15.3.2.1. Kinematics versus kinetics

Kinematics and kinetics are the two major components in biomechanics that need to be understood to understand human motion (see Figure 15.2). Kinematics is the physics of motion, including quantities such as displacement, velocity, acceleration and joint angles. Kinematic analysis is particularly useful for evaluating the mechanics of movement, detecting gait impairments and monitoring rehabilitation progress (Czamara et al. 2015).

In contrast, kinetics studies the internal and external forces acting on the body, such as the muscle forces, joint torques and ground reaction forces. This is important for the knowledge of joint loading, designing training protocols (Twumasi 2025) and avoiding overuse injuries.

The combined analysis of kinematic and kinetic data is always helpful to understand the movement science and direct intervention for diagnosis and rehabilitation (Orishimo et al. 2010; Sikorski and Czamara 2021). Functional movement screening is an example of a tool operating from a model similar to that of health and disease, using both types of data to identify compensatory patterns and evaluate the likelihood of injury in a population, with the aim of establishing individualized and effective rehabilitation protocols (Pan et al. 2011).

Refinements in motion capture systems and wearable inertial sensors have improved the usefulness of these for clinical practice. These instruments acquire real-time, objective data on joint motion and symmetry that contribute to clinical decision-making, particularly in the field of stroke rehabilitation (García 2022; Lee et al. 2022; LeMoyne and Mastroianni 2023). Kinematic and kinetic gait assessment provides a more complete understanding of motor control in health and disease (Lyu et al. 2021; Hui and Song 2023).

The kinetic chain model supports a system-based approach to understanding how joints and muscles work together, again emphasizing the need to consider the entire body in rehabilitation.Table 15.2 and Figure 15.2 summarize the structure of the developed framework to present an all-encompassing view of human movement that can contribute to the industry of clinical practice and sports performance optimization.

The reliance on the common motor framework means disruptions in one region can affect the whole system, which, in turn, highlights the importance of task-specific training and functional exercises that imitate real-life activities (Ličen et al. 2021; Brückner et al. 2023; Kim et al. 2023).

Novel technologies, such as robotic-assisted rehabilitation, enable accurate motion guidance and simultaneously collect real-time kinematic data, enhancing movement precision and therapeutic regularity (Lyu et al. 2021; Patel et al. 2021; Ceglia et al. 2023). Machine learning combined with kinetic analysis has also shown predictive ability for rehabilitation, opening a new frontier of data-driven personalized care (Halilaj et al. 2018; Shanshan et al. 2018).

Medical rehabilitation practitioners can thus use assessments that include aspects of both kinematics and kinetics, which have been proven to improve functional gains in patients.

15.3.2.2. Planes, axes, levers and muscle contraction

Physiotherapy and rehabilitation are closely tied with biomechanical concepts such as the planes of movement, axes of rotation, lever systems and types of muscle contractions and so on, and these concepts are the most fundamental for clinicians, as they provide the base in terms of movement analysis, design of individualized rehabilitation protocol and finally enhancing the functional outcomes for patients with a plethora of musculoskeletal and neurological conditions.

It is summarized in Table 15.2.

15.3.2.3. Lever systems in the body

The body functions as a system of levers, enabling efficient force generation and movement. These levers are categorized into:

• First-class levers, where the fulcrum lies between the load and the effort (e.g. neck extension), allowing balanced movement.
  
• Second-class levers, with the load between the fulcrum and the force (e.g. standing on tiptoes), optimizing force production.
  
• Third-class levers, the most common in the body, place the force between the fulcrum and load (e.g. bicep curls), maximizing speed and range of motion at the cost of mechanical advantage.

15.3.2.4. Center of gravity and balance

Center of gravity (CoG) and balance mechanics are important in physical rehabilitation, not only for the treatment of injuries but also in managing chronic conditions. The location of the CoG, which is where most of the body mass is focused, influences the balance control during dynamic and stationary tasks. Balance depends on proper CoG alignment relative to the base of support. CoG mechanics are of particular interest in rehabilitation, where therapies often aim to improve balance and minimize the risk of falls.

15.3.2.5. Balance training benefits in CoG alignment and muscle strength and Chronic Obstructive Pulmonary Disease (COPD)

Balance training aids in CoG alignment and strength training (Mkacher et al. 2015). Segmental assessment of trunk alignment during balance activity, particularly in a virtual reality environment, enhances stability (Feng et al. 2019) in individuals with Parkinson’s disease.

Wobble boards, commonly used in rehabilitation programs, specifically for older adults, focus on stimulating the CoG and proprioception, facilitating balance (Ogaya et al. 2011). Comprehensive balance training following hip fractures improves postural stability and lowers fall rates (Chang et al. 2024).

Balance control is a complex process that requires adequate muscle strength, neuromuscular coordination, dynamics and interaction with our environment. The vestibular system is also trained and strengthened, while the muscle strength improves to manage CoG over Base of Support (BoS) (Mkacher et al. 2015).

Modern technologies, including biofeedback and wearable sensors, have made it possible to visualize dynamic changes in CoG in real time, as patients posturally adjust to achieve improved functional balance (Sadura-Sieklucka et al. 2023; Museck et al. 2024). These are especially beneficial for at-risk populations, as they have a higher risk of falling while they recover.