Biomechanics of Resistance Exercise

Basic Definitions

n     Strength

n    Maximum force that a muscle or muscle group can generate at a specific velocity

n     Power

n    Maximum force that a muscle or muscle group can generate at a very high rate of speed

n     Torque

n    The “rotational” power of a muscle or muscle groups; i.e. movement against a resistance through angular displacement. Example, knee flexion or extension.

Factors Affecting Ability to
Generate Muscle Force

1.  Angle of Pennation

a. Angle at which muscle fibers align with tendon

n      The more parallel to the tendon, the greater the potential for speed of contraction
n      The more oblique (angled) to the tendon, the greater the potential for force of contraction

Factors Affecting Ability to
Generate Muscle Force

B. Types of pennation

n      Unipennate – one set of muscle fibers at oblique angle
n      Bipennate – muscle fibers aligned on either side of tendon
n      Multipennate – more than one tendon attachment with 2 or more angles in whole muscle group
n      Fusiform – muscle fibers are parallel to tendon
n      Longitundinal – muscle fibers are parallel but have intermediate attachments
n      Radiate – muscle fibers are spread out from single point

 

Factors Affecting Ability to
Generate Muscle Force

2. Angle of Muscle-Bone Attachment

   - a person whose tendons are inserted on the bone closer to the joint center should be able to move the body part faster through the ROM

   - a person whose tendons are inserted on the bone further from the joint center should be able to lift heavier weights

Factors Affecting Ability to
Generate Muscle Force

3. Length of Moment Arm

   - the longer the moment arm, the greater the mechanical advantage, i.e. easier to lift a weight

   - the shorter the moment arm, the lesser the mechanical advantage, i.e. more difficult to lift a weight

 

Factors Affecting Ability to
Generate Muscle Force

4. Neural Control

   - neural interaction can affect contraction output in various ways:

        A. more muscle force occurs when:

                1. more motor units are involved

                2. motor units are greater in size

                3. rate of motor unit firing is

                    faster

Factors Affecting Ability to
Generate Muscle Force

        B. Conditioning timeline

 

 

 

 

 

 

 

Factors Affecting Ability to
Generate Muscle Force

5. Muscle Cross-sectional Area

   - the force that a muscle can exert is directly related to its cross-sectional area rather than volume

        e.g. a tall athlete with longer arms can have large muscle volume, but a shorter athlete with less volume can have greater cross-sectional area

Factors Affecting Ability to
Generate Muscle Force

6. Muscle Contraction Velocity

   - force capability of muscle declines as velocity of contraction increases and vice versa

Factors Affecting Ability to
Generate Muscle Force

7. Muscle Action

    - Basic actions are:

          a. isometric – no change in muscle length

              since contractile force = resistive force;

                    e.g. abdominals during sit-up

          b. isotonic – change in muscle length

                   1. concentric – shortening

                   2. eccentric – lengthening

          c. isokinetic – change in muscle length while

             working against an accomodating resistance at a

             fixed velocity.

Factors Affecting Ability to
Generate Muscle Force

8. Strength to Mass ratio

   - strength of a muscle (relative to its mass) directly reflects an athlete’s ability to accelerate their body

        e.g. if an athlete’s muscle mass     by

              15% but force only    by 10%,

              ability to accelerate is reduced

Factors Affecting Ability to
Generate Muscle Force

9. Body Size

        - smaller athletes are generally stronger than taller athletes on a pound for pound basis

        Why?

                As body size   , muscle mass is

                increasing faster than muscle

                strength can keep up

Sources of Resistance to
 Muscle Contraction

1. Gravity

- downward force on an object

- object’s weight = object’s mass x acceleration

2. Positioning of Resistance

      - depending upon the placement of an object (barbell), different muscles may be affected

          e.g. in the squat, a more forward inclination of the trunk brings the weight closer to the knees; hence this reduces the resistive torque about the knees that the quad muscles must counteract

                             while at the same time

          the weight is farther from the hip, so the resistive torque about the hip increases making the gluteus and hamstrings work harder

Sources of Resistance to
 Muscle Contraction

3. Weight-stacked machines

    - provide support during lifting actions

    - can design certain ROM’s that can’t be provided by free weights

    - easy to use when selecting weights

 

4. Free-weights

    - provide means to do whole-body training

          i.e. cause certain muscle to become stabilizers

    - simulate real-life muscle actions

          i.e. more natural coordination occurring between

               muscle groups

Sources of Resistance to
 Muscle Contraction

5. Inertia

    - the force that must be overcome to move a weight

          a. can be used to one’s advantage as

              in the technique of bracketing*

                   * performing a sport movement

                      with lighter than normal and

                      heavier than normal implements

                             e.g. using a heavier shot put to

                                   train for strength or a lighter shot

                                   put to train for speed

Sources of Resistance to
 Muscle Contraction

6. Friction

    - creating resistance while gripping a barbell or a rope during a rope-climb

 

