kine 301 final

• Name three areas of motor control for which vision is important.  Short answer.

• Balance and posture - maintaining equilibrium by providing spatial orientation

• Locomotion - assists in navigating obstacles and adjusting steps while walking or running 

• Hand eye coordination - guides precise movements, such as reaching or catching 

• Is vision important in the maintenance of standing posture?

Yes, vision is important in the maintenance of standing posture. It provides information about the body’s position relative to the environment, helping to detect sway and make postural adjustments to stay balanced

• Does vision provide feedback that aids in the control of human movement

Yes, vision provides feedback that aids int he control of human movement by allowing individuals to monitor their actions, detect errors, and make real time corrections to improve accuracy and coordination

• The lecture presented 2 general types of augmented feedback. Name the two types of feedback. 

• Knowledge of results (KR): feedback about the outcome of the movement (ex. Whether the goal was achieved)

• Knowledge of performance (KP): feedback about the movement pattern of technique used to perform the skill

  • Then, choose a motor skill and give an example of both types of feedback that could be applied to that particular motor skill. Short answer.

    • KR: “you made 7 out of 10 shots”

    • KP: “your elbow was too far out during the shot”

    • KR: “the ball landed outside of the service box”

    • KP: “you didn't follow through with your swing”

• What are the types of augmented feedback that can be used for a motor skill?

• Knowledge of results (KR): information about the outcome of the movement

• Knowledge of performance (KP): information about the movement pattern or technique

• Verbal feedback: spoken cues or corrections

• Visual feedback: video replay or demonstrations

• Auditory feedback: sounds indicating success or errors (ex. Beep for correct action)

• Biofeedback: data from devices showing body functions (ex. Heart rate, muscle activity)

• What best describes “Knowledge of Results” feedback during motor learning?

• Knowledge of results (KR) feedback is information given about the outcome or success of a movement in achieving the goal. It tells the learner what happened as a result of their action, not how they performed it 

  • Ex: after a soccer kick, a coach says, “the ball missed the goal to the left.”

• What is a key reason why you may need to use augmented feedback?

• provide information that the learner cannot obtain on their own, especially when intrinsic feedback (like seeing or feeling the result) is limited or unclear

• This helps improve performance and learning by guiding corrections and reinforcing successful movements.

• When can augmented feedback hinder learning?

• Augmented feedback can hinder learning when it is:

  • Given too frequently, making the learner overly dependent on it

  • Too detailed of complex, causing confusion

  • Provided too early, before the learner has a chance to process intrinsic feedback

  • Inaccurate or inconsistent, leading to incorrect adjustments

• When giving augmented feedback after a motor skill is completed, which two timing intervals are important?

• Feedback delay interval: the time between the completion of the movement and the presentation of feedback

• Post feedback interval: the time between when feedback is given and when the next attempt is made

• Is feedback control used to make corrections during the movement?

• Yes, feedback control is used to make corrections during the movement, it relies on sensory information (vision or proprioception) received while the movement is happening to detect errors and adjust the action in real time

• Is the optimal frequency for giving augmented feedback all the time, every time?

• No, its often more effective to use reduced or faded feedback, giving it less often or gradually decreasing it over time, to encourage the learner to rely more on their own internal feedback and problem solving.

• Is focusing solely on what is done correctly sufficient to produce optimal motor learning?

• No, Learners benefit most from a balance of positive reinforcement and constructive error correction to refine their skills and develop accurate motor patterns 

·  Name the two dimensions and then each division of those dimensions for Gentile’s 2D taxonomy for classifying a motor skill. Clearly label each dimension and division in your answer.  Short answer.

• D1: environment context

  • Regulatory conditions:

    • Stationary

    • In motion

  • Intertrial variability 

    • Absent

    • Present

• D2: function of the action

  • Body orientation

    • Body stability

    • Body transport

  • Object manipulation

    • Absent

    • Present 

• ·  A general characteristic of Gentile’s model of learning in the later stage (stage 2) is decreased consistency. Is this true?

• False in later stages, movements becomes more consistent 

• · What are the characteristics of movement and muscle physiology that occur with aging?

• Characteristics of movement and muscle physiology that occur with aging include:

  • Dec muscle mass (sarcopenia)

  • Reduced muscle strength and power

  • Slower reaction time and movement speed

  • Dec flexibility and joint rom

  • Impaired balance and coordination

  • Reduced motor unit recruitment and firing rate

• ·  What are the differences in movement and muscle anatomy/physiology characteristics between a 65 year old and 20 year old?

