McCaw, S. T., & Melrose, D. R. (1999). Stance width and bar load effects on leg muscle activity during the parallel squat. Medicine and Science in Sports and Exercise, 31, 428-436.

The purpose of this study was to compare activity in six muscles crossing the hip and/or knee joints when the parallel squat was performed with different stances and bar loads. EMG data were collected when parallel squat stances were 75, 100, and 140% of shoulder width with 60 and 75% 1RM loads.

Some muscles were only sensitive to load changes, others interacted with load and stance. Stance width did not cause alterations within the quadriceps but did influence muscle activity on the medial thigh and buttock muscles. Load changes modified the quadriceps' responses.

Implication. Slight changes in exercise load and position, alter muscle functions across groups. Some muscles respond to load only, while others respond to load and position. These features support the specificity of training principle and demonstrate significant alterations at the neuromuscular level when exercises are "altered" or "varied."

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