fascinating is to see how the position of the foot also modifies the role of the psoas. A support on the forefoot in plantar flexion tends to increase the anterior lever arm. and thus to solicit more strongly the psoas and the rectus femoris. On the contrary, a heel support in dorsiflexion transfers a part of the work towards the hamstrings and reduces the anterior pelvic tilt. We then understand why the instruction keep the heel relatively immobile is so important. It is a way of limiting the undesired variations of lever and of protecting the heel. the neutrality of the pelvis. On the neurological level, the innervation of the psoas also clarifies their role. The major psoas, innervated by several lumbar segments, possesses a rich and modulable command. It can recruit progressively, provide tonic adjustments, or rapid contractions according to the need. The minor psoas, innervated mainly by L1, has a more limited command and behaves rather like a fine adjuster. Together they participate in a network of adjustments where precision counts as much as strength. The teaching of this exercise in Pilates must therefore insist on some clear pedagogical principles. Preactivate the lower abdomen before each push, as if preparing the ground. Keep the pelvis neutral and level, imagining the iliac crests parallel and immobile. Watch the lumbar lordosis to detect any excessive anterior tilt. Adjust the position of the foot according to the objective sought. More postural control with the heel, more phasic work with the forefoot. Finally, Limit the amplitude if necessary, because a movement too large can surpass the capacity of neuromuscular control and generate undesired compensations. The practical implications are numerous. In a practitioner suffering from weakness of the abductors, the single heel variation can be useful, but it will have to be completed by specific abduction work to prevent pelvic obliquity. in a person presenting hyperlordosis. dorsiflexion and limitation of amplitude will help to reduce the anterior traction of the psoas. In an athlete who seeks power, the reciprocal bicycle will develop timing and coordination but will require increased vigilance on pelvic neutrality. Each profile calls for an adaptation and that is what makes the pedagogical richness of this exercise. The unilateral bicycle is therefore not only a leg exercise. It is a real school of motor control, a training ground for the synergy between mobility and stability. It teaches us that the pelvis, far from being a simple support, is a crossroads of forces constantly influenced by the muscles that surround it and by the position of the distal segments. It also reminds us that the psoas, this deep muscle often misunderstood, and is at the same time an essential motor and a delicate stabilizer whose balance always depends on the global context. By integrating these principles, each repetition becomes a work of precision. Each push is an occasion to strengthen body awareness, to test the capacity of the body to produce a fluid movement without losing its center. This is, in the end, the essence of Pilates. to harmonize mobility and stability, to create a gesture at the same time powerful and light, conscious and durable. Thank you for having followed this dive into pelvic stability and the role of the psoas in the unilateral bicycle. See you soon for a next episode of BioPilates Deep Dive.