• Muscle mass: a 65 year old typically has reduced muscle mass compared to a 20 year old

• Strength and power: older adults show dec strength and slower power output 

• Joint mobility: flexibility and joint rom are dec w age 

• ·  What happens to reaction time as we age?

• Reaction time generally slows as we age. This is due to age related changes in the nervous system, such as slower nerve conduction, reduced motor unit recruitment, and dec processing speed in the brain

• ·  What are the results of strength training in elderly individuals?

• Inc muscle mass and strength

• Improved balance and coordination

• Enhance mobility and functions independence 

• Reduced risk of falls and injury 

• Better bone density and joint health

• Improved overall quality of life

• ·  What was the main objective of the research in the paper “Differences in lower extremity muscular activation during walking between healthy older and young adults” by Schmitz et al. 2009?

•  how age & walking speed influence activation patterns of lower-extremity muscles during walking

• comparing young and older adults to understand neuromuscular differences/underlying gait changes &/ aging

• ·  In the research paper “Differences in lower extremity muscular activation during walking between healthy older and young adults” by Schmitz et al. 2009, what was the main finding regarding differences in lower-extremity muscular activation between healthy older and young adults during walking in this study? 

• older adults exhibited greater coactivation of muscles about the ankle & knee across all speeds & relied less on soleus during push off at faster walking speeds. 

• Suggesting older adults adopt limb stiffening strategy to maintian stability, reducing push off power.

• ·  In the research paper “Differences in lower extremity muscular activation during walking between healthy older and young adults” by Schmitz et al. 2009, which muscle(s) showed significant differences in activation between older and young adults during the terminal stance and pre-swing phase of walking? 

• Significant differences in the soleus muscle which older adults showed decreased activation during push off at faster speeds

• The gastrocnemius muscles activation was comparable b/w groups during these phases 

• Soleus displayed notable decline among older adults 

• ·  What is the Dynamic Gait Index?

•  Dynamic Gait Index - clinical tool used to assess a person's ability to modify balance while walking in the presence of external demands. Evaluates gait, balance, and fall risk

  • Composed of 8 walking tasks:

    • Walking in different speeds

    • Walking w head turns

    • Stepping over and around obstacles

    • Pivot turns

    • Stair climbing

  • Each task is scored from 0 (severe impairment) to 3 (normal performance)

  • Max score is 24; scores below 19 show higher risk of falls 

• ·  In the research paper, “Active and passive contributions to joint kinetics during walking in older adults” by Silder et al. 2009, what was the main difference observed in the net joint work between young and older adults during walking?

• Older adults generated more net positive work at the hip across all walking speeds

• Older adults generated less net positive work at the ankle compared to younger adults

• ·  In the research paper, “Active and passive contributions to joint kinetics during walking in older adults” by Silder et al. 2009, what did the study findings suggest about the passive contributions to hip and ankle work between young and older adults?

The study found no significant differences in passive contributions to hip and ankle work between young and older adults. This suggests that the increased hip work observed in older adults is not due to enhanced passive mechanisms but rather to active muscular efforts.

• ·  In the research paper, “Active and passive contributions to joint kinetics during walking in older adults” by Silder et al. 2009, According to the study by Silder et al. (2008), what was significantly correlated with peak positive plantar-flexor power at both preferred and fast walking speeds in older adults?

• significantly correlated w maximum isokinetic ankle plantar flexor strength

• ·  In the research paper, “Active and passive contributions to joint kinetics during walking in older adults” by Silder et al. 2009, what was the preferred gait speed between both study groups?

• No significant difference in preferred walking speed between young and older adults

• Step length and cadence were also similar between the 2 groups

• Difference in joint kinetics were not due to walking speed

• Results highlight the age related changes in muscle function, not speed, drive gait differences

• Ensures fail comparison of joint mechanics across age groups at similar walking speeds 

• · What are the five general characteristics of motor learning?

