Strength and Flexibility Development Throughout the Lifespan

General Concepts of Strength and Flexibility

• Balance of Capabilities: To perform functional tasks throughout life, individuals require a combination of strength, flexibility, and skill. Muscle tissue must be both strong and flexible to optimize performance.
• Rate Limiters: Specific physical attributes can limit the ability to perform developmental or functional tasks.     * For infants, a primary rate limiter for initiating walking is strength.     * For older adults, a lack of strength can impede the ability to perform stairs or sit-to-stand transitions.
• Imbalance Risks:     * Flexibility without Strength: Can lead to excessive joint mobility, instability, lack of movement control, and an increased risk of injury.     * Strength without Flexibility: Can lead to limited functional capacity, reduced athletic performance (muscles cannot go through full excursion), and increased injury risk.     * Anecdotal Example: A bodybuilder teacher was so muscular but inflexible that he could not reach sticky notes placed on his back and struggled with basic tasks like putting on or taking off a shirt.

Defining Muscle Strength

• Scientific Definition: Strength is a measurement of force, specifically how much force a muscle can generate under tension.
• The Formula for Force: $Force = Mass \times Acceleration$
• Factors Influencing Muscle Force Production:     * Neurological Activation: The number of muscle fibers that have nerves capable of innervating them.     * Coordination and Timing: The synchronous firing of muscle fibers. Poor coordination can lead to twitching fasciculations rather than productive force.     * Cross-Sectional Area: Often referred to as muscle mass; generally, more mass allows for more force generation.     * Leverage and Sliding Filament Theory: Based on the interaction of actin and myosin heads.         * The mid-length of a muscle is where the most force is generated.         * At either extreme (fully lengthened or fully shortened), force generation is lower due to insufficient or excessive contact between filaments.         * Example: In a bicep curl, strength is lower at the very beginning and very end of the movement compared to the mid-range.

Strength and Muscle Mass Throughout the Lifespan

• General Progression:     * Infants: Early increase.     * Childhood: Steady advancement.     * Adolescence: Rapid increase or "spurt."     * Adulthood: Maintenance.     * Older Adults: Decline.
• Sarcopenia: The age-related decline in muscle mass and strength seen in older adults.
• Developmental Mechanics:     * After the first year of life, muscle mass increases via hypertrophy (increase in fiber size) rather than hyperplasia (increase in the number of fibers).     * Infancy: Infants gain strength and coordination through motor milestones: tummy time, reaching, lifting the head, rolling, sitting, creeping, crawling, cruising, and walking.     * Childhood: Steady increase in strength occurs until approximately age $13$. This is primarily due to the development of the neuromuscular system (timing and coordination) and is not directly linked to hypertrophy alone.     * Adolescence: Sex differences emerge around age $12$ to $13$.         * Males: Experience a pronounced increase in strength at age $13$. They tend to have more Type II muscle fibers (strength and power).         * Females: Experience a more gradual increase. They tend to have more Type I muscle fibers (endurance).         * By age $16$, males generally outperform females in average strength measurements (grip strength, elbow flexion, squats, etc.).
• Absolute vs. Relative Strength:     * Relative Strength: Gains relative to fat-free mass (muscle tissue composition) are similar between males and females.     * Absolute Strength: Males have greater absolute increases, particularly in the upper body.     * Upper Body: Females have roughly $40\% \text{ to } 60\%$ of the upper body strength of males.     * Lower Body: Females have roughly $60\% \text{ to } 80\%$ of the lower extremity strength of males.
• Adulthood: Strength peaks in the $20s$ and $30s$. A decrease in mass and strength typically begins in the $50th$ year of life (the fifth decade).     * Factors for decline include the "use it or lose it" principle, poor nutrition, inactivity, and disease.     * Hypertrophy Potential: The capacity to build muscle through hypertrophy remains even into the $90s$.

