What is the autonomic nervous system?
Your autonomic nervous system is a part of your overall nervous system that controls the automatic functions of your body that you need to survive. These are processes you don’t think about and that your brain manages while you’re awake or asleep.
The gait cycle describes the cyclic pattern of movement that occurs while walking. A single cycle of gait starts when the heel of one foot strikes the ground and ends when that same heel touches the ground again.
Walking requires the healthy functioning of several body systems including the musculoskeletal, *nervous,* *cardiovascular* and *respiratory systems.* These systems provide balance, mobility and stability as well as higher cognitive function and executive control. A loss of healthy gait function can lead to falls, injuries, loss of movement and personal freedom, and a significantly reduced quality of life.
When you lean or bend, your body must work harder to stay balanced.
Your hip, knee, and ankle joints change angles, and your muscles generate torque (rotational force) to prevent you from falling.
The more you bend or move, the more torque your muscles need to generate to bring you back into balance.
This system works together to keep your center of mass over your feet (your base of support), so you don’t fall down!
The torque generated by muscles around the ankle, knee, and hip joints depends on the angles of these joints. These torques help maintain balance by adjusting posture and controlling the body's center of mass (CoM) relative to the base of support (BoS). Here's a list of common joint angles and how they influence torque at each joint during balance control:
1. Ankle Joint Torque and Angles:
Neutral Position (90°):
The ankle is in a neutral position when the foot is flat on the ground, with the angle between the shin and foot close to 90°.
Torque: Minimal torque is required to maintain balance since the body is aligned vertically.
Dorsiflexion (less than 90°):
When you lean forward, the angle decreases (e.g., 80°). The anterior muscles (dorsiflexors) generate torque to prevent you from falling forward.
Torque: Increases as the angle decreases to pull the body back upright.
Plantarflexion (greater than 90°):
When you lean backward or rise onto your toes, the angle increases (e.g., 100°). The posterior muscles (plantarflexors, especially the calf muscles) generate torque to bring the body forward and stabilize.
Torque: Increases as the angle increases, especially when standing on your toes.
2. Knee Joint Torque and Angles:
Full Extension (180°):
When standing straight, the knee joint is fully extended at 180°. This is the most stable position for balance with minimal torque required.
Torque: Minimal torque, as the knee is locked in place and the muscles aren't working hard to keep the body upright.
Slight Flexion (160–170°):
In this position, the knee bends slightly, such as during slight forward lean or athletic stances.
Torque: The quadriceps and hamstrings produce more torque to stabilize the knee and control the body’s movement, particularly if the body is shifting forward or backward.
Deep Flexion (90° or less):
This occurs during a squat or when sitting. The angle of the knee decreases significantly.
Torque: High torque is required from the quadriceps and hamstrings to maintain balance, as the body’s weight is primarily supported by the legs in this position.
3. Hip Joint Torque and Angles:
Full Extension (180°):
When standing upright, the hip joint is fully extended (180°) with the torso aligned over the legs.
Torque: Minimal hip torque is required in this position, as the body’s mass is balanced directly over the feet.
Mild Flexion (150–170°):
When you lean forward slightly, the hip angle decreases as the torso bends forward. This occurs during small forward body movements, such as reaching or slight bending.
Torque: The hip flexors and extensors (gluteus maximus, iliopsoas) generate moderate torque to control the movement and keep the body upright.
Significant Flexion (90–120°):
When sitting, squatting, or bending forward significantly (e.g., picking something up), the hip joint flexes to about 90–120°.
Torque: High torque is generated by the hip extensors (glutes, hamstrings) to counteract the forward motion of the body and prevent falling.
Summary of Torque and Joint Angle Relationships:
Ankle Joint:
90° (Neutral): Minimal torque, stable balance.
< 90° (Dorsiflexion): Increased anterior torque (dorsiflexors) to prevent forward fall.
> 90° (Plantarflexion): Increased posterior torque (plantarflexors) to prevent backward fall.
Knee Joint:
180° (Full Extension): Minimal torque, stable position.
160–170° (Slight Flexion): Moderate torque, stabilizing position.
90° or less (Deep Flexion): High torque, required for squatting or bending.
