Amyotrophic Lateral Sclerosis (ALS) is a progressive neurodegenerative disease that primarily affects the nerve cells controlling voluntary muscle movement. These nerve cells, known as motor neurons, are located in the brain and spinal cord, and their function is to transmit signals from the brain to the muscles. When ALS causes these motor neurons to degenerate and die, the connection between the nervous system and muscle fibers is lost, which immediately complicates the process of muscle development.
How ALS Prevents Muscle Hypertrophy
True muscle hypertrophy, the process of muscle fibers increasing in size, relies entirely on successful communication between motor neurons and muscle tissue. Muscle growth is triggered by resistance exercise, which causes micro-damage to muscle fibers, followed by a repair process signaled by intact motor neurons. In ALS, the progressive death of both upper and lower motor neurons permanently severs this communication pathway, making traditional muscle building virtually impossible.
The physical disconnection process is known as denervation, where the muscle fiber is cut off from its nerve supply. Once denervated, the muscle fiber quickly begins to waste away, a process called atrophy. This muscle wasting is compounded by the activation of specific molecular pathways, such as the ubiquitin-proteasome system, which actively breaks down muscle proteins.
In the early stages, the body attempts a compensatory mechanism called reinnervation, where nearby, healthy motor neurons sprout new connections to rescue the orphaned muscle fibers. This temporary rescue helps mask the initial loss of strength and function. Unfortunately, this reinnervation process is eventually overwhelmed by ongoing motor neuron death, leading to a net loss of functional motor units.
The disease also involves the elevated presence of myostatin, a protein that naturally acts as a brake on muscle growth. This combination of denervation, active protein breakdown, and growth inhibition means the body is in a state of continuous muscle loss. Physical activity must therefore focus on preserving the function of remaining muscle, rather than building new mass.
Goals of Exercise: Maximizing Remaining Functional Strength
Since building new muscle mass is biologically impractical due to denervation, the primary goal of exercise shifts to maximizing the use of existing functional motor units and maintaining flexibility. The focus is on slowing the rate of disuse atrophy and preventing secondary complications, such as joint contractures. This approach requires a careful balance to avoid overexertion, which can cause detrimental fatigue and potentially accelerate the breakdown of remaining motor units.
Physical activity must be tailored to the individual’s specific stage of disease and overall level of function. Safe exercise modalities help maintain joint mobility and cardiovascular health without demanding excessive energy.
Safe Exercise Modalities ####
Safe exercise modalities typically include:
- Gentle range-of-motion exercises
- Passive or active stretching
- Light aerobic activity, such as walking or stationary cycling
If resistance training is used, it must be low-impact and submaximal, focusing on strength maintenance rather than strength gain, using light weights or resistance bands. The intensity should remain low to moderate, ensuring the person does not feel weaker or overly fatigued in the hours or days following the exercise session. Exercise must cease well before the point of exhaustion to prevent overuse weakness, which can occur when remaining motor units are pushed beyond their capacity.
Functional mobility exercises, such as assisted transfers or gait training, are also incorporated to keep daily activities possible for as long as possible. Specific breathing exercises are often introduced to help maintain the strength and capacity of respiratory muscles. A physical therapist is an important part of the care team, designing and modifying personalized programs as the disease progresses.
Nutritional Strategies for Muscle Maintenance
Maintaining existing muscle mass and overall body weight is a significant challenge in ALS due to the disease’s effects and the increased energy expenditure. The body experiences a hypermetabolic state, meaning it burns calories faster than expected, further accelerating muscle and weight loss. This demand must be met with sufficient caloric intake to prevent cachexia, which is severe muscle and fat wasting.
Adequate protein intake is necessary to support the maintenance and repair of muscle tissue, with recommendations often ranging between 0.8 and 1.5 grams of protein per kilogram of body weight per day. Consuming enough calories from carbohydrates and fats is also important, as this provides a “protein-sparing effect,” ensuring the consumed protein is used for tissue maintenance rather than being burned for energy.
A major complicating factor is dysphagia, or difficulty swallowing, which makes eating and drinking a struggle and increases the risk of aspiration. To address this, high-calorie, nutrient-dense foods should be prioritized in smaller, more frequent meals to conserve energy. Food texture and liquid consistency modifications, guided by a speech-language pathologist, are often implemented to make swallowing safer.
Strategies like using a chin-tuck maneuver during swallowing or blending foods into a softer consistency help ensure nutritional needs are met safely. Ultimately, the focus of nutritional management is on maintaining a stable weight and maximizing nutrient delivery to preserve the remaining physical function.