Training Strength Across the Lifespan

• Infants: Focus on movement and environment exploration. They should not be restrained (in high chairs or car seats) for more than one hour at a time. Tummy time is essential for building extensor strength.
• Young Childhood: Focus on motor competence and fun activities. Use playground equipment and bodyweight exercises. Avoid adding extra weight like kettlebells; light items like bean bags are acceptable.
• Older Children ($6 \text{ to } 10$ years): Introduction of formal resistance training with scaled equipment. Focus must be on form and motor patterns. Use light resistance; do not use maximal or near-maximal loading.
• Youth Resistance Training Guidelines:     * Frequency: No more than twice a week for younger children when adding external weights.     * Myth of Growth Plates: Research shows that supervised, carefully dosed training does not cause premature closure of growth plates or significant injury. Injuries usually result from poor form or excessive weight causing fractures.
• Adolescents: Muscle strengthening $2 \text{ to } 3$ times per week. They have higher recovery rates than adults.     * Supervision: Must account for growth spurts and "adolescent awkwardness" (temporary loss of coordination).     * Social Constraints: The "$1000$-pound club" in high school gyms can motivate students to use poor form to reach milestones, increasing injury risk.
• Adults and Older Adults: Minimum of twice a week.     * Older Adults: Multimodal training is preferred (combining balance, cardio, strength, and flexibility). Cardio-respiratory and orthopedic issues (e.g., osteoporosis) must be monitored.

Flexibility and Mobility

• Flexibility: The ability of a muscle or soft tissue to stretch (elasticity).
• Mobility: The ability of a joint to move through its full range of motion with control and coordination. It requires flexibility, strength, stability, and coordination.
• Anatomical Constraints: Joint structure (hinge vs. ball and socket) and soft tissue mass (e.g., large hamstrings limiting knee flexion) determine end-feel.
• Developmental Changes:     * Infants/Young Children: Highly flexible due to immature skeletons and high elastin content. Joints have more space to accommodate bone growth. This mobility typically decreases after age $9$.     * Sex Differences: Females generally have greater flexibility due to estrogen (affects connective tissue), participation in specific sports (dance/gymnastics), and lower muscle mass.     * Adolescence: A temporary decrease in flexibility may occur during growth spurts as bones lengthen before soft tissues can catch up.     * Adulthood: Declines begin in the $20s$ and $30s$ as collagen and elastin content change and the body stops signaling for length growth.     * Older Adults: Accelerated decline in the $50s$ and $60s$ (influenced by menopause in females). Significant declines in hip and shoulder mobility appear in the $70s$. Maintaining flexibility reduces fall and injury risk.

Training Flexibility and Mobility

• Frequency: For children through older adults, mobility training should occur at least $2 \text{ to } 3$ times per week.
• Infants: Active play and exploration; avoid restraint.
• Children: Use game-like activities (Twister), playground equipment (monkey bars for shoulder flexion), and dynamic or static stretches.
• Adolescents: Include dynamic warm-ups and static cool-downs. Yoga and structured stretching are appropriate.
• Adults: Participation in sports and regular structured stretching. New sports require supplementary mobility work.
• Older Adults: Static stretching is particularly beneficial, especially for arthritic joints (longer hold times).     * Cautions: For patients with osteoporosis, avoid combined end-range spinal movements (e.g., simultaneous rotation and flexion).     * Adaptations: Chair yoga or modified sports swings (e.g., golf) can improve safety while promoting mobility.

Questions & Discussion

• Video 1 (Toddler): A young child (age $2\text{-}3$) playing with a light weight, using the "tonic labyrinthian reflex" to lift. The speaker is not concerned as it is light and play-based.
• Video 2 (9-year-old girl): Girl performing a clean and jerk with heavy weights ($25$ lbs on each side). Concerns were raised regarding her right knee dropping in (valgus) during the eccentric phase and the risk to the spine during a near-maximal overhead load.
• Video 3 (Young boy): Performing cleans with a coach. Though the weight is heavy, the coach is providing hands-on support and cues, and the boy drops the weight properly. This was deemed "better" but still requires strict supervision.
• Growth Textbook Critique: The primary textbook used for the course contains outdated information (based on a study from $1978$). It incorrectly claims women do not gain strength after age $13$. The instructor advised using materials from the therapeutic exercise textbook instead of the growth textbook for these specific topics.