Hip Joint:
180° (Full Extension): Minimal torque, stable posture.
150–170° (Mild Flexion): Moderate torque, controls forward lean.
90–120° (Deep Flexion): High torque, stabilizing deep bends or sitting.
In balancing tasks, the body constantly adjusts these angles and torques to maintain CoM over the BoS. Each joint contributes dynamically depending on how far the body is leaning or bending, with the ankle usually fine-tuning small adjustments, and the knee and hip managing larger movements and posture changes.
In a free fall scenario (like falling forward or backward without restraint), the interaction of torque and joint angles at the hip, knee, and ankle joints with respect to the center of mass (CoM) and center of gravity (CoG) becomes critical in understanding how the body reacts. Here's a comparison of the torque and angle dynamics during free fall:
1. Free Fall and Center of Mass (CoM) vs. Center of Gravity (CoG):
Center of Mass (CoM): This is the point where the body’s mass is evenly distributed. In a standing position, it is typically located near the belly button, between the hips.
Center of Gravity (CoG): For practical purposes in this context, the CoM and CoG are almost the same. The CoG is the point where gravitational force acts.
In free fall, the body rotates around the CoM/CoG, and the torques at the hip, knee, and ankle joints determine how the body moves and positions itself relative to the ground.
2. Torque and Angle Relationship During Free Fall:
In free fall, the angles of the hip, knee, and ankle joints change as the body rotates. Since the body is falling, there's no ground reaction force to counteract gravity, and the muscles can’t generate enough torque to prevent movement. Here’s how the body responds:
a) Free Fall Forward (Leaning or Falling Forward):
When the body falls forward, the CoM moves ahead of the base of support (the feet), and gravity accelerates the body downward. The body rotates around the ankle, and the angles of the hip and knee joints change.
Ankle Joint:
As you fall forward, the ankle moves into dorsiflexion (decreasing from 90° to less than 90°). In free fall, torque at the ankle is minimal since the muscles can’t apply force quickly enough to stabilize.
Torque: Negligible because gravity dominates the motion. Without support, the torque generated by the muscles is too small to prevent forward motion.
Knee Joint:
The knee begins to bend as you fall forward, reducing the angle from 180° (fully extended) to a more flexed position (closer to 150° or less). In free fall, the knee's torque contribution is minimal, as the muscles don’t generate enough force to slow the fall.
Torque: The quadriceps and hamstrings may try to generate torque, but in free fall, this torque is insufficient to stop the motion.
Hip Joint:
The hip flexes as the body falls forward, reducing the angle from 180° (standing) to less than 90° as you bend forward. The glutes and hamstrings may attempt to generate torque to slow the fall, but they cannot overcome gravity.
Torque: The hip torque is negligible since the muscles can’t apply enough force during free fall.
b) Free Fall Backward (Leaning or Falling Backward):
When falling backward, the CoM moves behind the BoS. The body rotates backward around the ankle, and the hip and knee joints adjust to the motion.
Ankle Joint:
In free fall backward, the ankle moves into plantarflexion (increasing from 90° to more than 90°). The calf muscles would try to generate torque to pull the body forward, but in a free fall, this is ineffective.
Torque: Minimal torque is generated as the muscles can’t counteract the backward motion quickly enough.
Knee Joint:
As you fall backward, the knee joint may remain extended or slightly bend. The angle might shift from 180° (fully extended) to 160° or more, depending on how you fall.
Torque: The quadriceps may try to extend the knee, but the torque produced is insufficient to prevent backward rotation.
Hip Joint:
The hip moves into extension as you fall backward, increasing the angle from 180° to more than 180°. The hip extensors (glutes) might attempt to generate torque to resist the fall, but this torque is minimal compared to the force of gravity.
Torque: Like the other joints, hip torque is minimal during free fall.
3. Comparison of Joint Torques and Angles During Free Fall:
In free fall, the joint angles of the hip, knee, and ankle change dynamically as the body rotates under gravity. The torques generated by the muscles are minimal, as there is no ground reaction force to push against, meaning the muscles have little ability to control the fall.