7. Fluid Resistance

    - resistance encountered when moving through water

    - 2 types:

          a. surface drag – resistance on surface

          b. form drag – resistance when fluid presses

             against front or rear of object passing through it

Sources of Resistance to
 Muscle Contraction

8. Elasticity

    - resistance created through springs or bands

          e.g. therabands

                   - amount of resistance is

                     controlled by length of stretch or

                     thickness of band

    - problems with elastic devices:

          a. every movement begins with low resistance

             and ends with high resistance; may be

             contrary to human movement

          b. adjusting resistance

          c. not optimum for providing resistance in jumpers

Sources of Resistance to
 Muscle Contraction

9. Electronically-controlled devices

   - control rate of machine movement through a feedback mechanism

        examples:

        a. isokinetic devices – Biodex, Cybex,

            Orthotron, Kin Com, mini-gyms

        b. cycle ergometers

        c. others – control power output and

           acceleration

Weight Lifting Concerns

1. In general, weight lifting injuries are minimal and account for < 1% of in-season injuries

n    Most common injury- back region

n    Majority of injuries occur at L4-L5 and L5-S1

n   Occur due to mechanical disadvantage

n   Muscles must often exert forces at 10 times the weight lifted
n   Flat-back posture is better than rounded
n   Minimize L5-S1 compression forces and ligament sprain
n   Slightly arched back is also better than rounded back in avoiding injury; also capable of exerting greater forces

Weight Lifting Concerns

2. Intra-thoracic vs intra-abdominal Pressure

   - when glottis is closed while lifting a resistance, intra-thoracic (within thorax) pressure builds up; also known as Valsalva Manuever

        * this may provide temporary stabilization to

           the torso

   - not necessary to hold breath during lift, however “fluid ball” is created in intra-abdominal region due to contraction of deep abdominals and diaphragm.

        *”fluid ball” can actually support lower spine

Weight Lifting Concerns

2. Intra-thoracic vs intra-abdominal Pressure

   - holding breath can lead to     in venous return (due to compression on heart and blood vessels)

        * this results in    blood pressure and

           subsequently the possibility of

           passing out

Weight Lifting Concerns

3. Weight Belts

   - help to    intra-abdominal pressure

   - appear to be safe way of improving lifting technique

   - Caution

        * must be careful that low back

          muscles don’t weaken because of use

Weight Lifting Concerns

   Recommendations for use of weight belts:

   1. Weight belt not needed for exercises that don’t involve low back.

   2. May use weight belt for near-max or max sets but not for lighter sets of low back exercises.

   3. When sufficient level of low back strength is evident, may be best to not use weight belt.

Weight Lifting Concerns

4. Shoulder Stability

   - due to shallowness of glenoid cavity, improper lifting can contribute to shoulder injury

   - Recommendations:

        a. Warm up with light weights

        b. Follow a program of balanced exercises (i.e. exercises that incorporate ab- and adduction in frontal plane, transverse plane, and internal and external rotation)

 

Weight Lifting Concerns

5. Knee Stability

   - patella and surrounding tissue are most susceptible in lifting

   - research on efficiency of knee wraps is lacking

        a. may lead to chondromalacia

           (wearing down of underside of patella)

        b. one study suggests improvement in lifting

            performance – assists in squat lifting force

        c. general recommendation – should

            minimize use of knee wraps

General Safety Tips with
Weight Lifting

n           Perform 1 or more warm-up sets with light weights

n           Perform exercises through full ROM

n           Perform new exercises with light wts, particularly after layoff of 2 or more weeks

n           Don’t ignore pain

n           Don’t attempt max lifts w/o proper training

General Safety Tips with
Weight Lifting

n            Post-workout icing of joints may be helpful to reduce inflammation

n            Supplemental exercises are helpful in maintaining balance,

         e.g. ratio of knee flexion to extension

                torque

                - at 60o/sec, ratio should be .67-.77

                - at 180o/sec, ratio should be .80-.91

                - at 300o/sec, ratio should be .95-1.11

General Safety Tips with
Weight Lifting

n           Avoid bouncing in weight lifting movements

n           Gain sufficient strength before incorporating plyometrics

n           Don’t deviate knee from vertical plane when performing squat

n           Avoid using knee & elbow wraps when having strong muscles

 

General Safety Tips with
Weight Lifting

n           Perform exercises with different variations of same muscle group,

     e.g. flat bench press, incline bench

            press, decline bench press

n           Avoid explosive exercises (e.g. cleans, jerks, snatches) without proper supervision

n           Incorporate proper breathing mechanics

Weight Lifting RX Movements

n     Wrist - flexion-exten. (sagittal)

              radial/ulnar deviation (frontal)

n     Elbow - flexion-exten. (sagittal)

             pronation/supination (transverse)

n     Shoulder - flexion/extension (sagittal)

                - ab/adduction (frontal)

                - internal/ext. rotation (transverse)

                - horizontal ab/adduction (transverse)

Weight Lifting RX Movements

n     Neck - flexion-exten. (sagittal)

           - left/right rotation (tranverse)

           - left/right tilt (frontal)

n     Low Back - flexion-exten. (sagittal)

                 - left/right tilt (frontal)

                 - left/right rotation (tranverse)

n     Hip - flexion/extension (sagittal)

        - ab/adduction (frontal)

        - internal/ext. rotation (transverse)

        - horizontal ab/adduction (transverse)

Weight Lifting RX Movements

n    Knee - flexion-exten. (sagittal)

n    Ankle - dorsi-flex/plantar-flex (sagittal)

            - inversion/eversion (frontal)

       

Weight Lifting RX Movements

Assignment:

   1. Work with a partner

   2. Derive a sport activity in which each of

       the Rx movements is used.  Use an

       example other than in your textbook.

        Example:

                Movement               Sport Activity