Safe and effective weightlifting is all about technique. Proper form and muscle activation is crucial. For powerlifting movements like the deadlift and back squat, which are performed relatively slowly, maintaining proper form improves both safety and performance. For quicker Olympic weightlifting (for example, clean, jerk, and snatch) , in addition to form, the speed and dynamics of the movement execution are fundamental for success.
One way to ascertain weightlifting movement performance for standing exercises is by measuring the forces imparted by the feet to the ground. Newton's third law of motion tells us that when we lift an object (changing its velocity by exerting force on it), the object imparts a force equal in magnitude and opposite in direction on our body. If we are standing, that force is directed through our frame and ultimately to the ground through our feet. Thus, the forces on our feet carry information about how we are executing a lift. The distribution of forces front to rear and side to side indicate balance. The dynamics of the forces (how the forces vary with time) indicates speed and timing of the movements. For example, remembering that force equals mass times acceleration, a larger peak force will appear at the feet if a given weight is lifted quickly rather than slowly. Quick movements like the snatch depend upon rapid acceleration of the barbell early in the lift. The magnitude and timing of the acceleration shows up directly in the force applied by the feet to the ground.
In an attempt to study and better understand these movements, we have built a rudimentary dynamic force measurement device. The brassboard device consists of a rectangular plywood deck with a load cell located at each corner. Each load cell is sampled approximately 500 times per second with a resolution of about 0.25 lbs force (1.1 N), and the data is collected on a laptop for analysis. The deck is large enough so that movements which require a wide stance or shifting feet can be accommodated.
Initial calibration has been performed and preliminary software is written. Live weightlifting experiments will be performed soon to assess device performance and to suggest hardware and software improvements. We are looking forward to reviewing the data. The hope is that the data will reveal subtle differences in balance and technique between lifters, and will lead to possible methods for suggesting prescriptions for improved performance.
For anyone interested, here are some specifics on the hardware and software. The 3/4" birch plywood deck is aligned with a 1/4" MDF base plate with fixed 1/4" pins which mate with brass bushings in the deck. Electrically, each load cell is a Wheatstone bridge with piezoresistive elements, providing a differential analog signal proportional to the applied force. Signals from each cell are passed through instrumentation amplifiers to 12-bit analog-to-digital converters (ADCs). The ADCs are sampled by an Arduino-compatible microcontroller, which asynchronously sends force and timing data to a laptop via a USB port. Implementation is straightforward since the bandwidth is relatively low. On the laptop, the USB data stream is read with a Python script which uses calibration data to rescale the ADC counts to units of force. The data can then be analyzed to produce force and balance plots. We intend to synchronize the data plots with video of the lifts to provide a more visual interpretation of the dynamics.
If anyone is interested in schematics or code, feel free to contact us. Since the project is still evolving, details are subject to change.