Ankle:
Angle: Moves from 90° (neutral) to < 90° during forward fall (dorsiflexion) or > 90° during backward fall (plantarflexion).
Torque: Minimal, since muscles can't generate sufficient torque in free fall.
Knee:
Angle: Moves from 180° (full extension) to a more flexed position (closer to 160–150°) during both forward and backward falls.
Torque: Minimal, with little resistance from the muscles during the fall.
Hip:
Angle: Moves from 180° (standing) to < 90° (flexion) during forward fall or > 180° (extension) during backward fall.
Torque: Minimal, as the muscles cannot stop the fall.
4. Center of Mass (CoM) Movement During Free Fall:
During free fall, the CoM moves outside of the base of support (BoS), which is why the body falls.
When falling forward, the CoM moves ahead of the feet, causing a forward rotation.
When falling backward, the CoM shifts behind the feet, causing a backward rotation.
Since the CoM is the point around which the body rotates in free fall, the joint torques become irrelevant after a certain point, as gravity dictates the body’s motion.
5. Center of Gravity (CoG) and Torque in Free Fall:
The CoG is the point where gravitational forces act, usually around the torso or pelvis in a standing position.
As you fall, the CoG pulls the body downward, and the body's rotation depends on how the CoG moves relative to the BoS.
Torque at the joints can't overcome the downward pull of gravity once the body is in free fall, meaning the CoG dominates the motion and the body continues to rotate toward the ground.
Conclusion:
In free fall, the angles of the hip, knee, and ankle joints change dramatically as the body rotates around the center of mass (CoM). However, the torques generated by these joints are minimal, as muscles can't produce enough force to resist gravity. The body rotates based on how the CoM and center of gravity (CoG) move relative to the base of support.
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To understand the relationship between torque, joint angles (hip, knee, ankle), and balance in simple terms, let's break it down further using an easy-to-grasp example of standing and balancing. Here's how it works:
1. Balancing and Joint Angles:
When you stand upright, your body relies on the angles of your hip, knee, and ankle joints to control your balance. These angles change depending on your posture and movement, and the muscles around each joint create torque (rotational force) to keep you balanced.
Standing Straight:
In this position, your ankle, knee, and hip joints are nearly straight (0–10 degrees). You don't feel a lot of strain because your body is naturally balanced, and very little torque is needed to stay upright.
Leaning Forward or Backward:
As you lean forward or backward, your hip and ankle angles increase (e.g., from 10 to 20 degrees). The further you lean, the more your ankles and hips must adjust by creating torque to prevent you from falling.
Torque generated at your ankles pulls you back upright, while your hips stabilize your upper body to control the center of mass (CoM) and bring it back in line with your feet (base of support).
2. Degrees of Angles and Torque at Each Joint:
Think of the degrees of joint angles as how bent or straight each joint is:
Ankle: When standing, the ankle is almost at 90 degrees (between your foot and lower leg). If you lean forward slightly, the ankle joint angle decreases (closer to 80 degrees), and if you lean backward, the angle increases (closer to 100 degrees). The ankle muscles adjust their torque to bring you back to balance.
Knee: When standing upright, your knee is almost straight, with an angle close to 180 degrees. If you bend your knees (like in a squat), this angle decreases (e.g., down to 90 degrees), and your muscles work harder to keep you balanced, generating more torque.
Hip: Your hip is close to 180 degrees when standing. As you bend forward or backward, the hip angle reduces (e.g., to 160 or 150 degrees), and the muscles around the hip joint increase torque to control your upper body’s movement and prevent you from tipping over.
3. How Joint Angles and Torque Work Together to Keep You Balanced:
Let’s take three simple actions to explain the angle-torque-balance relationship:
a) Standing Upright:
Joint Angles: Hip, knee, and ankle are near neutral (180 degrees at the hip and knee, 90 degrees at the ankle).
Torque: Minimal torque is needed because your body is naturally aligned. Muscles around the joints are relaxed but still active to keep you steady.
CoM and CoP: Your center of mass (CoM) is directly above your feet (base of support), and the center of pressure (CoP) is balanced beneath your feet.
b) Leaning Forward:
Joint Angles: Your hip angle decreases (bends forward, say to 160 degrees), and your ankle angle decreases (closer to 80 degrees).
