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Biomechanics-of-Cycling

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					Reassessment Summer 2009
SP2004N Sports Biomechanics Roger Gossett

Summary Component Reassessment Task Reassessment Environment (include length if exam) 1200 words Handed in by standard deadline Exam 1 hour Additional info (optional)

CWK

EXU – Unseen Exam OTH – Practical Attendance

Non timeconstrained Practical report, (40%) Unseen timeconstrained examination (60%) Not reassessed

Details are attached together with data

n/a

Details of the Practical Report are provided below. Important – you must write-up the practical previously failed in Semester 1. The practical component of module SP2004N, Sports Biomechanics, counts 40% towards the assessment of the module. You are not required to write more than 1200 words for either practical write-up. Only one practical should be submitted for assessment: Practical 1 Using EMG and force measurements to evaluate force-angle relationships (DC) OR Practical 2 Force, Work and Power (RG). You must submit the same reassessment component that you previously attempted. Practical 1 Using EMG and force measurements to evaluate force-angle relationships (DC) Aim Evaluating isometric force production and muscle activation in the biceps in relation to joint angle. Equipment AD instruments EMG rig Strap dynamometer for force measurement. Goniometer for measuring joint angle Stopwatch for timing contraction Data entry sheets

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Aims In the biomechanics of resistance training lecture you were introduced to the concepts and the biomechanical and physiological causes of: 1) Muscle torque (rotational force) producing ability of muscle changes across the ROM (fig. 1) Which relates partly to the force-length relationship in muscle fibres (fig. 3) 2) resistive torque (ie. how hard it is to lift a constant load) varies across the ROM due to changes in the resistance arm (fig. 2) across the ROM of an exercise.
Fig 1. Changes in muscle torque across ROM Fig 2. Changes in resistance arm across ROM

D = length of resistance arm

Fig 3. Force v muscle fibre length

The aim is to quantify the effect of these underlying biomechanical and physiological principles in an intact joint. To this end force production and muscle activation have been measured in submaximal and maximal contractions at different joint angles. You will try explain your results in terms of the fundamental principles and original research in this area.

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Procedure One subject in every group has maximal isometric force tested in their dominant arm at three joint angles (45º, 90º, 120º) in random order. The subject(s) will also perform three isometric contractions holding a dumbbell in the three positions. In both cases EMG is recorded at the bicep. 1. Calibration As a form of calibration and to check that the system is working first do three tests at the same joint angle with three different resistance loads. 2. Strap dynamometer-maximal isometric test The length of the strap and consequently the joint angle and degree of flexion is adjusted using the goniometer to determine the joint angles. The axis of the goniometer should be aligned with the Olecranon process (elbow) with full extension being zero degrees and full flexion being in the region of 150 degrees. Try to create 0º between the strap ends- ie. the strap should fall vertically straight from where it is held by the subject to the other end of the strap, so someone else should stand on the free end. The subject should contract for 3 seconds with the highest force (N) recorded. One member of group should be checking that the joint positions are maintained during the contraction. One subject should watch the dynamometer screen and note the highest force achieved and one subject should give hold the stopwatch and give the start and end command. Allow the subject to rest for 60 seconds and use goniometer to set the next angle. Highlight the highest 1 second period on the EMG trace and record the mv value (using the data pad) for this alongside the force generated at that angle. Repeat to counterbalance the possible fatigue effects and record the mean for each angle in the table below. 3. Isometric-isoinertial test As above except that subject holds a dumbbell for three seconds at the joint angle selected instead of producing a maximal contraction against the strap. Joint positions for test The subject who is being tested should have their elbow tight to their side and upper arm in neutral (not flexed or extended), shoulders should be relaxed (not elevated). Feet should be together and knees full extended. These joint positions should be also be maintained during the test contraction. One end of the strap should be held in with a supinated position (palm up) and the other end under the mid-foot of the side being tested. Questions For all graphs except for the joint angle vs maximum isometric force and emg (Q.4) you are using data for one subject. For Q.4 calculate and use and graph the mean normalised values for the three subjects.

CALIBRATION
Three progressively heavier weights were held at the same joint angle (90°) using an isometric contraction. 1. Draw a graph of EMG (y axis) against load (x axis) using the data (10%) graph 1

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2. Why was EMG used in this practical and briefly what does in represent? (10%) CONSTANT LOAD-VARYING JOINT ANGLE A selected weight was held at three different joint angles with an isometric contraction, EMG measured. 3. Draw a graph of normalised EMG (y axis) against joint angle (x axis) graph 2 (10%) JOINT ANGLE vs MAXIMUM ISOMETRIC FORCE and EMG 4. Draw a graph of normalised maximum isometric force (y axis 1) and normalised EMG (y axis 2) against joint angle (x axis) (15%) graph 3 5. If these results were unexpected sketch the lines you were expecting for maximum isometric force and EMG. In either case, explain what and how the biomechanical and / or physiological effects that we have discussed in the lecture and intro to the practical may have influenced these results? (30%) 6. We measured maximum isometric force output at three different joint angles. What information would you need to calculate the torque at each angle and how would you use it? (15%) 7. Why did we use EMG normalised to maximum EMG rather than the absolute value? ` (10%) * “normalised” EMG and force is where the raw values have been recalculated as % of the maximum for each subject.

