Neuromuscular Mechanics

Transcription

1 Schematic Representation of Motor Units in Skeletal Muscle Neuromuscular Mechanics Hamill & Knutzen (Ch 4) Whatever text you read do not focus on motorneuron structure and sensory receptors Muscle Fibres and Motor Units Muscle # Muscle # Motor Av. # of Fibres Units Motor U. Platysma 27,100 1, Brachioradialis 130, Tibialis anterior 250, Gastrocnemius 1,120, ,000 Innervation ratios vary from 1:2000 to 1:6 (extraocular muscles) 1

2 Tension Fibre Type Fast twitch (gastrocnemius) Slow twitch (Soleus) Tension See Figure 4-8 page 119 Twitch Wave Summation Incomplete Tetanus Complete Tetanus (unfused) (fused) Time Increased Stimulation Electromyography (EMG) EMG Telemetry Indwelling electrodes are also common in Biomechanical research If a large range of motion is required, leads on EMG electrodes may hinder movement 2

3 MEGA data logger system. This is different from telemetry as the device I am holding can store the EMG data (4 channels for 2 hours, sampled at 1000 Hz averaged and stored every 100 ms) for later downloading to a computer via a fibre optic link. Electrodes Pre-amplifier 300 N 0 a: Force Output from Isometric Contraction 2 mv b & c: EMG for Isometric Contraction 2 mv 3

4 Electromechanical Delay Static 4

5 Elevation Depression Biceps Brachii EMG Eccentric phase Biceps Brachii Activity IEMG Concentric Eccentric Flexor Force (N) EMG A suitably calibrated linear envelop EMG can be a coarse predictor of muscle tension for muscles that are not changing length rapidly (Winter, 1979). EMG signal amplitude during negative and positive work is similar despite more work being possible during negative work. Still an indicator of muscle activity? Suitable for most ergonomic (and rehab) work when properly calibrated with the subject s maximum voluntary contraction (MVC). 5

6 EMG during fatigue Biomechanical Factors Kumar (1999) argues a theory of overexertion that states overexertion can be created by exceeding the normal physical and physiological in any one of: force (Fx), exposure time (Dy), range of motion (Mz). The weighting of these three functions is obscure but Kumar symbolically represents overexertion (OE) with the equation below. OE = ( Fx, Dy, Mz) Musculoskeletal Injury (MSI) 6

7 Repetition Repetition does cause friction wear damage in tissues and this does contribute to MSI problems. This is more prevalent if the frequent movements include poor postures where more stress is placed on these tissues. However, research suggests that the sustained postures are the major cause of MSI. Blood Supply Sustained posture (or very short rest periods) keeps the blood supply away form the working tissue reducing the rate of repair. When the effort reaches 50-60% of the maximum, the flow is almost zero. At 10% MVC there is little mechanical hindrance to blood flow. However, studies have shown significant perceived fatigue at 5% of MVC. The literature shows that static contractions maintained for more than one hour should not exceed 5% of MVC. Rohmert Curve. (von Rohmert 1968) Static Postures Endurance time (minutes) Exertion Level (% max. force) Frequent Rest Periods 7

8 Trapezius MVC for Trapezius Erector Spinae Limit Values for Muscular Load Based on studies of muscular endurance during constrained static and dynamic work, Jonsson (1978) suggested the following limit values for work of one hour or longer duration. Static Load ideally 2%MVC & must not >5%MVC Median Load ideally 10%MVC & not >14%MVC Peak Load ideally 50%MVC & not >70%MVC P 1 Limit Values for Muscular Load Limit for full day of work 1 hour of work 10 minutes of work % % MVC Curve should not exceed these levels 8

9 EMG as % of MVC A = table at correct height B = table too high, resulting in elevation of shoulders C = table too high, compensated for by abduction of the arms Extensor Exercise Right Arm Both Arms Left Arm Bilateral Deficit and Co-Contraction 9

10 Biceps Brachii Activity Biceps Brachii EMG IEMG Concentric Eccentric Flexor Force (N) Data from Edman, Concentric Eccentric Muscle stimulated at 150 Hz for 300 ms, after 100 ms muscle length was decreased, held constant or increased. 10

11 Next Slide Positive work done by (a) an isolated muscle during a shortening contraction preceded by an isometric contraction and (b) a shortening contraction preceded by a lengthening contraction. The work done by the muscle (area under phase c of the length force curve is greater for the lengthening-shortening contraction (stretch-shortening). Stretch-Shortening Cycle This was discussed in the Chapter 5, Work and Machines, lecture. Stretch-Shortening Cycle Enhancement of Positive Work Return of stored energy from passive elastic structures within the muscle (cross-bridges and connective tissue (70-75% of increase?) Prior activation (time to develop force reduced) Initial increased force potentiation (eccentric contraction) Reflex augmentation (stretch reflex) Small amplitude high velocity no delay 11

12 Effects of Plyometric Training Adaptations to muscles include increased rate of force development and hypertrophy of type IIx fibres. Training with rats (Dooley et al. 1990) 15% increase in tetanic force 3% increase in max rate of force devel. 15% increase in fatigability 4% decrease in type IIa fibres Elasticity and Force Transmission If we study Kangaroos we find out that for a given speed they are more efficient than a quadruped like a cheetah. They are not as fast but they use less oxygen for a given speed (very sub-maximal for a Cheetah) This is because the muscle can work almost isometrically controlling the stretch and recoil of a VERY long achilles tendon. Work loops for (a) the plantaris muscle of a wallaby during hoping and (b) the pectoralis muscle of a pigeon during flying. Comfort versus speed? So a highly cushioned shoe seems like a good safety feature but maybe not. Too much elasticity ( give ) may affect stability Too much give may slow you down (you have to compress the elasticity material of the midsole before you can drive off again) Static stretching has been shown to reduce force transmission as it increases tendon compliance 12