Torque: Your ankle muscles generate torque to pull you back upright. Hip muscles stabilize the upper body. If you lean too far, the torque at your hip and ankle will increase to keep the CoM aligned with your base of support.
CoM and CoP: As you lean, your CoM moves forward. To prevent falling, your CoP shifts forward too, using ankle torque to bring your CoM back.
c) Squatting:
Joint Angles: Your knee angle decreases significantly (from 180 degrees to around 90 degrees), and your hip angle reduces (from 180 to around 120 degrees). Your ankle also flexes slightly (to around 80 degrees).
Torque: Knee torque increases significantly as your quadriceps work harder to keep you balanced. Hip torque also rises to control your torso, and the ankle generates torque to keep your feet flat on the ground.
CoM and CoP: When you squat, your CoM lowers, making it easier to balance. Your CoP moves slightly backward to adjust for the shift in weight distribution.
4. Ground Reaction Force (GRF) and Joint Angles:
The ground reaction force (GRF) helps support your body as you balance. Here’s how GRF interacts with joint angles:
When you’re standing straight, the GRF pushes directly upward through your feet to balance the downward pull of gravity.
As you bend your joints (like in a squat), the GRF shifts to stay underneath your CoM, requiring more effort (torque) from your muscles to maintain balance. The more you bend, the more torque you need.
5. Everyday Example: Reaching for Something High
Let’s say you’re reaching up to grab something from a shelf:
Ankle Angle: When you lean forward, your ankle angle changes slightly (from 90 to 85 degrees).
Hip Angle: Your hip bends forward, reducing the angle (from 180 to about 170 degrees).
Torque: To prevent yourself from tipping over, your ankle and hip muscles generate torque. This keeps your CoM aligned with your feet. If you don’t engage these muscles, you’d lose your balance and fall forward.
6. Final Takeaway on Angles, Torque, and Balance:
Angles: The more you bend or lean, the more your joint angles change.
Torque: As angles change, your muscles produce torque to counterbalance the shift in weight and stabilize your CoM.
Balance: The goal is always to keep your CoM over your BoS. Your body uses torque at the hip, knee, and ankle to adjust your posture, correct imbalances, and keep you upright.
In Layman’s Terms:
When you lean or bend, your body must work harder to stay balanced.
Your hip, knee, and ankle joints change angles, and your muscles generate torque (rotational force) to prevent you from falling.
The more you bend or move, the more torque your muscles need to generate to bring you back into balance
This system works together to keep your center of mass over your feet (your base of support), so you don’t fall down!
The concepts of center of gravity (CoG), center of mass (CoM), center of pressure (CoP), and various segmental centers of mass (e.g., foot CoM, knee CoM, hip CoM) all play crucial roles in understanding human balancing. Here’s an overview of their interrelationships and how they contribute to balance, particularly in the context of ground reaction force (GRF):
1. Center of Gravity (CoG) and Center of Mass (CoM):
Center of Mass (CoM): This is the point where the mass of the body is equally distributed in all directions. For a standing human, it is typically located around the lower abdomen near the pelvis.
Center of Gravity (CoG): CoG is the vertical projection of the CoM onto the ground. For practical purposes in human balancing, these terms are often used interchangeably. The location of the CoG depends on posture, and if a person changes position, their CoG shifts as well.
2. Segmental Centers of Mass (Foot, Knee, Hip CoM):
The human body is often analyzed as a multi-segment system (foot, shank, thigh, trunk, etc.). Each segment has its own center of mass, such as the foot CoM, knee CoM, and hip CoM.
These segmental centers of mass contribute to the overall body CoM. The relative positions of these segmental CoMs change based on joint angles and posture, influencing the overall CoM location.
3. Center of Pressure (CoP):
Center of Pressure (CoP) refers to the point on the ground where the total force exerted by the body through the feet is applied. It is the point of interaction between the body and the ground, representing the average position of all the pressure points on the surface of contact.
The CoP is dynamic and constantly shifts to maintain balance. For instance, when you lean forward or backward, your CoP moves to prevent you from falling.