• is a set of processes

  • Motor learning unfolds over time and involves feedback, adaptation and repeated practice. Its dynamic and ongoing rather than a one time event

• not directly observable

  • The actual learning processes cannot be seen. Instead, learning is inferred through improvements in performance, consistency, and accuracy

• involves neural and behavioral changes

  • Motor learning reflects changes in both the central nervous system and in how movements are executed

• is practice dependent

  • Skill acquisition requires purposeful, task specific practice, and problem solving. Wo sufficient, appropriate practice, learning does not occur

• results in relatively permanent changes in capability

  • True motor learning leads to long lasting improvements in the capability to perform a skill, not just temporary performance gains that might disappear w time or fatigue

• ·  What are the methods used to assess motor learning?

• PRTTES 

• P – Performance Curves

  • Track progress during practice (but doesn’t confirm learning).

• R – Retention Test

  • Test after time off → shows if learning stuck.

• T – Transfer Test

  • Try the skill in a new situation → shows adaptability.

• T – Dual-Task Test

  • Do two things at once → shows how automatic the skill is.

• E – Error Correction

  • Can the person fix mistakes on their own?

• S – Savings Score

  • How fast do they relearn it after a break?

• ·  What are the definitions and important characteristics of a motor skill?

• Characteristics of motor skills - GO MAC

  • G – Goal-Oriented

    • Done to achieve a specific goal (e.g., hitting a target, running to a base).

  • O – Organized and Coordinated

    • Movements are not random — they are structured and involve the right timing and sequence.

  • M – Movement-Based

    • Requires voluntary body and limb movement (not automatic like a heartbeat)

  • A – Acquired through PracticE

    • Needs repetition and learning — not something you're just born doing perfectly.

  • C – Can be Measured and Improved

• You can assess accuracy, speed, consistency, and refine it over time.

• ·  If we were to classify a motor skill based on the size of the musculature required, which of the following motor skills would be considered a fine motor skill?

• Gross motor skills: Use large muscles (e.g., legs, arms); used for walking, jumping, running.

• Fine motor skills: Use small muscles, especially of the hands and fingers; used for tasks requiring precision.

• Fine motor skills ex:

  • typing on a keyboard

  • Writing with a pencil

  • Buttoning a shirt

  • Playing the piano

  • Threading a needle

• ·  If we were to classify a motor skill based on the beginning and ending, which of the following motor skills would be considered a discrete motor skill?

• Discrete motor skills 

  • Has a clear beginning and end

  • Usually brief and single movement

  • Examples:

• Throwing a ball

• Kicking a soccer ball

• Pressing a button

• ·  If we were to classify a motor skill based on the stability of the environmental context, which of the following best describes a closed motor skill?

• Closed motor skill

  • Performed in a stable, predictable environment

  • The environment does not change during the movement

  • The performer can plan their action ahead of time

  • Ex:

    • Shooting a free throw in basketball

    • Bowling

    • Typing on a keyboard

    • Gymnast doing a floor routine in a quiet gym

• ·  You practice Frisbee golf throws for an hour, wait for a week, and then throw a Frisbee again. What is the amount of motor skill lost over the week best called?

• Relative retention — it tells us how much of the motor skill was lost (or retained) after a period of no practice.

• ·  Internal models of motor control are commonly associated with which type of movement control?

feedforward control

• Why?

  • Feedforward control involves predicting and planning movements in advance, using internal models of the body and environment.

  • These internal models allow the brain to simulate outcomes before the movement happens, which is essential when:

    • Movements are too fast for real-time feedback

    • Precision is required

• ·  Does motor variability during motor learning equate with action exploration?

• Yes — motor variability during motor learning often equates with action exploration, especially in the early stages of learning.

• Why?

  • Motor variability refers to the natural differences in movement when someone is practicing a skill.

  • In early learning, this variability is not just "error" — it's a form of exploration, where the brain is testing out different movement strategies.

  • Through this trial-and-error process, the nervous system learns which movements work best, leading to refined and consistent performance over time.

• ·  What is the definition of motor learning?

• Motor learning - set of processes associated with practice or experience that leads to relatively permanent changes in the capability for skilled movement.

• ·  What is the definition of Persistence as it relates to motor learning?

• Persistence - the lasting improvement in performance that remains over time, even after a period without practice

• How it relates to motor learning:

  • It shows that the skill has been retained, not just temporarily performed well.

  • It's a sign of true motor learning, not just a short-term gain

  • Ex: If you practice juggling for a week, take a break, and can still juggle well later — that’s persistence

• ·  Can elderly adults learn a novel or new motor skill?

