Understanding mechanisms of cycling biomechanics allows recreational
cyclists to position themselves for optimal comfort and efficiency and competitive
cyclists to improve their performance in competition. It also helps people
undergoing physical therapy to attain maximum benefits from the use of stationary
bicycles, and reassures therapists that the demands placed on their patients
will improve their condition rather than hinder it.
An important force in cycling is power output with every cyclist
(recreational or competitive) having to overcome this in order to move the
bicycle in different conditions. Being able to apply pedalling forces effectively
is extremely important, while correct body positioning is crucial for successful
performance and injury prevention. This article considers some variables
of rider positioning and equipment set up that are important to all cyclists.
Saddle Height
Changing the saddle height alters a number of variables in cycling including
joint angles, muscle lengths and the force output muscles can produce. Research
has reported the optimal seat height (seated in an upright position) to
produce the most power output is 109% of leg length(1,2) (figure 1). This
height is considered most efficient for track sprint cyclists who have to
produce extremely high power outputs for very short periods. A 1% difference
in saddle height in either direction - from 109% of leg length - would produce
approximately 1% less power.
In moderate intensity cycling, which is performed under steady
state conditions, a saddle height between 105% and 107% of leg length requires
the lowest oxygen consumption(3,4). By using a lower oxygen consumption
for the same power output means an increased efficiency, which is important
for touring and endurance cyclists who have to ride for long periods.
From The Physiology and Biomechanics of Cycling (1)
In terms of performance, a saddle height of 109% of leg length is often
referred to as the ‘maximum saddle height’ and is recommended
for short-term power output, whereas a saddle height of 105-107% is known
as the ‘optimum saddle height’, regarded as most efficient for
events of lower intensity and longer duration.
The saddle height also affects the activity of the muscles
in the leg. As saddle height decreases the amount of muscle activity in
the quadriceps and hamstring muscle groups increases. A greater saddle height
allows cyclists to pedal with greater ease especially at high workloads.
Crank length
Crank length is another variable that contributes to force production. A
change in crank length can alter the distance between the saddle and the
pedal but with different consequences. Increasing the length of the crank
allows for greater torque production (force of the pedal rotating) and decreasing
the length of the crank would increase the amount of muscular tension involved
in force production, which could lead to earlier fatigue. Neither of these
occur if you just raise or lower the saddle height.
There are limitations on the length of the crank with longer
cranks assisting in hill climbing while shorter ones are best suited to
track events. The crank length must however allow the pedal to clear the
ground when cornering and must not interfere with steering of the front
wheel and must therefore be short enough for these circumstances.
Crank lengths can also affect the comfort of the cyclist.
If cranks are too long the hips and knees may have to flex (bend) too much
at top dead centre (TDC) position (highest point of the pedal) and can cause
great discomfort.
When changing crank length, the saddle-to-pedal distance must be altered
correspondingly to maintain the same effective saddle height. Since joint
and connective tissue injuries have been associated with over-long cranks
it is recommended caution is needed when experimenting with crank lengths.
Seat tube angle
The positive interaction between the cyclist and bicycle required to maximise
cycling performance is dependent on the design of the bicycle. The seat
tube angle (STA) between the top tube and the seat tube (see figure 2) is
important since it determines the rider’s comfort level.
The STAs cyclists use changes depending on the competition.
Competitive road cyclists claim that STAs between 72° and 76° are
most effective for optimal performance, while competitive triathletes often
ride with steeper STAs of 76-78°. A steeper STA allows a rider’s
body weight to be positioned further forward over the crank with triathletes
believing this allows for greater comfort, efficiency and power production
when using aerodynamic handlebars. There however may be advantages gained
from selecting a bicycle with an STA best suited to the chosen terrain.
Researchers examining the effects of STA during moderate intensity
cycling have reported more oxygen is used at shallower STAs (<76°),
compared to steeper STAs (>76°)(5,6) making performance easier when
using a steeper STA (>76°).
The standard recommendation for cyclists on STA has been to
select an angle that will result in the kneecap of the forward leg being
directly over the pedal axle when the cranks are horizontal. Although not
in the scientific research, it is believed that this position provides for
even weight distribution.