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Suggested textbooks to refer to (from your bibliography) Bartlett, R. (1997) Introduction to sports biomechanics. London: E. & F. N. Spon. Carr, G. (2004) Sport Mechanics for Coaches, (2nd ed.) Champaign, IL: Human Kinetics

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DATA CALIBRATION TABLE – GRAPH 1 EMG data (mv) subject who held a weight at 90 degrees of flexion. Three progressively heavier weights were used and the EMG measurements taken. CALIBRATION WEIGHT 1 (2kg) WEIGHT 2 (4kg) WEIGHT 3 (6kg) SUBJECT 1 12 25 45

CONSTANT WEIGHT HELD ISOMETRICALLY AT 3 JOINT ANGLES- DATA FOR GRAPH 2 The subjects held a weight at three different joint angles. The weight used was constant but the joint angle changed with EMG measured (mv) ANGLE 45 90 135 SUBJECT 1 52 27 29

MAXIMAL ISOMETRIC CONTRACTION - DATA FOR GRAPH 3 Subjects used the strap dynamometer and were asked to maximally contract at three different joint angles. At each angle, EMG (mv) and force (N) was measured. ANGLE Force / EMG Normalised to 100% Force EMG Force EMG Force EMG SUBJECT 1 SUBJECT 2 SUBJECT 3

45 90 135

100 100 74 87 91 82

91 77 81 100 100 51

100 80 67 100 87 97

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Practical 2 Force, Work and Power (RG) In this experiment physiological estimation of oxygen consumption is made in order to provide a measure of cycling efficiency. The more efficient a cyclist is at maintaining any given speed the less oxygen he or she will consume. Cycling is performed on a set of cycle rollers rather than a cycle ergometer. This is because the estimation is designed to be as close to the real cycling situation as possible (see notes). An investigation into cycling efficiency and cadence At any given speed cadence is affected by gear ratio. By using a lower gear ratio cycling becomes less strenuous but, to maintain this speed, the cyclist must adopt a higher cadence. The question that this experiment investigates is: “For any given speed is there an optimal gear ratio at which the efficiency of the cyclist is maximal?” The lowering of force per pedal stroke by the use of higher cadences would seem to be a mechanism whereby the recruitment of Type II fibres is minimised. Select a given speed, say 20 kilometres per hour (20 km•hr-1, which is approximately (20 x 5) / 8 = 12.5 mile per hour (12.5 mph). Ask the cyclist to pedal at this speed using a selection of gear ratios and measure efficiency for each ratio. Plot a graph of efficiency against gear ratio (or cadence) and look for an optimum value of efficiency. Selection of gear ratios: ask the cyclist what seems to be the most comfortable at the chosen speed then set the protocol by taking 3 ratios below and 3 ratios above the self-selected chosen value. Protocol; (i) monitor heart rate throughout the experiment, (ii) prepare the front wheel with a suitable marker and note the cadence at each gear ratio, (iii) measure the diameter of the front wheel. Practical write-up Provide answers to Questions 1-5. The total word count for this practical is 1200 words and Questions 1 and 2 count 60 words each. Question 3(i) should be answered in approximately 360 words, and Question 3(ii) in approximately 120 words. Questions 4 and 5 require answers of around 300 words each. The notes that accompanied this practical and the lecture „Cycling Biomechanics‟ provided all necessary background information. You should also incorporate further up-to-date information from the scientific literature where appropriate and reference this accordingly. The mark allocation for each Question is as follows: Qs 1-2, 5% each; Q 3(i) 30%; and, Q 3(ii) 10%; Qs 4 and 5, 25% each.

Q1

Calculate oxygen consumption (VO2) and plot a graph of oxygen consumption (Y-axis) against cadence (X-axis) and comment on the shape of the graph. Calculate the distance travelled with one revolution of the crank at each gear ratio – show clearly how you arrived at your answer: the diameter of the wheel = 600 mm. Most cyclists have a preferred cadence when racing along relatively flat terrain. (i) Use principles of physiology and biomechanics to explain why this may be so. Page 6 of 7

Q2

Q3

(ii)

Does a cyclist‟s preferred cadence necessarily correspond to that at which they are most efficient? Explain your answer

Q4

Use principles of biomechanics and physiology to explain why variation in tyre pressure affects average oxygen consumption during steady state cycling at a given speed and cadence. Use principles of biomechanics to explain why it requires more energy to cycle up-hill compared to cycling along a flat road.

Q5

Data Gear Ratio Cadence rpm 48:22 38:22 28:22 48rpm 60rpm 80rpm Expired Oxygen % 15.8% 16.2% 15.9% Ve L/min 25.6 20.7 24.2 VO2 L/min

Ve = Volume (L) of expired air per minute; VO2 = Volume (L) of oxygen consumed per minute Use the following equation to calculate VO2 VO2 = [(20.9 - Expired Oxygen %)/100] x Ve

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