• Yes

  • Why: 

    • Neuroplasticity — while it declines with age, the brain still retains the ability to form new motor pathways.

    • Practice and repetition still lead to improvements in skill and coordination.

    • Motivation, feedback, and task-specific training enhance learning in older adults

  • However:

    • Learning may take longer due to slower processing and reduced working memory.

    • Motor variability and fatigue may be higher.

    • Simplified instructions, increased practice time, and augmented feedback can help.

• ·  Is variable practice a more effective practice condition for discrete motor skills but not continuous skills?

• its generally effective for both discrete and continuous motor skills, but it may have different effects depending on the skill type and learning stage.

• ·  In the elderly, is the type of cognitive function most affected by fitness and exercise processing speed?

• Yes — in the elderly, processing speed is one of the cognitive functions most positively affected by fitness and exercise.

• Why?

  • Processing speed refers to how quickly someone can take in, interpret, and respond to information.

  • As we age, this ability naturally declines, but:

• Aerobic exercise, strength training, and movement-based activities like dance or tai chi have been shown to slow or even reverse this decline.

Improved cardiovascular health enhances blood flow to the brain, supporting faster cognitive function.

• ·  Does a negatively accelerated performance curve for motor learning show small amounts of improvement early in practice and large amounts of improvement later?

• shows large improvements early in practice, followed by smaller improvements later

  • Early practice brings rapid gains as the learner figures out the basics.

  • Over time, performance plateaus, and gains become more gradual

• ·  Does overestimation of learning happen when certain aids are available during practice that are not available during performance?

• Yes — overestimation of learning can happen when aids used during practice are not available during performance.

• Why?

When learners practice with assistive devices, visual guides, or extra feedback, they may perform better during practice. But if those aids are not available during the actual performance test (like in a real-world setting or retention test), their performance often drops.

• ·  During motor learning, there may be periods of time, known as plateaus, when performance does not improve but learning may still occur.  Is this true? 

• Yes. performance plateaus are common — these are periods where visible improvement stops or slows down, but that doesn’t mean learning has stopped.

• The brain may be:

  • Consolidating motor patterns

  • Integrating feedback

  • Adjusting movement strategies

• ·  Accelerometers and gyroscopes found in Inertial Measurement Units can be used to measure kinematics. Is this true?

• Yes

• Inertial Measurement Units (IMUs) contain:

  • Accelerometers – measure linear acceleration (e.g., speeding up, slowing down).

  • Gyroscopes – measure angular velocity (e.g., how fast something is rotating).

• IMU can measure: speed, direction, displacement, rotation

• ·  Beginners learning a new motor skill tend to use fewer muscles to perform the motor skill.  Is this true?

• No, Beginners learning a new motor skill tend to use more muscles than necessary — a phenomenon known as co-contraction.

• Why?

  • Beginners often recruit extra or incorrect muscles to stabilize or control the movement

  • This leads to:

    • Inefficient movement

• Excessive energy use

• Less smooth, more rigid motion

• ·  Does the location of brain activity change with practice?

• Yes, brain activity shifts from higher-level planning regions to more automatic motor control areas as learning progresses.

  • Early learning relies more on: prefrontal cortex (planning, attention, decision making), cognitive control areas, conscious effort

  • With practice, brain activity shifts to

    • Sensorimotor areas (more automatic control)

    • Cerebellum (timing and coordination)

    • Basil ganglia (habitual movement patterns)

  • Shift reflects the movement becoming: more efficient, less mentally demanding, more automatic

• ·  What are moderator variables and what are their influence on motor learning and performance as described in the paper “The Relationship Between Deliberate Practice and Performance in Sports: A Meta-Analysis” by MacNamara et al. 2016?

• Moderator variables are factors that can influence the strength or the nature of the relationship b/w deliberate practice & sports performance 

• ey moderator variables and their influence:

  • Type of sport skill

    • Closed skill sports (ex. bowling) -> stronger link to deliberate practice

    • Open skill sports (soccer) -> weaker link due to unpredictable environments

  • Pacing of sport

    • Internally paced sports (e.g., gymnastics) → stronger relationship with practice

    • Externally paced sports (e.g., tennis) → weaker relationship

  • Skill lvl of athlete

  • Age group

  • Measurement method

• ·  How does the relationship between deliberate practice and performance differ between elite and sub-elite athletes according to “The Relationship Between Deliberate Practice and Performance in Sports: A Meta-Analysis” by MacNamara et al. 2016?

