Bike Fit

Oct 23, 2019

Cycling has always been a popular sport and recreation in Australia. Its popularity continues to grow as a means of fitness and eco friendly transportation. Recent years has seen its popularity rise amongst middle aged men keen to improve their fitness without the stress on the joints that running or other impact sports cause. Groups of “MAMILS” aka middle-aged men in lycra can be seen weaving through the dawn streets in Canberra. Apart from the obvious health benefits, these regular cycling sessions are a great way of maintaining social contact and support. The potential mental health benefits that something as simple as a regular cycle session with mates shouldn’t be underestimated. This explosion in popularity has also seen an increase in cycling related overuse injuries. Many of these injuries are preventable with adequate preparation, training and having optimal bike fit and rider position. Injuries occur through repetitive and excessive loading of joints and soft tissue and both intrinsic and extrinsic factors contribute. Intrinsic factors include the riders anatomic alignment and fitness. Extrinsic factors include the bike, equipment, training and riding technique.

Proper bike fit is very important in optimising riding technique and efficiency and in decreasing the incidence of overuse injuries. Your local bike shop will offer a fitting service as do a number of physiotherapists. Frame size, saddle height and position, handlebar height, crankshaft  length and foot position are the basic fit-related adjustments that should be made for every rider.

1. Frame size & saddle position

Frame size can be determined by measuring the distance between the crotch of the rider and the top tube of the frame. For road bikes a distance of 2.5-5cm is ideal and for off road bikes the clearance should be 7.5-15cm.

Correct positioning of the seat is very important for efficient cycling mechanics. Improper seat height and position can lead to excessive strain and loads being placed on the hip, knee and ankle joints and contribute to repetitive injuries including tendonitis and back and knee pain. Multiplying the inseam measurement by 1.09 will provide a seat height that will optimise cycling mechanics. The simplest and most common method of determining seat height is to have the rider seated on a secured bike. The seat height is then adjusted so that the knee is flexed to 30 degrees when the pedal is at its lowest position.

The forward position of the seat is determined by dropping a plumb line from the tibial knot (tubercle) such that it crosses the axle of the forward pedal when the pedals are in the 3 and 9 O’clock positions.

2. Handle bar position

Once the seat position is determined, positioning of the handlebar will determine the reach of the rider. Poor positioning of the handlebars can contribute to lower back discomfort. A simple estimate of appropriate handlebar position is determined by measuring the distance from the riders tip of elbow (olecranon) to the tip of the long finger. The handlebars should be adjusted so that the distance from the forward tip of the seat to the centre of the bars matches this measurement. Typically the height of the handlebars is lower than that of the seat. For road bikes the handle bars are 3-10 cm lower. The lower the bars will allow the rider to assume a more crouched aerodynamic position.

3. Crank length & cleats

Crank length is the distance from the centre of the chain ring to the axle of the pedal. This distance will determine the arc that is made during the revolution. A longer crank length provides mechanical advantage but also put the hip and knee joints through a larger range of motion. A good point to start with is by measuring the distance from the greater trochanter to the ground and multiplying by 0.185 and setting this as the crank length. Small adjustments can then be made to suit the comfort of the rider.

Poor positioning of toe clips and cleats can lead to foot pain and mechanical disadvantage. They should be positioned so that the ball of the foot is over the pedal axle. Cleats should also be well centred on the shoe so that the foot is not forced into a rotated position.

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