Saddle fore-aft position
The fore-aft position refers to the location of the rear of the saddle behind
a vertical line drawn to the centre of the crank axle (point of crank attachment).
The saddle is positioned to allow a plumb-line dropped from the patella
to bisect the pedal axle when the crank is in the horizontal forward position.
This enables the hip and knee extensor and flexor muscles to be fully utilised,
which minimises strain on the knee joint; any change in the fore-aft position
of the saddle will change the joint angles.
The position of the knee over the pedal axle should be regarded
as the starting point from which minor adjustments can be made. A saddle
positioned too far forward will decrease the knee angle at TDC and threfore
increase the activity of the quadriceps to extend the knee, raising the
risk of injuries. Conversely, a saddle positioned too far backward reduces
the effective working of the hamstrings, gluteus maximus and gastrocnemius
muscles.
Triathletes and time trialists have experimented with the
position of the saddle, with many adopting an extreme forward posture combined
with ‘maximum saddle height’ with the aim to reduce air resistance
while maximising power output. For safety reasons, such extreme positions
are confined to time trial and track events only.
Shoe-pedal interface
One of the main reasons why cyclists obtain knee injuries is due to the
connection of the foot with the pedal. It is accepted that the use of cleated
(wedged) cycling shoes on pedals with toe clips or the more recent clipless
pedal system allows a better knee alignment and greater power to be produced
compared against regular sport shoes.
Toe clips are also used to enhance performance by enhancing
efficiency early in the pedalling cycle. However the introduction of clipless
pedals has made injuries associated with incorrect positioning and alignment
of the foot more evident as fixing the cyclists foot to a pedal during cycling
places stress on the knee during pedalling. As a result, a pedal system
was designed to provide ‘float’ – a rotational allowance
of the shoe in relation to the pedal.
Placing the ball of the foot directly over the pedal helps
to decrease stresses across the knee ligaments and is the most efficient
cycling position. A more forward foot position, with the pedal either in
the arch of the foot or under the heel, increases hip extensor and flexor
activity but does not allow the full range of ankle motion needed for effective
force production.
Some cyclists prefer to mount the cleats on their cycling
shoes in such a way that the ball of the foot is slightly in front of or
behind the pedal axle. When the ball of the foot is ahead of the axle, the
effective lever arm from the ankle to the pedal axle is shortened, requiring
less force to stabilise the foot on the pedal and putting less strain on
the Achilles tendon and gastrocnemius. Triathletes and time trial specialists
favour this position as it allows them to produce more force when using
large gears, even though it limits their ability to pedal at high cadences.
Positioning the ball of the foot behind the pedal axle effectively
lengthens the lever arm from the ankle to the pedal axle, making it more
difficult for the foot to act as a rigid lever, and the Achilles tendon
and gastrocnemius have to work harder to stabilise the foot on the pedal.
Track cyclists favour this position since it allows them to pedal at higher
cadences during fixed gear events.
Upper body position
The handlebar design popular in triathlons and time trials was developed
to improve the aerodynamics of cyclists by allowing them to adopt a ‘tuck’
position similar to that used in downhill skiing. An aero handlebar with
elbow rests allows the rider to adopt a position with a flatter back and
reduced frontal area. Ultimately, the flexibility of the cyclist may determine
the most comfortable position on the bicycle
To achieve optimum power output and avoid injury it is important
that cyclists are positioned correctly and the bicycle is set up properly.
Saddle height, crank length, seat tube angle, saddle fore-aft position,
shoe-pedal interface and upper body position are all factors to be considered
to improve cycling performance and reduce injury.
References
1. The Physiology and Biomechanics of Cycling. New York: John Wiley and
Sons, 1978
2. Exercise and Sport Sciences Reviews, vol 19, pp 127-169, 1991
3. Medicine and Science in Sports and Exercise, vol 9, pp 113-117, 1977
4. Medicine and Science in Sports and Exercise, vol 8, pp 119-126, 1976
5. Medicine and Science in Sports and Exercise, vol 27, pp 730-735, 1995
6. Journal of Sports Sciences, vol 15, pp 395-402, 1997
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