• Sub elite athletes 

  • Deliberate practice explained a larger proportion of performance differences

  • Practice plays a significant role in helping athletes reach competitive levels

  • Stronger correlation between amount of practice and measured performance.

• Elite athletes

  • Deliberate practice explained only about 1% of performance variance

  • At the highest levels, other factors (e.g., genetics, decision-making speed, psychological traits) contribute more

• ·  In the paper “The Relationship Between Deliberate Practice and Performance in Sports: A Meta-Analysis” by MacNamara et al. 2016, how much was sport performance explained by deliberate practice?

• Deliberate practice accounted 18% of the variance in sports performance

• Means practice is important but so are many other things like physical ability, genetics, mental skill is a big role in how good someone can become

• ·  What is the function of transfer tests when assessing motor learning?

• Transfer test - measures how well a learned skill can be applied in a new or different contex

• Function:

  • Assess adaptability: Determines if the learner can use the skill in real-world or altered environments

  • Reveal depth of learning: Shows whether the skill is flexible and not just memorized under one condition

  • Differentiate true learning from memorized routines

  • Support generalization: Evaluate how well learning transfers to related tasks (e.g., using a forehand in both tennis and pickleball)

• The cerebrocerebellum is the functional division of the cerebellum that plays a role in the planning and timing of voluntary movement.

• Functions:

  • Planning and timing of voluntary movements

  • Coordination of complex motor tasks, especially those requiring precision

  • Motor learning and adaptation

  • Cognitive functions such as attention and language (to a lesser extent)

• Location:

  • Located in the lateral hemispheres of the cerebellum

  • Receives input from the cerebral cortex via the pontine nuclei

  • Sends output to the motor and premotor areas through the dentate nucleus and thalamus

• What is the type of neuron in the cerebellum that has a very large dendrite tree and provides inhibitory connections?

• Purkinje cells

  • Purkinje cells = large dendritic trees + inhibitory output = essential for fine-tuning motor control

• Does the cerebellum contribute to the maintenance of posture and balance?

• Yes

• How the cerebellum contributes:

  • he vestibulocerebellum (aka flocculonodular lobe) is the primary region responsible

  •  It processes input from the vestibular system (inner ear), which detects head position and motion

  • Helps coordinate balance, equilibrium, and eye movements.

  • Sends outputs to maintain muscle tone, postural stability, and reflexes that keep you upright

• Does the cerebellum act like a feed-forward controller?

• Yes the cerebellum functions like a feed forward controller, predicting and fine tuning movement before it occurs

  • Uses internal models to predict the sensory consequences of a movement

  • Adjusts motor commands before errors happen

  • Helps coordinate timing, force, and accuracy of movement in real time.

• Is the Purkinje cell the primary input of the cerebellar cortex?

• No, the purkinje cell is the primary OUTput neuron of the cerebellar cortex

• What are the three stages of the Fitts and Posner Model of learning?

• C -> A -> A 

• Cognitive:

  • What happens: learner is trying to understand the task

  • High mental effort and many errors

  • Thinking and trial and error dominate

  • Ex: a beginner learning to ride a bike is focused on balancing, pedaling, and steering all at once

• Associative stage

  • What happens: movement become more refined and consistent

  • Learner starts to associate actions with outcomes

  • Fewer eros, better timing, and more efficient movement

  • Ex: the rider can now stay balanced and pedal smoothly but still focuses on improving technique 

• Autonomous stage

  • What happens: Skill becomes automatic and effortless

  • Little conscious thought is needed to perform the task

  • Performer can focus on strategy or other aspects

  • Ex: The rider can now ride while talking or navigating obstacles

• Early practice is characterized by large gains in improvement

• Early practice is characterized by large gains in improvement

  • Yes

  • Why:

    • Performance improves rapidly a first as the learner figures out the basic mechanics

    • Over time, improvements slow down, following a negatively accelerated curve

    • Applies to both motor and cognitive skills

    • Early practice = steep gains

• Late practice is characterized by small gains in improvement

• Yes

• Why:

  • As learner becomes more skilled, their performance improvement slow down

  • The rate of learner dec even though practice continues

  • Smaller gains often reflect 

    • Fine tuning of technique

    • Inc consistency

    • Improved efficiency 

  • Late practice = gradual plateau

• ·  Where do memory and coordination of multi-joint motions and multi-limb function occur?