4. Ground Reaction Force (GRF):
The ground reaction force (GRF) is the force exerted by the ground on the body in response to the body’s weight and movement. The direction and magnitude of GRF change based on posture, movement, and foot placement.
GRF is essential in maintaining balance. It helps counteract the force due to gravity that pulls the body down. The point where the GRF acts on the body is critical for understanding balance.
5. Human Balancing:
Balancing is about keeping the CoG within the base of support (BoS)—the area between the feet when standing. To maintain balance, the body constantly adjusts its posture to ensure that the CoG stays above the BoS.
The CoP plays a critical role in this process. When the CoG moves near the edge of the BoS, the CoP shifts to compensate, generating corrective forces to prevent a loss of balance.
The GRF contributes to this process by providing the counterforce necessary to keep the body upright. If the CoG shifts too far from the BoS, the person may need to step or reposition their body to regain balance.
6. Coordination of Segmental Centers (Foot, Knee, Hip CoM):
Balancing involves the coordination of different body segments. For example, when standing on one leg, the foot CoM aligns with the overall CoM to stabilize balance.
The knee CoM and hip CoM influence the control of posture and movement. The brain continuously processes sensory feedback to adjust muscle activity, ensuring that the CoM stays within a safe zone relative to the BoS.
For instance, when someone sways, adjustments occur at the hip, knee, and ankle to shift the CoM and control the CoP within the BoS.
Summary of Interrelationships:
CoM and CoG: Represent the central point of the body’s mass, which must be kept within the BoS to maintain balance.
CoP: Indicates the point of force application on the ground, which shifts to maintain balance when the CoM moves.
GRF: Provides the counterforce to keep the body upright and balanced.
Segmental CoMs (foot, knee, hip): These local centers of mass dynamically contribute to the overall CoM and help in the fine-tuning of balance.
Together, these elements describe the dynamic, complex process of human balance, where various feedback mechanisms ensure that the CoM, CoP, and GRF are constantly aligned to maintain stability.
In human balancing, the torques at the hip, knee, and ankle joints play a crucial role in maintaining stability and controlling the position of the center of mass (CoM) relative to the base of support (BoS). These torques result from muscular forces acting on the joints, and they help counteract the external forces that cause instability, such as gravity and the shifting ground reaction force (GRF). Here's how these concepts interrelate with the previously mentioned elements:
1. Torque and Joint Control in Balancing:
Torque is the rotational force produced by muscles acting around a joint. In human balancing:
Hip Torque: Controls the position of the pelvis and upper body. By generating torque at the hip, the body can adjust the CoM over the legs, helping to maintain an upright posture.
Knee Torque: Plays a role in maintaining the alignment of the thigh and lower leg. Adjustments at the knee help to fine-tune the position of the CoM by controlling the relationship between the thigh and lower leg, especially during movements like squatting or bending.
Ankle Torque: Crucial for fine balance control. When standing, the ankles make constant small adjustments to the position of the CoP relative to the CoM to prevent falling. The ankle acts like a pivot point, with torque helping to keep the CoM within the BoS.
2. Interrelationship Between Joint Torques and Center of Mass (CoM):
The hip, knee, and ankle work together to keep the CoM stable within the BoS by generating appropriate torques:
Hip Torque and CoM Control: When there is a shift in the CoM, say forward or backward, the hip muscles (such as the gluteus maximus, hamstrings, and iliopsoas) generate torque to pull the torso back to a stable position. This controls the upper body's alignment to prevent the CoM from moving too far forward or backward beyond the BoS.
Knee Torque and CoM Stabilization: The quadriceps and hamstrings create torque at the knee to stabilize the leg. If the CoM moves forward, the knee torque prevents the leg from collapsing under the body weight, which helps to keep the CoM aligned with the BoS. During activities like standing from a squat or stepping, knee torque is essential for shifting the CoM.
Ankle Torque and CoM-Correction: The ankle is the last line of defense in balance control. When the CoM drifts slightly out of the BoS, the ankle plantarflexors (calf muscles) or dorsiflexors (shin muscles) generate torque to pull the body back upright. This torque corrects small sway movements by adjusting the CoP and controlling how the GRF aligns with the CoM.
3. Interaction Between Joint Torques and Center of Pressure (CoP):
The center of pressure (CoP) is where the ground reaction force (GRF) is applied to the body and is directly influenced by ankle, knee, and hip torque. For balance:
Ankle Torque and CoP Movement: Ankle torque directly shifts the CoP. For instance, when the ankle plantarflexors contract, the CoP moves forward; when the dorsiflexors contract, the CoP moves backward. This shifting of the CoP allows the body to maintain stability by counteracting the movement of the CoM.
Knee Torque and CoP Stabilization: The knee also plays a role in adjusting the CoP indirectly by controlling the alignment of the foot. If the knees bend or extend, they help manage how the weight is distributed through the feet, influencing the CoP.
Hip Torque and CoP Adjustment: At the hip, torque adjusts the position of the pelvis and trunk, influencing how the CoM shifts within the BoS. In movements such as swaying or leaning, the hip torque moves the upper body, which in turn changes the distribution of pressure on the feet, slightly adjusting the CoP.
4. Ground Reaction Force (GRF) and Torque Interaction:
The GRF is the external force exerted by the ground in response to body weight. It plays a significant role in balance control:
GRF Counteracts Gravity: The GRF acts upward, opposing the downward force of gravity, and its magnitude and direction are influenced by the position of the CoM and CoP. When the CoM moves away from the BoS, the GRF shifts to realign the body's weight distribution.
Joint Torques Modulate GRF: The torques generated at the hip, knee, and ankle help control how the GRF interacts with the body. For example, if the body sways forward, the ankle torque moves the CoP forward to keep the GRF aligned with the CoM, preventing a fall.
5. Human Balancing and Joint Torque Coordination:
To maintain balance, the body must coordinate torques at all three joints—ankle, knee, and hip:
Ankle Strategy: In quiet standing, balance is often maintained with minor torque adjustments at the ankle joint. This ankle strategy controls the CoM and CoP within the BoS using small shifts in ankle torque.
Hip Strategy: When larger perturbations occur (such as leaning far forward or backward), the hip torque becomes more prominent. This hip strategy adjusts the upper body's alignment to bring the CoM back over the BoS.
Knee Strategy: The knee plays a crucial role in transitioning between the ankle and hip strategies, particularly when the body undergoes significant postural adjustments. For instance, knee torque is essential when bending, walking, or making corrective steps to prevent a fall.
Summary of Interrelationships:
Joint Torques (Hip, Knee, Ankle): These torques are necessary to adjust posture, stabilize the CoM, and shift the CoP to maintain balance.
CoM and CoP: The CoM must remain within the BoS to maintain balance. Joint torques ensure that the CoM is controlled, while the CoP shifts to counteract any instability.
GRF: The ground reaction force provides external support to keep the body upright. Joint torques control how the GRF is applied to maintain the alignment between the CoM and BoS.
These elements work together in a continuous feedback loop to enable smooth balance control, ensuring that the body remains stable despite external forces and changes in posture.
Maintenance of balance and posture. The cerebellum is important for making postural adjustments in order to maintain balance. Through its input from vestibular receptors and proprioceptors, it modulates commands to motor neurons to compensate for shifts in body position or changes in load upon muscles. Patients with cerebellar damage suffer from balance disorders, and they often develop stereotyped postural strategies to compensate for this problem (e.g., a wide-based stance).
Coordination of voluntary movements. Most movements are composed of a number of different muscle groups acting together in a temporally coordinated fashion. One major function of the cerebellum is to coordinate the *timing* and *force of these different muscle groups* to produce fluid limb or body movements.
Motor learning. The cerebellum is important for motor learning. The cerebellum plays a major role in adapting and fine-tuning motor programs to make accurate movements through a trial-and-error process (e.g., learning to hit a baseball).
Resources
https://my.clevelandclinic.org/health/body/22638-brain
https://www.kenhub.com/en/library/anatomy/gait-cycle
https://www.physio-pedia.com/The_Gait_Cycle
https://www.physio-pedia.com/Joint_Range_of_Motion_During_Gait#