• Cerebellum

  • Coordinates timing and precision of multi joint/limb movements

  • Stores internal models for smooth, efficient coordination

  • Critical for motor learning and adaptation

• Primary motor cortex 

  • Executes voluntary movement

  • Organizes and activates multiple muscle groups

• Basal ganglia

  • Involved in movement initiation and movement memory 

• Premotor cortex

  • Helps w movement planning, especially in response to sensory cues

• ·  Information about a particular motor skill is likely stored in which type of memory?

• Procedural memory - responsible for storing how to perform tasks or motor skills (unconsciously)

• ·  What are the subsystems of long-term memory?

• Declarative (semantic + episodic)

  • General facts

  • Personal experiences 

• non declarative (procedural)

  • Use wo conscious awareness (tied to motor skills)

• ·  What type of motor skills would likely show better retention?

• Continuous motor skills 

• ·  What three directions were performed for our lab data collection? Short answer.

• Anterior, posteromedial, and posterolateral

• ·  What is the purpose of measuring leg length for the star excursion balance test?

• To normalize reach distances so that balance performance can be fairly compared between individuals of different sizes 

• Makes comparisons more accurate 

• ·  In the paper, “Systematic review and meta-analysis of the y-balance test lower quarter: reliability, discriminant validity, and predictive validity,” by Plisky et al. 2021, what was an important recommendation for accurate injury risk prediction?

• Use population specific cut points for injury risk prediction

• Avoid using general or one size fits all thresholds across all groups

remember this

Label this correctly.  Which one is most common for human motor learning?

Top right is the most common pattern in human motor learning 

What are the characteristics of fatigue that can be seen with an EMG signal? 

Fatigue in an emg signal is shown by a dec in freq and an inc in amplitude as the muscle tires and body tires to maintain output

• Middle picture shows start of fatigue

Explain the different types of muscle contractions below. In your description, include the concepts of Internal and External Torque.

•  Isometric contraction 

  • Muscle length: stays the same (no joint movement)

  • Torque relationship: internal torque = external torque 

  • Muscle is active and generating force, but there is no movement bc the torque produced = torque from external load

• Concentric contraction

  • Muscle length: shortens

  • Torque relationship: internal torque > external torque

  • Muscle overcomes the external load, causing the joint to move in direction of the muscle action

• Eccentric contraction 

  • Muscle length: lengthens

  • Torque relationship: external torque > internal torque

  • Muscle is active but is lengthened by a stronger external force 

• • The primary motor cortex is contained in which area of the brain?

• Precentral gyrus of the frontal lobe 

• • What is the function of an afferent neuron? Where in the spinal cord do they live?

• Afferent neuron carries sensory information from body to cns 

• Lives in the dorsal root ganglia and enters spinal cord via dorsal horn

• • What is the function of an efferent neuron? Where in the spinal cord do they live?

• Efferent neuron carries motor commands from the cns to muscle or glands

• Lives in ventral horn of spinal cord and exits spinal cord via ventral root

• • Neuroplasticity can be defined as changes in neural pathways and synapses that can be caused by changes in what?

• Behavior, environment, neural processes, thinking, emotions, and injury 

• • During quiet standing, humans have limits to their postural stability, commonly represented as angles of sway. In which direction do humans generally have the least amount of angular motion before stability is lost

• Humans generally have the least amount of angular motion before stability is lost in the lateral (side to side) direction 

• • What are more response choices to a given stimulus likely to do?

• Inc reaction time

• More response choices = slower reaction time

• • Increasing the accuracy requirements of a movement will increase reaction time. An increase in movement accuracy is most likely to affect which of the following stages of the information processing model?

• Response programming stage 

• • Swing phase is approximately what percentage of the total gait cycle (1 stride)?

• 40%

• • Decreasing movement speed will generally increase timing accuracy. Is this true?

• False, slowing down generally hurts timing accuracy but it might improve spatial accuracy 

• • Is there a neurological component to fatigue?

• Yes

• Occurs in brain or spinal cord

• Dec motor unit recruitment, slower firing rates, and reduced coordination